compiler/rustc_hir_typeck/src/demand.rs

   1 use crate::FnCtxt;
   2 use rustc_ast::util::parser::PREC_POSTFIX;
   3 use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
   4 use rustc_hir as hir;
   5 use rustc_hir::def::CtorKind;
   6 use rustc_hir::lang_items::LangItem;
   7 use rustc_hir::{is_range_literal, Node};
   8 use rustc_infer::infer::InferOk;
   9 use rustc_middle::lint::in_external_macro;
  10 use rustc_middle::middle::stability::EvalResult;
  11 use rustc_middle::ty::adjustment::AllowTwoPhase;
  12 use rustc_middle::ty::error::{ExpectedFound, TypeError};
  13 use rustc_middle::ty::print::with_no_trimmed_paths;
  14 use rustc_middle::ty::{self, Article, AssocItem, Ty, TypeAndMut};
  15 use rustc_span::symbol::{sym, Symbol};
  16 use rustc_span::{BytePos, Span};
  17 use rustc_trait_selection::infer::InferCtxtExt as _;
  18 use rustc_trait_selection::traits::ObligationCause;
  19
  20 use super::method::probe;
  21
  22 use std::iter;
  23
  24 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  25     pub fn emit_coerce_suggestions(
  26         &self,
  27         err: &mut Diagnostic,
  28         expr: &hir::Expr<'tcx>,
  29         expr_ty: Ty<'tcx>,
  30         expected: Ty<'tcx>,
  31         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
  32         error: Option<TypeError<'tcx>>,
  33     ) {
  34         if expr_ty == expected {
  35             return;
  36         }
  37
  38         self.annotate_expected_due_to_let_ty(err, expr, error);
  39
  40         // Use `||` to give these suggestions a precedence
  41         let _ = self.suggest_missing_parentheses(err, expr)
  42             || self.suggest_deref_ref_or_into(err, expr, expected, expr_ty, expected_ty_expr)
  43             || self.suggest_compatible_variants(err, expr, expected, expr_ty)
  44             || self.suggest_non_zero_new_unwrap(err, expr, expected, expr_ty)
  45             || self.suggest_calling_boxed_future_when_appropriate(err, expr, expected, expr_ty)
  46             || self.suggest_no_capture_closure(err, expected, expr_ty)
  47             || self.suggest_boxing_when_appropriate(err, expr, expected, expr_ty)
  48             || self.suggest_block_to_brackets_peeling_refs(err, expr, expr_ty, expected)
  49             || self.suggest_copied_or_cloned(err, expr, expr_ty, expected)
  50             || self.suggest_into(err, expr, expr_ty, expected)
  51             || self.suggest_option_to_bool(err, expr, expr_ty, expected)
  52             || self.suggest_floating_point_literal(err, expr, expected);
  53
  54         self.note_type_is_not_clone(err, expected, expr_ty, expr);
  55         self.note_need_for_fn_pointer(err, expected, expr_ty);
  56         self.note_internal_mutation_in_method(err, expr, expected, expr_ty);
  57     }
  58
  59     // Requires that the two types unify, and prints an error message if
  60     // they don't.
  61     pub fn demand_suptype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  62         if let Some(mut e) = self.demand_suptype_diag(sp, expected, actual) {
  63             e.emit();
  64         }
  65     }
  66
  67     pub fn demand_suptype_diag(
  68         &self,
  69         sp: Span,
  70         expected: Ty<'tcx>,
  71         actual: Ty<'tcx>,
  72     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
  73         self.demand_suptype_with_origin(&self.misc(sp), expected, actual)
  74     }
  75
  76     #[instrument(skip(self), level = "debug")]
  77     pub fn demand_suptype_with_origin(
  78         &self,
  79         cause: &ObligationCause<'tcx>,
  80         expected: Ty<'tcx>,
  81         actual: Ty<'tcx>,
  82     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
  83         match self.at(cause, self.param_env).sup(expected, actual) {
  84             Ok(InferOk { obligations, value: () }) => {
  85                 self.register_predicates(obligations);
  86                 None
  87             }
  88             Err(e) => Some(self.err_ctxt().report_mismatched_types(&cause, expected, actual, e)),
  89         }
  90     }
  91
  92     pub fn demand_eqtype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  93         if let Some(mut err) = self.demand_eqtype_diag(sp, expected, actual) {
  94             err.emit();
  95         }
  96     }
  97
  98     pub fn demand_eqtype_diag(
  99         &self,
 100         sp: Span,
 101         expected: Ty<'tcx>,
 102         actual: Ty<'tcx>,
 103     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
 104         self.demand_eqtype_with_origin(&self.misc(sp), expected, actual)
 105     }
 106
 107     pub fn demand_eqtype_with_origin(
 108         &self,
 109         cause: &ObligationCause<'tcx>,
 110         expected: Ty<'tcx>,
 111         actual: Ty<'tcx>,
 112     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
 113         match self.at(cause, self.param_env).eq(expected, actual) {
 114             Ok(InferOk { obligations, value: () }) => {
 115                 self.register_predicates(obligations);
 116                 None
 117             }
 118             Err(e) => Some(self.err_ctxt().report_mismatched_types(cause, expected, actual, e)),
 119         }
 120     }
 121
 122     pub fn demand_coerce(
 123         &self,
 124         expr: &hir::Expr<'tcx>,
 125         checked_ty: Ty<'tcx>,
 126         expected: Ty<'tcx>,
 127         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 128         allow_two_phase: AllowTwoPhase,
 129     ) -> Ty<'tcx> {
 130         let (ty, err) =
 131             self.demand_coerce_diag(expr, checked_ty, expected, expected_ty_expr, allow_two_phase);
 132         if let Some(mut err) = err {
 133             err.emit();
 134         }
 135         ty
 136     }
 137
 138     /// Checks that the type of `expr` can be coerced to `expected`.
 139     ///
 140     /// N.B., this code relies on `self.diverges` to be accurate. In particular, assignments to `!`
 141     /// will be permitted if the diverges flag is currently "always".
 142     #[instrument(level = "debug", skip(self, expr, expected_ty_expr, allow_two_phase))]
 143     pub fn demand_coerce_diag(
 144         &self,
 145         expr: &hir::Expr<'tcx>,
 146         checked_ty: Ty<'tcx>,
 147         expected: Ty<'tcx>,
 148         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 149         allow_two_phase: AllowTwoPhase,
 150     ) -> (Ty<'tcx>, Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>>) {
 151         let expected = self.resolve_vars_with_obligations(expected);
 152
 153         let e = match self.try_coerce(expr, checked_ty, expected, allow_two_phase, None) {
 154             Ok(ty) => return (ty, None),
 155             Err(e) => e,
 156         };
 157
 158         self.set_tainted_by_errors(self.tcx.sess.delay_span_bug(
 159             expr.span,
 160             "`TypeError` when attempting coercion but no error emitted",
 161         ));
 162         let expr = expr.peel_drop_temps();
 163         let cause = self.misc(expr.span);
 164         let expr_ty = self.resolve_vars_with_obligations(checked_ty);
 165         let mut err = self.err_ctxt().report_mismatched_types(&cause, expected, expr_ty, e.clone());
 166
 167         let is_insufficiently_polymorphic =
 168             matches!(e, TypeError::RegionsInsufficientlyPolymorphic(..));
 169
 170         // FIXME(#73154): For now, we do leak check when coercing function
 171         // pointers in typeck, instead of only during borrowck. This can lead
 172         // to these `RegionsInsufficientlyPolymorphic` errors that aren't helpful.
 173         if !is_insufficiently_polymorphic {
 174             self.emit_coerce_suggestions(
 175                 &mut err,
 176                 expr,
 177                 expr_ty,
 178                 expected,
 179                 expected_ty_expr,
 180                 Some(e),
 181             );
 182         }
 183
 184         (expected, Some(err))
 185     }
 186
 187     fn annotate_expected_due_to_let_ty(
 188         &self,
 189         err: &mut Diagnostic,
 190         expr: &hir::Expr<'_>,
 191         error: Option<TypeError<'_>>,
 192     ) {
 193         let parent = self.tcx.hir().get_parent_node(expr.hir_id);
 194         match (self.tcx.hir().find(parent), error) {
 195             (Some(hir::Node::Local(hir::Local { ty: Some(ty), init: Some(init), .. })), _)
 196                 if init.hir_id == expr.hir_id =>
 197             {
 198                 // Point at `let` assignment type.
 199                 err.span_label(ty.span, "expected due to this");
 200             }
 201             (
 202                 Some(hir::Node::Expr(hir::Expr {
 203                     kind: hir::ExprKind::Assign(lhs, rhs, _), ..
 204                 })),
 205                 Some(TypeError::Sorts(ExpectedFound { expected, .. })),
 206             ) if rhs.hir_id == expr.hir_id && !expected.is_closure() => {
 207                 // We ignore closures explicitly because we already point at them elsewhere.
 208                 // Point at the assigned-to binding.
 209                 let mut primary_span = lhs.span;
 210                 let mut secondary_span = lhs.span;
 211                 let mut post_message = "";
 212                 match lhs.kind {
 213                     hir::ExprKind::Path(hir::QPath::Resolved(
 214                         None,
 215                         hir::Path {
 216                             res:
 217                                 hir::def::Res::Def(
 218                                     hir::def::DefKind::Static(_) | hir::def::DefKind::Const,
 219                                     def_id,
 220                                 ),
 221                             ..
 222                         },
 223                     )) => {
 224                         if let Some(hir::Node::Item(hir::Item {
 225                             ident,
 226                             kind: hir::ItemKind::Static(ty, ..) | hir::ItemKind::Const(ty, ..),
 227                             ..
 228                         })) = self.tcx.hir().get_if_local(*def_id)
 229                         {
 230                             primary_span = ty.span;
 231                             secondary_span = ident.span;
 232                             post_message = " type";
 233                         }
 234                     }
 235                     hir::ExprKind::Path(hir::QPath::Resolved(
 236                         None,
 237                         hir::Path { res: hir::def::Res::Local(hir_id), .. },
 238                     )) => {
 239                         if let Some(hir::Node::Pat(pat)) = self.tcx.hir().find(*hir_id) {
 240                             let parent = self.tcx.hir().get_parent_node(pat.hir_id);
 241                             primary_span = pat.span;
 242                             secondary_span = pat.span;
 243                             match self.tcx.hir().find(parent) {
 244                                 Some(hir::Node::Local(hir::Local { ty: Some(ty), .. })) => {
 245                                     primary_span = ty.span;
 246                                     post_message = " type";
 247                                 }
 248                                 Some(hir::Node::Local(hir::Local { init: Some(init), .. })) => {
 249                                     primary_span = init.span;
 250                                     post_message = " value";
 251                                 }
 252                                 Some(hir::Node::Param(hir::Param { ty_span, .. })) => {
 253                                     primary_span = *ty_span;
 254                                     post_message = " parameter type";
 255                                 }
 256                                 _ => {}
 257                             }
 258                         }
 259                     }
 260                     _ => {}
 261                 }
 262
 263                 if primary_span != secondary_span
 264                     && self
 265                         .tcx
 266                         .sess
 267                         .source_map()
 268                         .is_multiline(secondary_span.shrink_to_hi().until(primary_span))
 269                 {
 270                     // We are pointing at the binding's type or initializer value, but it's pattern
 271                     // is in a different line, so we point at both.
 272                     err.span_label(secondary_span, "expected due to the type of this binding");
 273                     err.span_label(primary_span, &format!("expected due to this{post_message}"));
 274                 } else if post_message == "" {
 275                     // We are pointing at either the assignment lhs or the binding def pattern.
 276                     err.span_label(primary_span, "expected due to the type of this binding");
 277                 } else {
 278                     // We are pointing at the binding's type or initializer value.
 279                     err.span_label(primary_span, &format!("expected due to this{post_message}"));
 280                 }
 281
 282                 if !lhs.is_syntactic_place_expr() {
 283                     // We already emitted E0070 "invalid left-hand side of assignment", so we
 284                     // silence this.
 285                     err.downgrade_to_delayed_bug();
 286                 }
 287             }
 288             _ => {}
 289         }
 290     }
 291
 292     /// If the expected type is an enum (Issue #55250) with any variants whose
 293     /// sole field is of the found type, suggest such variants. (Issue #42764)
 294     fn suggest_compatible_variants(
 295         &self,
 296         err: &mut Diagnostic,
 297         expr: &hir::Expr<'_>,
 298         expected: Ty<'tcx>,
 299         expr_ty: Ty<'tcx>,
 300     ) -> bool {
 301         if let ty::Adt(expected_adt, substs) = expected.kind() {
 302             if let hir::ExprKind::Field(base, ident) = expr.kind {
 303                 let base_ty = self.typeck_results.borrow().expr_ty(base);
 304                 if self.can_eq(self.param_env, base_ty, expected).is_ok()
 305                     && let Some(base_span) = base.span.find_ancestor_inside(expr.span)
 306                 {
 307                     err.span_suggestion_verbose(
 308                         expr.span.with_lo(base_span.hi()),
 309                         format!("consider removing the tuple struct field `{ident}`"),
 310                         "",
 311                         Applicability::MaybeIncorrect,
 312                     );
 313                     return true;
 314                 }
 315             }
 316
 317             // If the expression is of type () and it's the return expression of a block,
 318             // we suggest adding a separate return expression instead.
 319             // (To avoid things like suggesting `Ok(while .. { .. })`.)
 320             if expr_ty.is_unit() {
 321                 let mut id = expr.hir_id;
 322                 let mut parent;
 323
 324                 // Unroll desugaring, to make sure this works for `for` loops etc.
 325                 loop {
 326                     parent = self.tcx.hir().get_parent_node(id);
 327                     if let Some(parent_span) = self.tcx.hir().opt_span(parent) {
 328                         if parent_span.find_ancestor_inside(expr.span).is_some() {
 329                             // The parent node is part of the same span, so is the result of the
 330                             // same expansion/desugaring and not the 'real' parent node.
 331                             id = parent;
 332                             continue;
 333                         }
 334                     }
 335                     break;
 336                 }
 337
 338                 if let Some(hir::Node::Block(&hir::Block {
 339                     span: block_span, expr: Some(e), ..
 340                 })) = self.tcx.hir().find(parent)
 341                 {
 342                     if e.hir_id == id {
 343                         if let Some(span) = expr.span.find_ancestor_inside(block_span) {
 344                             let return_suggestions = if self
 345                                 .tcx
 346                                 .is_diagnostic_item(sym::Result, expected_adt.did())
 347                             {
 348                                 vec!["Ok(())"]
 349                             } else if self.tcx.is_diagnostic_item(sym::Option, expected_adt.did()) {
 350                                 vec!["None", "Some(())"]
 351                             } else {
 352                                 return false;
 353                             };
 354                             if let Some(indent) =
 355                                 self.tcx.sess.source_map().indentation_before(span.shrink_to_lo())
 356                             {
 357                                 // Add a semicolon, except after `}`.
 358                                 let semicolon =
 359                                     match self.tcx.sess.source_map().span_to_snippet(span) {
 360                                         Ok(s) if s.ends_with('}') => "",
 361                                         _ => ";",
 362                                     };
 363                                 err.span_suggestions(
 364                                     span.shrink_to_hi(),
 365                                     "try adding an expression at the end of the block",
 366                                     return_suggestions
 367                                         .into_iter()
 368                                         .map(|r| format!("{semicolon}\n{indent}{r}")),
 369                                     Applicability::MaybeIncorrect,
 370                                 );
 371                             }
 372                             return true;
 373                         }
 374                     }
 375                 }
 376             }
 377
 378             let compatible_variants: Vec<(String, _, _, Option<String>)> = expected_adt
 379                 .variants()
 380                 .iter()
 381                 .filter(|variant| {
 382                     variant.fields.len() == 1
 383                 })
 384                 .filter_map(|variant| {
 385                     let sole_field = &variant.fields[0];
 386
 387                     let field_is_local = sole_field.did.is_local();
 388                     let field_is_accessible =
 389                         sole_field.vis.is_accessible_from(expr.hir_id.owner.def_id, self.tcx)
 390                         // Skip suggestions for unstable public fields (for example `Pin::pointer`)
 391                         && matches!(self.tcx.eval_stability(sole_field.did, None, expr.span, None), EvalResult::Allow | EvalResult::Unmarked);
 392
 393                     if !field_is_local && !field_is_accessible {
 394                         return None;
 395                     }
 396
 397                     let note_about_variant_field_privacy = (field_is_local && !field_is_accessible)
 398                         .then(|| format!(" (its field is private, but it's local to this crate and its privacy can be changed)"));
 399
 400                     let sole_field_ty = sole_field.ty(self.tcx, substs);
 401                     if self.can_coerce(expr_ty, sole_field_ty) {
 402                         let variant_path =
 403                             with_no_trimmed_paths!(self.tcx.def_path_str(variant.def_id));
 404                         // FIXME #56861: DRYer prelude filtering
 405                         if let Some(path) = variant_path.strip_prefix("std::prelude::")
 406                             && let Some((_, path)) = path.split_once("::")
 407                         {
 408                             return Some((path.to_string(), variant.ctor_kind(), sole_field.name, note_about_variant_field_privacy));
 409                         }
 410                         Some((variant_path, variant.ctor_kind(), sole_field.name, note_about_variant_field_privacy))
 411                     } else {
 412                         None
 413                     }
 414                 })
 415                 .collect();
 416
 417             let suggestions_for = |variant: &_, ctor_kind, field_name| {
 418                 let prefix = match self.maybe_get_struct_pattern_shorthand_field(expr) {
 419                     Some(ident) => format!("{ident}: "),
 420                     None => String::new(),
 421                 };
 422
 423                 let (open, close) = match ctor_kind {
 424                     Some(CtorKind::Fn) => ("(".to_owned(), ")"),
 425                     None => (format!(" {{ {field_name}: "), " }"),
 426
 427                     // unit variants don't have fields
 428                     Some(CtorKind::Const) => unreachable!(),
 429                 };
 430
 431                 // Suggest constructor as deep into the block tree as possible.
 432                 // This fixes https://github.com/rust-lang/rust/issues/101065,
 433                 // and also just helps make the most minimal suggestions.
 434                 let mut expr = expr;
 435                 while let hir::ExprKind::Block(block, _) = &expr.kind
 436                     && let Some(expr_) = &block.expr
 437                 {
 438                     expr = expr_
 439                 }
 440
 441                 vec![
 442                     (expr.span.shrink_to_lo(), format!("{prefix}{variant}{open}")),
 443                     (expr.span.shrink_to_hi(), close.to_owned()),
 444                 ]
 445             };
 446
 447             match &compatible_variants[..] {
 448                 [] => { /* No variants to format */ }
 449                 [(variant, ctor_kind, field_name, note)] => {
 450                     // Just a single matching variant.
 451                     err.multipart_suggestion_verbose(
 452                         &format!(
 453                             "try wrapping the expression in `{variant}`{note}",
 454                             note = note.as_deref().unwrap_or("")
 455                         ),
 456                         suggestions_for(&**variant, *ctor_kind, *field_name),
 457                         Applicability::MaybeIncorrect,
 458                     );
 459                     return true;
 460                 }
 461                 _ => {
 462                     // More than one matching variant.
 463                     err.multipart_suggestions(
 464                         &format!(
 465                             "try wrapping the expression in a variant of `{}`",
 466                             self.tcx.def_path_str(expected_adt.did())
 467                         ),
 468                         compatible_variants.into_iter().map(
 469                             |(variant, ctor_kind, field_name, _)| {
 470                                 suggestions_for(&variant, ctor_kind, field_name)
 471                             },
 472                         ),
 473                         Applicability::MaybeIncorrect,
 474                     );
 475                     return true;
 476                 }
 477             }
 478         }
 479
 480         false
 481     }
 482
 483     fn suggest_non_zero_new_unwrap(
 484         &self,
 485         err: &mut Diagnostic,
 486         expr: &hir::Expr<'_>,
 487         expected: Ty<'tcx>,
 488         expr_ty: Ty<'tcx>,
 489     ) -> bool {
 490         let tcx = self.tcx;
 491         let (adt, unwrap) = match expected.kind() {
 492             // In case Option<NonZero*> is wanted, but * is provided, suggest calling new
 493             ty::Adt(adt, substs) if tcx.is_diagnostic_item(sym::Option, adt.did()) => {
 494                 // Unwrap option
 495                 let ty::Adt(adt, _) = substs.type_at(0).kind() else { return false; };
 496
 497                 (adt, "")
 498             }
 499             // In case NonZero* is wanted, but * is provided also add `.unwrap()` to satisfy types
 500             ty::Adt(adt, _) => (adt, ".unwrap()"),
 501             _ => return false,
 502         };
 503
 504         let map = [
 505             (sym::NonZeroU8, tcx.types.u8),
 506             (sym::NonZeroU16, tcx.types.u16),
 507             (sym::NonZeroU32, tcx.types.u32),
 508             (sym::NonZeroU64, tcx.types.u64),
 509             (sym::NonZeroU128, tcx.types.u128),
 510             (sym::NonZeroI8, tcx.types.i8),
 511             (sym::NonZeroI16, tcx.types.i16),
 512             (sym::NonZeroI32, tcx.types.i32),
 513             (sym::NonZeroI64, tcx.types.i64),
 514             (sym::NonZeroI128, tcx.types.i128),
 515         ];
 516
 517         let Some((s, _)) = map
 518             .iter()
 519             .find(|&&(s, t)| self.tcx.is_diagnostic_item(s, adt.did()) && self.can_coerce(expr_ty, t))
 520             else { return false; };
 521
 522         let path = self.tcx.def_path_str(adt.non_enum_variant().def_id);
 523
 524         err.multipart_suggestion(
 525             format!("consider calling `{s}::new`"),
 526             vec![
 527                 (expr.span.shrink_to_lo(), format!("{path}::new(")),
 528                 (expr.span.shrink_to_hi(), format!("){unwrap}")),
 529             ],
 530             Applicability::MaybeIncorrect,
 531         );
 532
 533         true
 534     }
 535
 536     pub fn get_conversion_methods(
 537         &self,
 538         span: Span,
 539         expected: Ty<'tcx>,
 540         checked_ty: Ty<'tcx>,
 541         hir_id: hir::HirId,
 542     ) -> Vec<AssocItem> {
 543         let methods = self.probe_for_return_type(
 544             span,
 545             probe::Mode::MethodCall,
 546             expected,
 547             checked_ty,
 548             hir_id,
 549             |m| {
 550                 self.has_only_self_parameter(m)
 551                     && self
 552                         .tcx
 553                         // This special internal attribute is used to permit
 554                         // "identity-like" conversion methods to be suggested here.
 555                         //
 556                         // FIXME (#46459 and #46460): ideally
 557                         // `std::convert::Into::into` and `std::borrow:ToOwned` would
 558                         // also be `#[rustc_conversion_suggestion]`, if not for
 559                         // method-probing false-positives and -negatives (respectively).
 560                         //
 561                         // FIXME? Other potential candidate methods: `as_ref` and
 562                         // `as_mut`?
 563                         .has_attr(m.def_id, sym::rustc_conversion_suggestion)
 564             },
 565         );
 566
 567         methods
 568     }
 569
 570     /// This function checks whether the method is not static and does not accept other parameters than `self`.
 571     fn has_only_self_parameter(&self, method: &AssocItem) -> bool {
 572         match method.kind {
 573             ty::AssocKind::Fn => {
 574                 method.fn_has_self_parameter
 575                     && self.tcx.fn_sig(method.def_id).inputs().skip_binder().len() == 1
 576             }
 577             _ => false,
 578         }
 579     }
 580
 581     /// Identify some cases where `as_ref()` would be appropriate and suggest it.
 582     ///
 583     /// Given the following code:
 584     /// ```compile_fail,E0308
 585     /// struct Foo;
 586     /// fn takes_ref(_: &Foo) {}
 587     /// let ref opt = Some(Foo);
 588     ///
 589     /// opt.map(|param| takes_ref(param));
 590     /// ```
 591     /// Suggest using `opt.as_ref().map(|param| takes_ref(param));` instead.
 592     ///
 593     /// It only checks for `Option` and `Result` and won't work with
 594     /// ```ignore (illustrative)
 595     /// opt.map(|param| { takes_ref(param) });
 596     /// ```
 597     fn can_use_as_ref(&self, expr: &hir::Expr<'_>) -> Option<(Span, &'static str, String)> {
 598         let hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) = expr.kind else {
 599             return None;
 600         };
 601
 602         let hir::def::Res::Local(local_id) = path.res else {
 603             return None;
 604         };
 605
 606         let local_parent = self.tcx.hir().get_parent_node(local_id);
 607         let Some(Node::Param(hir::Param { hir_id: param_hir_id, .. })) = self.tcx.hir().find(local_parent) else {
 608             return None;
 609         };
 610
 611         let param_parent = self.tcx.hir().get_parent_node(*param_hir_id);
 612         let Some(Node::Expr(hir::Expr {
 613             hir_id: expr_hir_id,
 614             kind: hir::ExprKind::Closure(hir::Closure { fn_decl: closure_fn_decl, .. }),
 615             ..
 616         })) = self.tcx.hir().find(param_parent) else {
 617             return None;
 618         };
 619
 620         let expr_parent = self.tcx.hir().get_parent_node(*expr_hir_id);
 621         let hir = self.tcx.hir().find(expr_parent);
 622         let closure_params_len = closure_fn_decl.inputs.len();
 623         let (
 624             Some(Node::Expr(hir::Expr {
 625                 kind: hir::ExprKind::MethodCall(method_path, receiver, ..),
 626                 ..
 627             })),
 628             1,
 629         ) = (hir, closure_params_len) else {
 630             return None;
 631         };
 632
 633         let self_ty = self.typeck_results.borrow().expr_ty(receiver);
 634         let name = method_path.ident.name;
 635         let is_as_ref_able = match self_ty.peel_refs().kind() {
 636             ty::Adt(def, _) => {
 637                 (self.tcx.is_diagnostic_item(sym::Option, def.did())
 638                     || self.tcx.is_diagnostic_item(sym::Result, def.did()))
 639                     && (name == sym::map || name == sym::and_then)
 640             }
 641             _ => false,
 642         };
 643         match (is_as_ref_able, self.sess().source_map().span_to_snippet(method_path.ident.span)) {
 644             (true, Ok(src)) => {
 645                 let suggestion = format!("as_ref().{}", src);
 646                 Some((method_path.ident.span, "consider using `as_ref` instead", suggestion))
 647             }
 648             _ => None,
 649         }
 650     }
 651
 652     pub(crate) fn maybe_get_struct_pattern_shorthand_field(
 653         &self,
 654         expr: &hir::Expr<'_>,
 655     ) -> Option<Symbol> {
 656         let hir = self.tcx.hir();
 657         let local = match expr {
 658             hir::Expr {
 659                 kind:
 660                     hir::ExprKind::Path(hir::QPath::Resolved(
 661                         None,
 662                         hir::Path {
 663                             res: hir::def::Res::Local(_),
 664                             segments: [hir::PathSegment { ident, .. }],
 665                             ..
 666                         },
 667                     )),
 668                 ..
 669             } => Some(ident),
 670             _ => None,
 671         }?;
 672
 673         match hir.find(hir.get_parent_node(expr.hir_id))? {
 674             Node::ExprField(field) => {
 675                 if field.ident.name == local.name && field.is_shorthand {
 676                     return Some(local.name);
 677                 }
 678             }
 679             _ => {}
 680         }
 681
 682         None
 683     }
 684
 685     /// If the given `HirId` corresponds to a block with a trailing expression, return that expression
 686     pub(crate) fn maybe_get_block_expr(
 687         &self,
 688         expr: &hir::Expr<'tcx>,
 689     ) -> Option<&'tcx hir::Expr<'tcx>> {
 690         match expr {
 691             hir::Expr { kind: hir::ExprKind::Block(block, ..), .. } => block.expr,
 692             _ => None,
 693         }
 694     }
 695
 696     /// Returns whether the given expression is an `else if`.
 697     pub(crate) fn is_else_if_block(&self, expr: &hir::Expr<'_>) -> bool {
 698         if let hir::ExprKind::If(..) = expr.kind {
 699             let parent_id = self.tcx.hir().get_parent_node(expr.hir_id);
 700             if let Some(Node::Expr(hir::Expr {
 701                 kind: hir::ExprKind::If(_, _, Some(else_expr)),
 702                 ..
 703             })) = self.tcx.hir().find(parent_id)
 704             {
 705                 return else_expr.hir_id == expr.hir_id;
 706             }
 707         }
 708         false
 709     }
 710
 711     /// This function is used to determine potential "simple" improvements or users' errors and
 712     /// provide them useful help. For example:
 713     ///
 714     /// ```compile_fail,E0308
 715     /// fn some_fn(s: &str) {}
 716     ///
 717     /// let x = "hey!".to_owned();
 718     /// some_fn(x); // error
 719     /// ```
 720     ///
 721     /// No need to find every potential function which could make a coercion to transform a
 722     /// `String` into a `&str` since a `&` would do the trick!
 723     ///
 724     /// In addition of this check, it also checks between references mutability state. If the
 725     /// expected is mutable but the provided isn't, maybe we could just say "Hey, try with
 726     /// `&mut`!".
 727     pub fn check_ref(
 728         &self,
 729         expr: &hir::Expr<'tcx>,
 730         checked_ty: Ty<'tcx>,
 731         expected: Ty<'tcx>,
 732     ) -> Option<(
 733         Span,
 734         String,
 735         String,
 736         Applicability,
 737         bool, /* verbose */
 738         bool, /* suggest `&` or `&mut` type annotation */
 739     )> {
 740         let sess = self.sess();
 741         let sp = expr.span;
 742
 743         // If the span is from an external macro, there's no suggestion we can make.
 744         if in_external_macro(sess, sp) {
 745             return None;
 746         }
 747
 748         let sm = sess.source_map();
 749
 750         let replace_prefix = |s: &str, old: &str, new: &str| {
 751             s.strip_prefix(old).map(|stripped| new.to_string() + stripped)
 752         };
 753
 754         // `ExprKind::DropTemps` is semantically irrelevant for these suggestions.
 755         let expr = expr.peel_drop_temps();
 756
 757         match (&expr.kind, expected.kind(), checked_ty.kind()) {
 758             (_, &ty::Ref(_, exp, _), &ty::Ref(_, check, _)) => match (exp.kind(), check.kind()) {
 759                 (&ty::Str, &ty::Array(arr, _) | &ty::Slice(arr)) if arr == self.tcx.types.u8 => {
 760                     if let hir::ExprKind::Lit(_) = expr.kind
 761                         && let Ok(src) = sm.span_to_snippet(sp)
 762                         && replace_prefix(&src, "b\"", "\"").is_some()
 763                     {
 764                                 let pos = sp.lo() + BytePos(1);
 765                                 return Some((
 766                                     sp.with_hi(pos),
 767                                     "consider removing the leading `b`".to_string(),
 768                                     String::new(),
 769                                     Applicability::MachineApplicable,
 770                                     true,
 771                                     false,
 772                                 ));
 773                             }
 774                         }
 775                 (&ty::Array(arr, _) | &ty::Slice(arr), &ty::Str) if arr == self.tcx.types.u8 => {
 776                     if let hir::ExprKind::Lit(_) = expr.kind
 777                         && let Ok(src) = sm.span_to_snippet(sp)
 778                         && replace_prefix(&src, "\"", "b\"").is_some()
 779                     {
 780                                 return Some((
 781                                     sp.shrink_to_lo(),
 782                                     "consider adding a leading `b`".to_string(),
 783                                     "b".to_string(),
 784                                     Applicability::MachineApplicable,
 785                                     true,
 786                                     false,
 787                                 ));
 788                     }
 789                 }
 790                 _ => {}
 791             },
 792             (_, &ty::Ref(_, _, mutability), _) => {
 793                 // Check if it can work when put into a ref. For example:
 794                 //
 795                 // ```
 796                 // fn bar(x: &mut i32) {}
 797                 //
 798                 // let x = 0u32;
 799                 // bar(&x); // error, expected &mut
 800                 // ```
 801                 let ref_ty = match mutability {
 802                     hir::Mutability::Mut => {
 803                         self.tcx.mk_mut_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 804                     }
 805                     hir::Mutability::Not => {
 806                         self.tcx.mk_imm_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 807                     }
 808                 };
 809                 if self.can_coerce(ref_ty, expected) {
 810                     let mut sugg_sp = sp;
 811                     if let hir::ExprKind::MethodCall(ref segment, receiver, args, _) = expr.kind {
 812                         let clone_trait =
 813                             self.tcx.require_lang_item(LangItem::Clone, Some(segment.ident.span));
 814                         if args.is_empty()
 815                             && self.typeck_results.borrow().type_dependent_def_id(expr.hir_id).map(
 816                                 |did| {
 817                                     let ai = self.tcx.associated_item(did);
 818                                     ai.trait_container(self.tcx) == Some(clone_trait)
 819                                 },
 820                             ) == Some(true)
 821                             && segment.ident.name == sym::clone
 822                         {
 823                             // If this expression had a clone call when suggesting borrowing
 824                             // we want to suggest removing it because it'd now be unnecessary.
 825                             sugg_sp = receiver.span;
 826                         }
 827                     }
 828                     if let Ok(src) = sm.span_to_snippet(sugg_sp) {
 829                         let needs_parens = match expr.kind {
 830                             // parenthesize if needed (Issue #46756)
 831                             hir::ExprKind::Cast(_, _) | hir::ExprKind::Binary(_, _, _) => true,
 832                             // parenthesize borrows of range literals (Issue #54505)
 833                             _ if is_range_literal(expr) => true,
 834                             _ => false,
 835                         };
 836
 837                         if let Some(sugg) = self.can_use_as_ref(expr) {
 838                             return Some((
 839                                 sugg.0,
 840                                 sugg.1.to_string(),
 841                                 sugg.2,
 842                                 Applicability::MachineApplicable,
 843                                 false,
 844                                 false,
 845                             ));
 846                         }
 847
 848                         let prefix = match self.maybe_get_struct_pattern_shorthand_field(expr) {
 849                             Some(ident) => format!("{ident}: "),
 850                             None => String::new(),
 851                         };
 852
 853                         if let Some(hir::Node::Expr(hir::Expr {
 854                             kind: hir::ExprKind::Assign(..),
 855                             ..
 856                         })) = self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
 857                         {
 858                             if mutability == hir::Mutability::Mut {
 859                                 // Suppressing this diagnostic, we'll properly print it in `check_expr_assign`
 860                                 return None;
 861                             }
 862                         }
 863
 864                         let sugg_expr = if needs_parens { format!("({src})") } else { src };
 865                         return Some(match mutability {
 866                             hir::Mutability::Mut => (
 867                                 sp,
 868                                 "consider mutably borrowing here".to_string(),
 869                                 format!("{prefix}&mut {sugg_expr}"),
 870                                 Applicability::MachineApplicable,
 871                                 false,
 872                                 false,
 873                             ),
 874                             hir::Mutability::Not => (
 875                                 sp,
 876                                 "consider borrowing here".to_string(),
 877                                 format!("{prefix}&{sugg_expr}"),
 878                                 Applicability::MachineApplicable,
 879                                 false,
 880                                 false,
 881                             ),
 882                         });
 883                     }
 884                 }
 885             }
 886             (
 887                 hir::ExprKind::AddrOf(hir::BorrowKind::Ref, _, ref expr),
 888                 _,
 889                 &ty::Ref(_, checked, _),
 890             ) if self.can_sub(self.param_env, checked, expected).is_ok() => {
 891                 // We have `&T`, check if what was expected was `T`. If so,
 892                 // we may want to suggest removing a `&`.
 893                 if sm.is_imported(expr.span) {
 894                     // Go through the spans from which this span was expanded,
 895                     // and find the one that's pointing inside `sp`.
 896                     //
 897                     // E.g. for `&format!("")`, where we want the span to the
 898                     // `format!()` invocation instead of its expansion.
 899                     if let Some(call_span) =
 900                         iter::successors(Some(expr.span), |s| s.parent_callsite())
 901                             .find(|&s| sp.contains(s))
 902                         && sm.is_span_accessible(call_span)
 903                     {
 904                         return Some((
 905                             sp.with_hi(call_span.lo()),
 906                             "consider removing the borrow".to_string(),
 907                             String::new(),
 908                             Applicability::MachineApplicable,
 909                             true,
 910                             true
 911                         ));
 912                     }
 913                     return None;
 914                 }
 915                 if sp.contains(expr.span)
 916                     && sm.is_span_accessible(expr.span)
 917                 {
 918                     return Some((
 919                         sp.with_hi(expr.span.lo()),
 920                         "consider removing the borrow".to_string(),
 921                         String::new(),
 922                         Applicability::MachineApplicable,
 923                         true,
 924                         true,
 925                     ));
 926                 }
 927             }
 928             (
 929                 _,
 930                 &ty::RawPtr(TypeAndMut { ty: ty_b, mutbl: mutbl_b }),
 931                 &ty::Ref(_, ty_a, mutbl_a),
 932             ) => {
 933                 if let Some(steps) = self.deref_steps(ty_a, ty_b)
 934                     // Only suggest valid if dereferencing needed.
 935                     && steps > 0
 936                     // The pointer type implements `Copy` trait so the suggestion is always valid.
 937                     && let Ok(src) = sm.span_to_snippet(sp)
 938                 {
 939                     let derefs = "*".repeat(steps);
 940                     if let Some((span, src, applicability)) = match mutbl_b {
 941                         hir::Mutability::Mut => {
 942                             let new_prefix = "&mut ".to_owned() + &derefs;
 943                             match mutbl_a {
 944                                 hir::Mutability::Mut => {
 945                                     replace_prefix(&src, "&mut ", &new_prefix).map(|_| {
 946                                         let pos = sp.lo() + BytePos(5);
 947                                         let sp = sp.with_lo(pos).with_hi(pos);
 948                                         (sp, derefs, Applicability::MachineApplicable)
 949                                     })
 950                                 }
 951                                 hir::Mutability::Not => {
 952                                     replace_prefix(&src, "&", &new_prefix).map(|_| {
 953                                         let pos = sp.lo() + BytePos(1);
 954                                         let sp = sp.with_lo(pos).with_hi(pos);
 955                                         (
 956                                             sp,
 957                                             format!("mut {derefs}"),
 958                                             Applicability::Unspecified,
 959                                         )
 960                                     })
 961                                 }
 962                             }
 963                         }
 964                         hir::Mutability::Not => {
 965                             let new_prefix = "&".to_owned() + &derefs;
 966                             match mutbl_a {
 967                                 hir::Mutability::Mut => {
 968                                     replace_prefix(&src, "&mut ", &new_prefix).map(|_| {
 969                                         let lo = sp.lo() + BytePos(1);
 970                                         let hi = sp.lo() + BytePos(5);
 971                                         let sp = sp.with_lo(lo).with_hi(hi);
 972                                         (sp, derefs, Applicability::MachineApplicable)
 973                                     })
 974                                 }
 975                                 hir::Mutability::Not => {
 976                                     replace_prefix(&src, "&", &new_prefix).map(|_| {
 977                                         let pos = sp.lo() + BytePos(1);
 978                                         let sp = sp.with_lo(pos).with_hi(pos);
 979                                         (sp, derefs, Applicability::MachineApplicable)
 980                                     })
 981                                 }
 982                             }
 983                         }
 984                     } {
 985                         return Some((
 986                             span,
 987                             "consider dereferencing".to_string(),
 988                             src,
 989                             applicability,
 990                             true,
 991                             false,
 992                         ));
 993                     }
 994                 }
 995             }
 996             _ if sp == expr.span => {
 997                 if let Some(mut steps) = self.deref_steps(checked_ty, expected) {
 998                     let mut expr = expr.peel_blocks();
 999                     let mut prefix_span = expr.span.shrink_to_lo();
1000                     let mut remove = String::new();
1001
1002                     // Try peeling off any existing `&` and `&mut` to reach our target type
1003                     while steps > 0 {
1004                         if let hir::ExprKind::AddrOf(_, mutbl, inner) = expr.kind {
1005                             // If the expression has `&`, removing it would fix the error
1006                             prefix_span = prefix_span.with_hi(inner.span.lo());
1007                             expr = inner;
1008                             remove += match mutbl {
1009                                 hir::Mutability::Not => "&",
1010                                 hir::Mutability::Mut => "&mut ",
1011                             };
1012                             steps -= 1;
1013                         } else {
1014                             break;
1015                         }
1016                     }
1017                     // If we've reached our target type with just removing `&`, then just print now.
1018                     if steps == 0 {
1019                         return Some((
1020                             prefix_span,
1021                             format!("consider removing the `{}`", remove.trim()),
1022                             String::new(),
1023                             // Do not remove `&&` to get to bool, because it might be something like
1024                             // { a } && b, which we have a separate fixup suggestion that is more
1025                             // likely correct...
1026                             if remove.trim() == "&&" && expected == self.tcx.types.bool {
1027                                 Applicability::MaybeIncorrect
1028                             } else {
1029                                 Applicability::MachineApplicable
1030                             },
1031                             true,
1032                             false,
1033                         ));
1034                     }
1035
1036                     // For this suggestion to make sense, the type would need to be `Copy`,
1037                     // or we have to be moving out of a `Box<T>`
1038                     if self.type_is_copy_modulo_regions(self.param_env, expected, sp)
1039                         // FIXME(compiler-errors): We can actually do this if the checked_ty is
1040                         // `steps` layers of boxes, not just one, but this is easier and most likely.
1041                         || (checked_ty.is_box() && steps == 1)
1042                     {
1043                         let deref_kind = if checked_ty.is_box() {
1044                             "unboxing the value"
1045                         } else if checked_ty.is_region_ptr() {
1046                             "dereferencing the borrow"
1047                         } else {
1048                             "dereferencing the type"
1049                         };
1050
1051                         // Suggest removing `&` if we have removed any, otherwise suggest just
1052                         // dereferencing the remaining number of steps.
1053                         let message = if remove.is_empty() {
1054                             format!("consider {deref_kind}")
1055                         } else {
1056                             format!(
1057                                 "consider removing the `{}` and {} instead",
1058                                 remove.trim(),
1059                                 deref_kind
1060                             )
1061                         };
1062
1063                         let prefix = match self.maybe_get_struct_pattern_shorthand_field(expr) {
1064                             Some(ident) => format!("{ident}: "),
1065                             None => String::new(),
1066                         };
1067
1068                         let (span, suggestion) = if self.is_else_if_block(expr) {
1069                             // Don't suggest nonsense like `else *if`
1070                             return None;
1071                         } else if let Some(expr) = self.maybe_get_block_expr(expr) {
1072                             // prefix should be empty here..
1073                             (expr.span.shrink_to_lo(), "*".to_string())
1074                         } else {
1075                             (prefix_span, format!("{}{}", prefix, "*".repeat(steps)))
1076                         };
1077
1078                         return Some((
1079                             span,
1080                             message,
1081                             suggestion,
1082                             Applicability::MachineApplicable,
1083                             true,
1084                             false,
1085                         ));
1086                     }
1087                 }
1088             }
1089             _ => {}
1090         }
1091         None
1092     }
1093
1094     pub fn check_for_cast(
1095         &self,
1096         err: &mut Diagnostic,
1097         expr: &hir::Expr<'_>,
1098         checked_ty: Ty<'tcx>,
1099         expected_ty: Ty<'tcx>,
1100         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
1101     ) -> bool {
1102         if self.tcx.sess.source_map().is_imported(expr.span) {
1103             // Ignore if span is from within a macro.
1104             return false;
1105         }
1106
1107         let Ok(src) = self.tcx.sess.source_map().span_to_snippet(expr.span) else {
1108             return false;
1109         };
1110
1111         // If casting this expression to a given numeric type would be appropriate in case of a type
1112         // mismatch.
1113         //
1114         // We want to minimize the amount of casting operations that are suggested, as it can be a
1115         // lossy operation with potentially bad side effects, so we only suggest when encountering
1116         // an expression that indicates that the original type couldn't be directly changed.
1117         //
1118         // For now, don't suggest casting with `as`.
1119         let can_cast = false;
1120
1121         let mut sugg = vec![];
1122
1123         if let Some(hir::Node::ExprField(field)) =
1124             self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
1125         {
1126             // `expr` is a literal field for a struct, only suggest if appropriate
1127             if field.is_shorthand {
1128                 // This is a field literal
1129                 sugg.push((field.ident.span.shrink_to_lo(), format!("{}: ", field.ident)));
1130             } else {
1131                 // Likely a field was meant, but this field wasn't found. Do not suggest anything.
1132                 return false;
1133             }
1134         };
1135
1136         if let hir::ExprKind::Call(path, args) = &expr.kind
1137             && let (hir::ExprKind::Path(hir::QPath::TypeRelative(base_ty, path_segment)), 1) =
1138                 (&path.kind, args.len())
1139             // `expr` is a conversion like `u32::from(val)`, do not suggest anything (#63697).
1140             && let (hir::TyKind::Path(hir::QPath::Resolved(None, base_ty_path)), sym::from) =
1141                 (&base_ty.kind, path_segment.ident.name)
1142         {
1143             if let Some(ident) = &base_ty_path.segments.iter().map(|s| s.ident).next() {
1144                 match ident.name {
1145                     sym::i128
1146                     | sym::i64
1147                     | sym::i32
1148                     | sym::i16
1149                     | sym::i8
1150                     | sym::u128
1151                     | sym::u64
1152                     | sym::u32
1153                     | sym::u16
1154                     | sym::u8
1155                     | sym::isize
1156                     | sym::usize
1157                         if base_ty_path.segments.len() == 1 =>
1158                     {
1159                         return false;
1160                     }
1161                     _ => {}
1162                 }
1163             }
1164         }
1165
1166         let msg = format!(
1167             "you can convert {} `{}` to {} `{}`",
1168             checked_ty.kind().article(),
1169             checked_ty,
1170             expected_ty.kind().article(),
1171             expected_ty,
1172         );
1173         let cast_msg = format!(
1174             "you can cast {} `{}` to {} `{}`",
1175             checked_ty.kind().article(),
1176             checked_ty,
1177             expected_ty.kind().article(),
1178             expected_ty,
1179         );
1180         let lit_msg = format!(
1181             "change the type of the numeric literal from `{checked_ty}` to `{expected_ty}`",
1182         );
1183
1184         let close_paren = if expr.precedence().order() < PREC_POSTFIX {
1185             sugg.push((expr.span.shrink_to_lo(), "(".to_string()));
1186             ")"
1187         } else {
1188             ""
1189         };
1190
1191         let mut cast_suggestion = sugg.clone();
1192         cast_suggestion.push((expr.span.shrink_to_hi(), format!("{close_paren} as {expected_ty}")));
1193         let mut into_suggestion = sugg.clone();
1194         into_suggestion.push((expr.span.shrink_to_hi(), format!("{close_paren}.into()")));
1195         let mut suffix_suggestion = sugg.clone();
1196         suffix_suggestion.push((
1197             if matches!(
1198                 (&expected_ty.kind(), &checked_ty.kind()),
1199                 (ty::Int(_) | ty::Uint(_), ty::Float(_))
1200             ) {
1201                 // Remove fractional part from literal, for example `42.0f32` into `42`
1202                 let src = src.trim_end_matches(&checked_ty.to_string());
1203                 let len = src.split('.').next().unwrap().len();
1204                 expr.span.with_lo(expr.span.lo() + BytePos(len as u32))
1205             } else {
1206                 let len = src.trim_end_matches(&checked_ty.to_string()).len();
1207                 expr.span.with_lo(expr.span.lo() + BytePos(len as u32))
1208             },
1209             if expr.precedence().order() < PREC_POSTFIX {
1210                 // Readd `)`
1211                 format!("{expected_ty})")
1212             } else {
1213                 expected_ty.to_string()
1214             },
1215         ));
1216         let literal_is_ty_suffixed = |expr: &hir::Expr<'_>| {
1217             if let hir::ExprKind::Lit(lit) = &expr.kind { lit.node.is_suffixed() } else { false }
1218         };
1219         let is_negative_int =
1220             |expr: &hir::Expr<'_>| matches!(expr.kind, hir::ExprKind::Unary(hir::UnOp::Neg, ..));
1221         let is_uint = |ty: Ty<'_>| matches!(ty.kind(), ty::Uint(..));
1222
1223         let in_const_context = self.tcx.hir().is_inside_const_context(expr.hir_id);
1224
1225         let suggest_fallible_into_or_lhs_from =
1226             |err: &mut Diagnostic, exp_to_found_is_fallible: bool| {
1227                 // If we know the expression the expected type is derived from, we might be able
1228                 // to suggest a widening conversion rather than a narrowing one (which may
1229                 // panic). For example, given x: u8 and y: u32, if we know the span of "x",
1230                 //   x > y
1231                 // can be given the suggestion "u32::from(x) > y" rather than
1232                 // "x > y.try_into().unwrap()".
1233                 let lhs_expr_and_src = expected_ty_expr.and_then(|expr| {
1234                     self.tcx
1235                         .sess
1236                         .source_map()
1237                         .span_to_snippet(expr.span)
1238                         .ok()
1239                         .map(|src| (expr, src))
1240                 });
1241                 let (msg, suggestion) = if let (Some((lhs_expr, lhs_src)), false) =
1242                     (lhs_expr_and_src, exp_to_found_is_fallible)
1243                 {
1244                     let msg = format!(
1245                         "you can convert `{lhs_src}` from `{expected_ty}` to `{checked_ty}`, matching the type of `{src}`",
1246                     );
1247                     let suggestion = vec![
1248                         (lhs_expr.span.shrink_to_lo(), format!("{checked_ty}::from(")),
1249                         (lhs_expr.span.shrink_to_hi(), ")".to_string()),
1250                     ];
1251                     (msg, suggestion)
1252                 } else {
1253                     let msg = format!("{msg} and panic if the converted value doesn't fit");
1254                     let mut suggestion = sugg.clone();
1255                     suggestion.push((
1256                         expr.span.shrink_to_hi(),
1257                         format!("{close_paren}.try_into().unwrap()"),
1258                     ));
1259                     (msg, suggestion)
1260                 };
1261                 err.multipart_suggestion_verbose(
1262                     &msg,
1263                     suggestion,
1264                     Applicability::MachineApplicable,
1265                 );
1266             };
1267
1268         let suggest_to_change_suffix_or_into =
1269             |err: &mut Diagnostic,
1270              found_to_exp_is_fallible: bool,
1271              exp_to_found_is_fallible: bool| {
1272                 let exp_is_lhs =
1273                     expected_ty_expr.map(|e| self.tcx.hir().is_lhs(e.hir_id)).unwrap_or(false);
1274
1275                 if exp_is_lhs {
1276                     return;
1277                 }
1278
1279                 let always_fallible = found_to_exp_is_fallible
1280                     && (exp_to_found_is_fallible || expected_ty_expr.is_none());
1281                 let msg = if literal_is_ty_suffixed(expr) {
1282                     &lit_msg
1283                 } else if always_fallible && (is_negative_int(expr) && is_uint(expected_ty)) {
1284                     // We now know that converting either the lhs or rhs is fallible. Before we
1285                     // suggest a fallible conversion, check if the value can never fit in the
1286                     // expected type.
1287                     let msg = format!("`{src}` cannot fit into type `{expected_ty}`");
1288                     err.note(&msg);
1289                     return;
1290                 } else if in_const_context {
1291                     // Do not recommend `into` or `try_into` in const contexts.
1292                     return;
1293                 } else if found_to_exp_is_fallible {
1294                     return suggest_fallible_into_or_lhs_from(err, exp_to_found_is_fallible);
1295                 } else {
1296                     &msg
1297                 };
1298                 let suggestion = if literal_is_ty_suffixed(expr) {
1299                     suffix_suggestion.clone()
1300                 } else {
1301                     into_suggestion.clone()
1302                 };
1303                 err.multipart_suggestion_verbose(msg, suggestion, Applicability::MachineApplicable);
1304             };
1305
1306         match (&expected_ty.kind(), &checked_ty.kind()) {
1307             (&ty::Int(ref exp), &ty::Int(ref found)) => {
1308                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1309                 {
1310                     (Some(exp), Some(found)) if exp < found => (true, false),
1311                     (Some(exp), Some(found)) if exp > found => (false, true),
1312                     (None, Some(8 | 16)) => (false, true),
1313                     (Some(8 | 16), None) => (true, false),
1314                     (None, _) | (_, None) => (true, true),
1315                     _ => (false, false),
1316                 };
1317                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1318                 true
1319             }
1320             (&ty::Uint(ref exp), &ty::Uint(ref found)) => {
1321                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1322                 {
1323                     (Some(exp), Some(found)) if exp < found => (true, false),
1324                     (Some(exp), Some(found)) if exp > found => (false, true),
1325                     (None, Some(8 | 16)) => (false, true),
1326                     (Some(8 | 16), None) => (true, false),
1327                     (None, _) | (_, None) => (true, true),
1328                     _ => (false, false),
1329                 };
1330                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1331                 true
1332             }
1333             (&ty::Int(exp), &ty::Uint(found)) => {
1334                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1335                 {
1336                     (Some(exp), Some(found)) if found < exp => (false, true),
1337                     (None, Some(8)) => (false, true),
1338                     _ => (true, true),
1339                 };
1340                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1341                 true
1342             }
1343             (&ty::Uint(exp), &ty::Int(found)) => {
1344                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1345                 {
1346                     (Some(exp), Some(found)) if found > exp => (true, false),
1347                     (Some(8), None) => (true, false),
1348                     _ => (true, true),
1349                 };
1350                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1351                 true
1352             }
1353             (&ty::Float(ref exp), &ty::Float(ref found)) => {
1354                 if found.bit_width() < exp.bit_width() {
1355                     suggest_to_change_suffix_or_into(err, false, true);
1356                 } else if literal_is_ty_suffixed(expr) {
1357                     err.multipart_suggestion_verbose(
1358                         &lit_msg,
1359                         suffix_suggestion,
1360                         Applicability::MachineApplicable,
1361                     );
1362                 } else if can_cast {
1363                     // Missing try_into implementation for `f64` to `f32`
1364                     err.multipart_suggestion_verbose(
1365                         &format!("{cast_msg}, producing the closest possible value"),
1366                         cast_suggestion,
1367                         Applicability::MaybeIncorrect, // lossy conversion
1368                     );
1369                 }
1370                 true
1371             }
1372             (&ty::Uint(_) | &ty::Int(_), &ty::Float(_)) => {
1373                 if literal_is_ty_suffixed(expr) {
1374                     err.multipart_suggestion_verbose(
1375                         &lit_msg,
1376                         suffix_suggestion,
1377                         Applicability::MachineApplicable,
1378                     );
1379                 } else if can_cast {
1380                     // Missing try_into implementation for `{float}` to `{integer}`
1381                     err.multipart_suggestion_verbose(
1382                         &format!("{msg}, rounding the float towards zero"),
1383                         cast_suggestion,
1384                         Applicability::MaybeIncorrect, // lossy conversion
1385                     );
1386                 }
1387                 true
1388             }
1389             (&ty::Float(ref exp), &ty::Uint(ref found)) => {
1390                 // if `found` is `None` (meaning found is `usize`), don't suggest `.into()`
1391                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
1392                     err.multipart_suggestion_verbose(
1393                         &format!(
1394                             "{msg}, producing the floating point representation of the integer",
1395                         ),
1396                         into_suggestion,
1397                         Applicability::MachineApplicable,
1398                     );
1399                 } else if literal_is_ty_suffixed(expr) {
1400                     err.multipart_suggestion_verbose(
1401                         &lit_msg,
1402                         suffix_suggestion,
1403                         Applicability::MachineApplicable,
1404                     );
1405                 } else {
1406                     // Missing try_into implementation for `{integer}` to `{float}`
1407                     err.multipart_suggestion_verbose(
1408                         &format!(
1409                             "{cast_msg}, producing the floating point representation of the integer, \
1410                                  rounded if necessary",
1411                         ),
1412                         cast_suggestion,
1413                         Applicability::MaybeIncorrect, // lossy conversion
1414                     );
1415                 }
1416                 true
1417             }
1418             (&ty::Float(ref exp), &ty::Int(ref found)) => {
1419                 // if `found` is `None` (meaning found is `isize`), don't suggest `.into()`
1420                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
1421                     err.multipart_suggestion_verbose(
1422                         &format!(
1423                             "{}, producing the floating point representation of the integer",
1424                             &msg,
1425                         ),
1426                         into_suggestion,
1427                         Applicability::MachineApplicable,
1428                     );
1429                 } else if literal_is_ty_suffixed(expr) {
1430                     err.multipart_suggestion_verbose(
1431                         &lit_msg,
1432                         suffix_suggestion,
1433                         Applicability::MachineApplicable,
1434                     );
1435                 } else {
1436                     // Missing try_into implementation for `{integer}` to `{float}`
1437                     err.multipart_suggestion_verbose(
1438                         &format!(
1439                             "{}, producing the floating point representation of the integer, \
1440                                 rounded if necessary",
1441                             &msg,
1442                         ),
1443                         cast_suggestion,
1444                         Applicability::MaybeIncorrect, // lossy conversion
1445                     );
1446                 }
1447                 true
1448             }
1449             (
1450                 &ty::Uint(ty::UintTy::U32 | ty::UintTy::U64 | ty::UintTy::U128)
1451                 | &ty::Int(ty::IntTy::I32 | ty::IntTy::I64 | ty::IntTy::I128),
1452                 &ty::Char,
1453             ) => {
1454                 err.multipart_suggestion_verbose(
1455                     &format!("{cast_msg}, since a `char` always occupies 4 bytes"),
1456                     cast_suggestion,
1457                     Applicability::MachineApplicable,
1458                 );
1459                 true
1460             }
1461             _ => false,
1462         }
1463     }
1464 }