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