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