compiler/rustc_typeck/src/check/demand.rs

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