compiler/rustc_typeck/src/check/demand.rs

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