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.def_id, 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                 // Suggest constructor as deep into the block tree as possible.
 421                 // This fixes https://github.com/rust-lang/rust/issues/101065,
 422                 // and also just helps make the most minimal suggestions.
 423                 let mut expr = expr;
 424                 while let hir::ExprKind::Block(block, _) = &expr.kind
 425                     && let Some(expr_) = &block.expr
 426                 {
 427                     expr = expr_
 428                 }
 429
 430                 vec![
 431                     (expr.span.shrink_to_lo(), format!("{prefix}{variant}{open}")),
 432                     (expr.span.shrink_to_hi(), close.to_owned()),
 433                 ]
 434             };
 435
 436             match &compatible_variants[..] {
 437                 [] => { /* No variants to format */ }
 438                 [(variant, ctor_kind, field_name, note)] => {
 439                     // Just a single matching variant.
 440                     err.multipart_suggestion_verbose(
 441                         &format!(
 442                             "try wrapping the expression in `{variant}`{note}",
 443                             note = note.as_deref().unwrap_or("")
 444                         ),
 445                         suggestions_for(&**variant, *ctor_kind, *field_name),
 446                         Applicability::MaybeIncorrect,
 447                     );
 448                 }
 449                 _ => {
 450                     // More than one matching variant.
 451                     err.multipart_suggestions(
 452                         &format!(
 453                             "try wrapping the expression in a variant of `{}`",
 454                             self.tcx.def_path_str(expected_adt.did())
 455                         ),
 456                         compatible_variants.into_iter().map(
 457                             |(variant, ctor_kind, field_name, _)| {
 458                                 suggestions_for(&variant, ctor_kind, field_name)
 459                             },
 460                         ),
 461                         Applicability::MaybeIncorrect,
 462                     );
 463                 }
 464             }
 465         }
 466     }
 467
 468     fn suggest_non_zero_new_unwrap(
 469         &self,
 470         err: &mut Diagnostic,
 471         expr: &hir::Expr<'_>,
 472         expected: Ty<'tcx>,
 473         expr_ty: Ty<'tcx>,
 474     ) {
 475         let tcx = self.tcx;
 476         let (adt, unwrap) = match expected.kind() {
 477             // In case Option<NonZero*> is wanted, but * is provided, suggest calling new
 478             ty::Adt(adt, substs) if tcx.is_diagnostic_item(sym::Option, adt.did()) => {
 479                 // Unwrap option
 480                 let ty::Adt(adt, _) = substs.type_at(0).kind() else { return };
 481
 482                 (adt, "")
 483             }
 484             // In case NonZero* is wanted, but * is provided also add `.unwrap()` to satisfy types
 485             ty::Adt(adt, _) => (adt, ".unwrap()"),
 486             _ => return,
 487         };
 488
 489         let map = [
 490             (sym::NonZeroU8, tcx.types.u8),
 491             (sym::NonZeroU16, tcx.types.u16),
 492             (sym::NonZeroU32, tcx.types.u32),
 493             (sym::NonZeroU64, tcx.types.u64),
 494             (sym::NonZeroU128, tcx.types.u128),
 495             (sym::NonZeroI8, tcx.types.i8),
 496             (sym::NonZeroI16, tcx.types.i16),
 497             (sym::NonZeroI32, tcx.types.i32),
 498             (sym::NonZeroI64, tcx.types.i64),
 499             (sym::NonZeroI128, tcx.types.i128),
 500         ];
 501
 502         let Some((s, _)) = map
 503             .iter()
 504             .find(|&&(s, t)| self.tcx.is_diagnostic_item(s, adt.did()) && self.can_coerce(expr_ty, t))
 505             else { return };
 506
 507         let path = self.tcx.def_path_str(adt.non_enum_variant().def_id);
 508
 509         err.multipart_suggestion(
 510             format!("consider calling `{s}::new`"),
 511             vec![
 512                 (expr.span.shrink_to_lo(), format!("{path}::new(")),
 513                 (expr.span.shrink_to_hi(), format!("){unwrap}")),
 514             ],
 515             Applicability::MaybeIncorrect,
 516         );
 517     }
 518
 519     pub fn get_conversion_methods(
 520         &self,
 521         span: Span,
 522         expected: Ty<'tcx>,
 523         checked_ty: Ty<'tcx>,
 524         hir_id: hir::HirId,
 525     ) -> Vec<AssocItem> {
 526         let mut methods =
 527             self.probe_for_return_type(span, probe::Mode::MethodCall, expected, checked_ty, hir_id);
 528         methods.retain(|m| {
 529             self.has_only_self_parameter(m)
 530                 && self
 531                     .tcx
 532                     // This special internal attribute is used to permit
 533                     // "identity-like" conversion methods to be suggested here.
 534                     //
 535                     // FIXME (#46459 and #46460): ideally
 536                     // `std::convert::Into::into` and `std::borrow:ToOwned` would
 537                     // also be `#[rustc_conversion_suggestion]`, if not for
 538                     // method-probing false-positives and -negatives (respectively).
 539                     //
 540                     // FIXME? Other potential candidate methods: `as_ref` and
 541                     // `as_mut`?
 542                     .has_attr(m.def_id, sym::rustc_conversion_suggestion)
 543         });
 544
 545         methods
 546     }
 547
 548     /// This function checks whether the method is not static and does not accept other parameters than `self`.
 549     fn has_only_self_parameter(&self, method: &AssocItem) -> bool {
 550         match method.kind {
 551             ty::AssocKind::Fn => {
 552                 method.fn_has_self_parameter
 553                     && self.tcx.fn_sig(method.def_id).inputs().skip_binder().len() == 1
 554             }
 555             _ => false,
 556         }
 557     }
 558
 559     /// Identify some cases where `as_ref()` would be appropriate and suggest it.
 560     ///
 561     /// Given the following code:
 562     /// ```compile_fail,E0308
 563     /// struct Foo;
 564     /// fn takes_ref(_: &Foo) {}
 565     /// let ref opt = Some(Foo);
 566     ///
 567     /// opt.map(|param| takes_ref(param));
 568     /// ```
 569     /// Suggest using `opt.as_ref().map(|param| takes_ref(param));` instead.
 570     ///
 571     /// It only checks for `Option` and `Result` and won't work with
 572     /// ```ignore (illustrative)
 573     /// opt.map(|param| { takes_ref(param) });
 574     /// ```
 575     fn can_use_as_ref(&self, expr: &hir::Expr<'_>) -> Option<(Span, &'static str, String)> {
 576         let hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) = expr.kind else {
 577             return None;
 578         };
 579
 580         let hir::def::Res::Local(local_id) = path.res else {
 581             return None;
 582         };
 583
 584         let local_parent = self.tcx.hir().get_parent_node(local_id);
 585         let Some(Node::Param(hir::Param { hir_id: param_hir_id, .. })) = self.tcx.hir().find(local_parent) else {
 586             return None;
 587         };
 588
 589         let param_parent = self.tcx.hir().get_parent_node(*param_hir_id);
 590         let Some(Node::Expr(hir::Expr {
 591             hir_id: expr_hir_id,
 592             kind: hir::ExprKind::Closure(hir::Closure { fn_decl: closure_fn_decl, .. }),
 593             ..
 594         })) = self.tcx.hir().find(param_parent) else {
 595             return None;
 596         };
 597
 598         let expr_parent = self.tcx.hir().get_parent_node(*expr_hir_id);
 599         let hir = self.tcx.hir().find(expr_parent);
 600         let closure_params_len = closure_fn_decl.inputs.len();
 601         let (
 602             Some(Node::Expr(hir::Expr {
 603                 kind: hir::ExprKind::MethodCall(method_path, receiver, ..),
 604                 ..
 605             })),
 606             1,
 607         ) = (hir, closure_params_len) else {
 608             return None;
 609         };
 610
 611         let self_ty = self.typeck_results.borrow().expr_ty(receiver);
 612         let name = method_path.ident.name;
 613         let is_as_ref_able = match self_ty.peel_refs().kind() {
 614             ty::Adt(def, _) => {
 615                 (self.tcx.is_diagnostic_item(sym::Option, def.did())
 616                     || self.tcx.is_diagnostic_item(sym::Result, def.did()))
 617                     && (name == sym::map || name == sym::and_then)
 618             }
 619             _ => false,
 620         };
 621         match (is_as_ref_able, self.sess().source_map().span_to_snippet(method_path.ident.span)) {
 622             (true, Ok(src)) => {
 623                 let suggestion = format!("as_ref().{}", src);
 624                 Some((method_path.ident.span, "consider using `as_ref` instead", suggestion))
 625             }
 626             _ => None,
 627         }
 628     }
 629
 630     pub(crate) fn maybe_get_struct_pattern_shorthand_field(
 631         &self,
 632         expr: &hir::Expr<'_>,
 633     ) -> Option<Symbol> {
 634         let hir = self.tcx.hir();
 635         let local = match expr {
 636             hir::Expr {
 637                 kind:
 638                     hir::ExprKind::Path(hir::QPath::Resolved(
 639                         None,
 640                         hir::Path {
 641                             res: hir::def::Res::Local(_),
 642                             segments: [hir::PathSegment { ident, .. }],
 643                             ..
 644                         },
 645                     )),
 646                 ..
 647             } => Some(ident),
 648             _ => None,
 649         }?;
 650
 651         match hir.find(hir.get_parent_node(expr.hir_id))? {
 652             Node::ExprField(field) => {
 653                 if field.ident.name == local.name && field.is_shorthand {
 654                     return Some(local.name);
 655                 }
 656             }
 657             _ => {}
 658         }
 659
 660         None
 661     }
 662
 663     /// If the given `HirId` corresponds to a block with a trailing expression, return that expression
 664     pub(crate) fn maybe_get_block_expr(
 665         &self,
 666         expr: &hir::Expr<'tcx>,
 667     ) -> Option<&'tcx hir::Expr<'tcx>> {
 668         match expr {
 669             hir::Expr { kind: hir::ExprKind::Block(block, ..), .. } => block.expr,
 670             _ => None,
 671         }
 672     }
 673
 674     /// Returns whether the given expression is an `else if`.
 675     pub(crate) fn is_else_if_block(&self, expr: &hir::Expr<'_>) -> bool {
 676         if let hir::ExprKind::If(..) = expr.kind {
 677             let parent_id = self.tcx.hir().get_parent_node(expr.hir_id);
 678             if let Some(Node::Expr(hir::Expr {
 679                 kind: hir::ExprKind::If(_, _, Some(else_expr)),
 680                 ..
 681             })) = self.tcx.hir().find(parent_id)
 682             {
 683                 return else_expr.hir_id == expr.hir_id;
 684             }
 685         }
 686         false
 687     }
 688
 689     /// This function is used to determine potential "simple" improvements or users' errors and
 690     /// provide them useful help. For example:
 691     ///
 692     /// ```compile_fail,E0308
 693     /// fn some_fn(s: &str) {}
 694     ///
 695     /// let x = "hey!".to_owned();
 696     /// some_fn(x); // error
 697     /// ```
 698     ///
 699     /// No need to find every potential function which could make a coercion to transform a
 700     /// `String` into a `&str` since a `&` would do the trick!
 701     ///
 702     /// In addition of this check, it also checks between references mutability state. If the
 703     /// expected is mutable but the provided isn't, maybe we could just say "Hey, try with
 704     /// `&mut`!".
 705     pub fn check_ref(
 706         &self,
 707         expr: &hir::Expr<'tcx>,
 708         checked_ty: Ty<'tcx>,
 709         expected: Ty<'tcx>,
 710     ) -> Option<(Span, String, String, Applicability, bool /* verbose */)> {
 711         let sess = self.sess();
 712         let sp = expr.span;
 713
 714         // If the span is from an external macro, there's no suggestion we can make.
 715         if in_external_macro(sess, sp) {
 716             return None;
 717         }
 718
 719         let sm = sess.source_map();
 720
 721         let replace_prefix = |s: &str, old: &str, new: &str| {
 722             s.strip_prefix(old).map(|stripped| new.to_string() + stripped)
 723         };
 724
 725         // `ExprKind::DropTemps` is semantically irrelevant for these suggestions.
 726         let expr = expr.peel_drop_temps();
 727
 728         match (&expr.kind, expected.kind(), checked_ty.kind()) {
 729             (_, &ty::Ref(_, exp, _), &ty::Ref(_, check, _)) => match (exp.kind(), check.kind()) {
 730                 (&ty::Str, &ty::Array(arr, _) | &ty::Slice(arr)) if arr == self.tcx.types.u8 => {
 731                     if let hir::ExprKind::Lit(_) = expr.kind
 732                         && let Ok(src) = sm.span_to_snippet(sp)
 733                         && replace_prefix(&src, "b\"", "\"").is_some()
 734                     {
 735                                 let pos = sp.lo() + BytePos(1);
 736                                 return Some((
 737                                     sp.with_hi(pos),
 738                                     "consider removing the leading `b`".to_string(),
 739                                     String::new(),
 740                                     Applicability::MachineApplicable,
 741                                     true,
 742                                 ));
 743                             }
 744                         }
 745                 (&ty::Array(arr, _) | &ty::Slice(arr), &ty::Str) if arr == self.tcx.types.u8 => {
 746                     if let hir::ExprKind::Lit(_) = expr.kind
 747                         && let Ok(src) = sm.span_to_snippet(sp)
 748                         && replace_prefix(&src, "\"", "b\"").is_some()
 749                     {
 750                                 return Some((
 751                                     sp.shrink_to_lo(),
 752                                     "consider adding a leading `b`".to_string(),
 753                                     "b".to_string(),
 754                                     Applicability::MachineApplicable,
 755                                     true,
 756                                 ));
 757                     }
 758                 }
 759                 _ => {}
 760             },
 761             (_, &ty::Ref(_, _, mutability), _) => {
 762                 // Check if it can work when put into a ref. For example:
 763                 //
 764                 // ```
 765                 // fn bar(x: &mut i32) {}
 766                 //
 767                 // let x = 0u32;
 768                 // bar(&x); // error, expected &mut
 769                 // ```
 770                 let ref_ty = match mutability {
 771                     hir::Mutability::Mut => {
 772                         self.tcx.mk_mut_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 773                     }
 774                     hir::Mutability::Not => {
 775                         self.tcx.mk_imm_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 776                     }
 777                 };
 778                 if self.can_coerce(ref_ty, expected) {
 779                     let mut sugg_sp = sp;
 780                     if let hir::ExprKind::MethodCall(ref segment, receiver, args, _) = expr.kind {
 781                         let clone_trait =
 782                             self.tcx.require_lang_item(LangItem::Clone, Some(segment.ident.span));
 783                         if args.is_empty()
 784                             && self.typeck_results.borrow().type_dependent_def_id(expr.hir_id).map(
 785                                 |did| {
 786                                     let ai = self.tcx.associated_item(did);
 787                                     ai.trait_container(self.tcx) == Some(clone_trait)
 788                                 },
 789                             ) == Some(true)
 790                             && segment.ident.name == sym::clone
 791                         {
 792                             // If this expression had a clone call when suggesting borrowing
 793                             // we want to suggest removing it because it'd now be unnecessary.
 794                             sugg_sp = receiver.span;
 795                         }
 796                     }
 797                     if let Ok(src) = sm.span_to_snippet(sugg_sp) {
 798                         let needs_parens = match expr.kind {
 799                             // parenthesize if needed (Issue #46756)
 800                             hir::ExprKind::Cast(_, _) | hir::ExprKind::Binary(_, _, _) => true,
 801                             // parenthesize borrows of range literals (Issue #54505)
 802                             _ if is_range_literal(expr) => true,
 803                             _ => false,
 804                         };
 805
 806                         if let Some(sugg) = self.can_use_as_ref(expr) {
 807                             return Some((
 808                                 sugg.0,
 809                                 sugg.1.to_string(),
 810                                 sugg.2,
 811                                 Applicability::MachineApplicable,
 812                                 false,
 813                             ));
 814                         }
 815
 816                         let prefix = match self.maybe_get_struct_pattern_shorthand_field(expr) {
 817                             Some(ident) => format!("{ident}: "),
 818                             None => String::new(),
 819                         };
 820
 821                         if let Some(hir::Node::Expr(hir::Expr {
 822                             kind: hir::ExprKind::Assign(..),
 823                             ..
 824                         })) = self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
 825                         {
 826                             if mutability == hir::Mutability::Mut {
 827                                 // Suppressing this diagnostic, we'll properly print it in `check_expr_assign`
 828                                 return None;
 829                             }
 830                         }
 831
 832                         let sugg_expr = if needs_parens { format!("({src})") } else { src };
 833                         return Some(match mutability {
 834                             hir::Mutability::Mut => (
 835                                 sp,
 836                                 "consider mutably borrowing here".to_string(),
 837                                 format!("{prefix}&mut {sugg_expr}"),
 838                                 Applicability::MachineApplicable,
 839                                 false,
 840                             ),
 841                             hir::Mutability::Not => (
 842                                 sp,
 843                                 "consider borrowing here".to_string(),
 844                                 format!("{prefix}&{sugg_expr}"),
 845                                 Applicability::MachineApplicable,
 846                                 false,
 847                             ),
 848                         });
 849                     }
 850                 }
 851             }
 852             (
 853                 hir::ExprKind::AddrOf(hir::BorrowKind::Ref, _, ref expr),
 854                 _,
 855                 &ty::Ref(_, checked, _),
 856             ) if self.can_sub(self.param_env, checked, expected).is_ok() => {
 857                 // We have `&T`, check if what was expected was `T`. If so,
 858                 // we may want to suggest removing a `&`.
 859                 if sm.is_imported(expr.span) {
 860                     // Go through the spans from which this span was expanded,
 861                     // and find the one that's pointing inside `sp`.
 862                     //
 863                     // E.g. for `&format!("")`, where we want the span to the
 864                     // `format!()` invocation instead of its expansion.
 865                     if let Some(call_span) =
 866                         iter::successors(Some(expr.span), |s| s.parent_callsite())
 867                             .find(|&s| sp.contains(s))
 868                         && sm.is_span_accessible(call_span)
 869                     {
 870                         return Some((
 871                             sp.with_hi(call_span.lo()),
 872                             "consider removing the borrow".to_string(),
 873                             String::new(),
 874                             Applicability::MachineApplicable,
 875                             true,
 876                         ));
 877                     }
 878                     return None;
 879                 }
 880                 if sp.contains(expr.span)
 881                     && sm.is_span_accessible(expr.span)
 882                 {
 883                     return Some((
 884                         sp.with_hi(expr.span.lo()),
 885                         "consider removing the borrow".to_string(),
 886                         String::new(),
 887                         Applicability::MachineApplicable,
 888                         true,
 889                     ));
 890                 }
 891             }
 892             (
 893                 _,
 894                 &ty::RawPtr(TypeAndMut { ty: ty_b, mutbl: mutbl_b }),
 895                 &ty::Ref(_, ty_a, mutbl_a),
 896             ) => {
 897                 if let Some(steps) = self.deref_steps(ty_a, ty_b)
 898                     // Only suggest valid if dereferencing needed.
 899                     && steps > 0
 900                     // The pointer type implements `Copy` trait so the suggestion is always valid.
 901                     && let Ok(src) = sm.span_to_snippet(sp)
 902                 {
 903                     let derefs = "*".repeat(steps);
 904                     if let Some((span, src, applicability)) = match mutbl_b {
 905                         hir::Mutability::Mut => {
 906                             let new_prefix = "&mut ".to_owned() + &derefs;
 907                             match mutbl_a {
 908                                 hir::Mutability::Mut => {
 909                                     replace_prefix(&src, "&mut ", &new_prefix).map(|_| {
 910                                         let pos = sp.lo() + BytePos(5);
 911                                         let sp = sp.with_lo(pos).with_hi(pos);
 912                                         (sp, derefs, Applicability::MachineApplicable)
 913                                     })
 914                                 }
 915                                 hir::Mutability::Not => {
 916                                     replace_prefix(&src, "&", &new_prefix).map(|_| {
 917                                         let pos = sp.lo() + BytePos(1);
 918                                         let sp = sp.with_lo(pos).with_hi(pos);
 919                                         (
 920                                             sp,
 921                                             format!("mut {derefs}"),
 922                                             Applicability::Unspecified,
 923                                         )
 924                                     })
 925                                 }
 926                             }
 927                         }
 928                         hir::Mutability::Not => {
 929                             let new_prefix = "&".to_owned() + &derefs;
 930                             match mutbl_a {
 931                                 hir::Mutability::Mut => {
 932                                     replace_prefix(&src, "&mut ", &new_prefix).map(|_| {
 933                                         let lo = sp.lo() + BytePos(1);
 934                                         let hi = sp.lo() + BytePos(5);
 935                                         let sp = sp.with_lo(lo).with_hi(hi);
 936                                         (sp, derefs, Applicability::MachineApplicable)
 937                                     })
 938                                 }
 939                                 hir::Mutability::Not => {
 940                                     replace_prefix(&src, "&", &new_prefix).map(|_| {
 941                                         let pos = sp.lo() + BytePos(1);
 942                                         let sp = sp.with_lo(pos).with_hi(pos);
 943                                         (sp, derefs, Applicability::MachineApplicable)
 944                                     })
 945                                 }
 946                             }
 947                         }
 948                     } {
 949                         return Some((
 950                             span,
 951                             "consider dereferencing".to_string(),
 952                             src,
 953                             applicability,
 954                             true,
 955                         ));
 956                     }
 957                 }
 958             }
 959             _ if sp == expr.span => {
 960                 if let Some(mut steps) = self.deref_steps(checked_ty, expected) {
 961                     let mut expr = expr.peel_blocks();
 962                     let mut prefix_span = expr.span.shrink_to_lo();
 963                     let mut remove = String::new();
 964
 965                     // Try peeling off any existing `&` and `&mut` to reach our target type
 966                     while steps > 0 {
 967                         if let hir::ExprKind::AddrOf(_, mutbl, inner) = expr.kind {
 968                             // If the expression has `&`, removing it would fix the error
 969                             prefix_span = prefix_span.with_hi(inner.span.lo());
 970                             expr = inner;
 971                             remove += match mutbl {
 972                                 hir::Mutability::Not => "&",
 973                                 hir::Mutability::Mut => "&mut ",
 974                             };
 975                             steps -= 1;
 976                         } else {
 977                             break;
 978                         }
 979                     }
 980                     // If we've reached our target type with just removing `&`, then just print now.
 981                     if steps == 0 {
 982                         return Some((
 983                             prefix_span,
 984                             format!("consider removing the `{}`", remove.trim()),
 985                             String::new(),
 986                             // Do not remove `&&` to get to bool, because it might be something like
 987                             // { a } && b, which we have a separate fixup suggestion that is more
 988                             // likely correct...
 989                             if remove.trim() == "&&" && expected == self.tcx.types.bool {
 990                                 Applicability::MaybeIncorrect
 991                             } else {
 992                                 Applicability::MachineApplicable
 993                             },
 994                             true,
 995                         ));
 996                     }
 997
 998                     // For this suggestion to make sense, the type would need to be `Copy`,
 999                     // or we have to be moving out of a `Box<T>`
1000                     if self.type_is_copy_modulo_regions(self.param_env, expected, sp)
1001                         // FIXME(compiler-errors): We can actually do this if the checked_ty is
1002                         // `steps` layers of boxes, not just one, but this is easier and most likely.
1003                         || (checked_ty.is_box() && steps == 1)
1004                     {
1005                         let deref_kind = if checked_ty.is_box() {
1006                             "unboxing the value"
1007                         } else if checked_ty.is_region_ptr() {
1008                             "dereferencing the borrow"
1009                         } else {
1010                             "dereferencing the type"
1011                         };
1012
1013                         // Suggest removing `&` if we have removed any, otherwise suggest just
1014                         // dereferencing the remaining number of steps.
1015                         let message = if remove.is_empty() {
1016                             format!("consider {deref_kind}")
1017                         } else {
1018                             format!(
1019                                 "consider removing the `{}` and {} instead",
1020                                 remove.trim(),
1021                                 deref_kind
1022                             )
1023                         };
1024
1025                         let prefix = match self.maybe_get_struct_pattern_shorthand_field(expr) {
1026                             Some(ident) => format!("{ident}: "),
1027                             None => String::new(),
1028                         };
1029
1030                         let (span, suggestion) = if self.is_else_if_block(expr) {
1031                             // Don't suggest nonsense like `else *if`
1032                             return None;
1033                         } else if let Some(expr) = self.maybe_get_block_expr(expr) {
1034                             // prefix should be empty here..
1035                             (expr.span.shrink_to_lo(), "*".to_string())
1036                         } else {
1037                             (prefix_span, format!("{}{}", prefix, "*".repeat(steps)))
1038                         };
1039
1040                         return Some((
1041                             span,
1042                             message,
1043                             suggestion,
1044                             Applicability::MachineApplicable,
1045                             true,
1046                         ));
1047                     }
1048                 }
1049             }
1050             _ => {}
1051         }
1052         None
1053     }
1054
1055     pub fn check_for_cast(
1056         &self,
1057         err: &mut Diagnostic,
1058         expr: &hir::Expr<'_>,
1059         checked_ty: Ty<'tcx>,
1060         expected_ty: Ty<'tcx>,
1061         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
1062     ) -> bool {
1063         if self.tcx.sess.source_map().is_imported(expr.span) {
1064             // Ignore if span is from within a macro.
1065             return false;
1066         }
1067
1068         let Ok(src) = self.tcx.sess.source_map().span_to_snippet(expr.span) else {
1069             return false;
1070         };
1071
1072         // If casting this expression to a given numeric type would be appropriate in case of a type
1073         // mismatch.
1074         //
1075         // We want to minimize the amount of casting operations that are suggested, as it can be a
1076         // lossy operation with potentially bad side effects, so we only suggest when encountering
1077         // an expression that indicates that the original type couldn't be directly changed.
1078         //
1079         // For now, don't suggest casting with `as`.
1080         let can_cast = false;
1081
1082         let mut sugg = vec![];
1083
1084         if let Some(hir::Node::ExprField(field)) =
1085             self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
1086         {
1087             // `expr` is a literal field for a struct, only suggest if appropriate
1088             if field.is_shorthand {
1089                 // This is a field literal
1090                 sugg.push((field.ident.span.shrink_to_lo(), format!("{}: ", field.ident)));
1091             } else {
1092                 // Likely a field was meant, but this field wasn't found. Do not suggest anything.
1093                 return false;
1094             }
1095         };
1096
1097         if let hir::ExprKind::Call(path, args) = &expr.kind
1098             && let (hir::ExprKind::Path(hir::QPath::TypeRelative(base_ty, path_segment)), 1) =
1099                 (&path.kind, args.len())
1100             // `expr` is a conversion like `u32::from(val)`, do not suggest anything (#63697).
1101             && let (hir::TyKind::Path(hir::QPath::Resolved(None, base_ty_path)), sym::from) =
1102                 (&base_ty.kind, path_segment.ident.name)
1103         {
1104             if let Some(ident) = &base_ty_path.segments.iter().map(|s| s.ident).next() {
1105                 match ident.name {
1106                     sym::i128
1107                     | sym::i64
1108                     | sym::i32
1109                     | sym::i16
1110                     | sym::i8
1111                     | sym::u128
1112                     | sym::u64
1113                     | sym::u32
1114                     | sym::u16
1115                     | sym::u8
1116                     | sym::isize
1117                     | sym::usize
1118                         if base_ty_path.segments.len() == 1 =>
1119                     {
1120                         return false;
1121                     }
1122                     _ => {}
1123                 }
1124             }
1125         }
1126
1127         let msg = format!(
1128             "you can convert {} `{}` to {} `{}`",
1129             checked_ty.kind().article(),
1130             checked_ty,
1131             expected_ty.kind().article(),
1132             expected_ty,
1133         );
1134         let cast_msg = format!(
1135             "you can cast {} `{}` to {} `{}`",
1136             checked_ty.kind().article(),
1137             checked_ty,
1138             expected_ty.kind().article(),
1139             expected_ty,
1140         );
1141         let lit_msg = format!(
1142             "change the type of the numeric literal from `{checked_ty}` to `{expected_ty}`",
1143         );
1144
1145         let close_paren = if expr.precedence().order() < PREC_POSTFIX {
1146             sugg.push((expr.span.shrink_to_lo(), "(".to_string()));
1147             ")"
1148         } else {
1149             ""
1150         };
1151
1152         let mut cast_suggestion = sugg.clone();
1153         cast_suggestion.push((expr.span.shrink_to_hi(), format!("{close_paren} as {expected_ty}")));
1154         let mut into_suggestion = sugg.clone();
1155         into_suggestion.push((expr.span.shrink_to_hi(), format!("{close_paren}.into()")));
1156         let mut suffix_suggestion = sugg.clone();
1157         suffix_suggestion.push((
1158             if matches!(
1159                 (&expected_ty.kind(), &checked_ty.kind()),
1160                 (ty::Int(_) | ty::Uint(_), ty::Float(_))
1161             ) {
1162                 // Remove fractional part from literal, for example `42.0f32` into `42`
1163                 let src = src.trim_end_matches(&checked_ty.to_string());
1164                 let len = src.split('.').next().unwrap().len();
1165                 expr.span.with_lo(expr.span.lo() + BytePos(len as u32))
1166             } else {
1167                 let len = src.trim_end_matches(&checked_ty.to_string()).len();
1168                 expr.span.with_lo(expr.span.lo() + BytePos(len as u32))
1169             },
1170             if expr.precedence().order() < PREC_POSTFIX {
1171                 // Readd `)`
1172                 format!("{expected_ty})")
1173             } else {
1174                 expected_ty.to_string()
1175             },
1176         ));
1177         let literal_is_ty_suffixed = |expr: &hir::Expr<'_>| {
1178             if let hir::ExprKind::Lit(lit) = &expr.kind { lit.node.is_suffixed() } else { false }
1179         };
1180         let is_negative_int =
1181             |expr: &hir::Expr<'_>| matches!(expr.kind, hir::ExprKind::Unary(hir::UnOp::Neg, ..));
1182         let is_uint = |ty: Ty<'_>| matches!(ty.kind(), ty::Uint(..));
1183
1184         let in_const_context = self.tcx.hir().is_inside_const_context(expr.hir_id);
1185
1186         let suggest_fallible_into_or_lhs_from =
1187             |err: &mut Diagnostic, exp_to_found_is_fallible: bool| {
1188                 // If we know the expression the expected type is derived from, we might be able
1189                 // to suggest a widening conversion rather than a narrowing one (which may
1190                 // panic). For example, given x: u8 and y: u32, if we know the span of "x",
1191                 //   x > y
1192                 // can be given the suggestion "u32::from(x) > y" rather than
1193                 // "x > y.try_into().unwrap()".
1194                 let lhs_expr_and_src = expected_ty_expr.and_then(|expr| {
1195                     self.tcx
1196                         .sess
1197                         .source_map()
1198                         .span_to_snippet(expr.span)
1199                         .ok()
1200                         .map(|src| (expr, src))
1201                 });
1202                 let (msg, suggestion) = if let (Some((lhs_expr, lhs_src)), false) =
1203                     (lhs_expr_and_src, exp_to_found_is_fallible)
1204                 {
1205                     let msg = format!(
1206                         "you can convert `{lhs_src}` from `{expected_ty}` to `{checked_ty}`, matching the type of `{src}`",
1207                     );
1208                     let suggestion = vec![
1209                         (lhs_expr.span.shrink_to_lo(), format!("{checked_ty}::from(")),
1210                         (lhs_expr.span.shrink_to_hi(), ")".to_string()),
1211                     ];
1212                     (msg, suggestion)
1213                 } else {
1214                     let msg = format!("{msg} and panic if the converted value doesn't fit");
1215                     let mut suggestion = sugg.clone();
1216                     suggestion.push((
1217                         expr.span.shrink_to_hi(),
1218                         format!("{close_paren}.try_into().unwrap()"),
1219                     ));
1220                     (msg, suggestion)
1221                 };
1222                 err.multipart_suggestion_verbose(
1223                     &msg,
1224                     suggestion,
1225                     Applicability::MachineApplicable,
1226                 );
1227             };
1228
1229         let suggest_to_change_suffix_or_into =
1230             |err: &mut Diagnostic,
1231              found_to_exp_is_fallible: bool,
1232              exp_to_found_is_fallible: bool| {
1233                 let exp_is_lhs =
1234                     expected_ty_expr.map(|e| self.tcx.hir().is_lhs(e.hir_id)).unwrap_or(false);
1235
1236                 if exp_is_lhs {
1237                     return;
1238                 }
1239
1240                 let always_fallible = found_to_exp_is_fallible
1241                     && (exp_to_found_is_fallible || expected_ty_expr.is_none());
1242                 let msg = if literal_is_ty_suffixed(expr) {
1243                     &lit_msg
1244                 } else if always_fallible && (is_negative_int(expr) && is_uint(expected_ty)) {
1245                     // We now know that converting either the lhs or rhs is fallible. Before we
1246                     // suggest a fallible conversion, check if the value can never fit in the
1247                     // expected type.
1248                     let msg = format!("`{src}` cannot fit into type `{expected_ty}`");
1249                     err.note(&msg);
1250                     return;
1251                 } else if in_const_context {
1252                     // Do not recommend `into` or `try_into` in const contexts.
1253                     return;
1254                 } else if found_to_exp_is_fallible {
1255                     return suggest_fallible_into_or_lhs_from(err, exp_to_found_is_fallible);
1256                 } else {
1257                     &msg
1258                 };
1259                 let suggestion = if literal_is_ty_suffixed(expr) {
1260                     suffix_suggestion.clone()
1261                 } else {
1262                     into_suggestion.clone()
1263                 };
1264                 err.multipart_suggestion_verbose(msg, suggestion, Applicability::MachineApplicable);
1265             };
1266
1267         match (&expected_ty.kind(), &checked_ty.kind()) {
1268             (&ty::Int(ref exp), &ty::Int(ref found)) => {
1269                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1270                 {
1271                     (Some(exp), Some(found)) if exp < found => (true, false),
1272                     (Some(exp), Some(found)) if exp > found => (false, true),
1273                     (None, Some(8 | 16)) => (false, true),
1274                     (Some(8 | 16), None) => (true, false),
1275                     (None, _) | (_, None) => (true, true),
1276                     _ => (false, false),
1277                 };
1278                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1279                 true
1280             }
1281             (&ty::Uint(ref exp), &ty::Uint(ref found)) => {
1282                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1283                 {
1284                     (Some(exp), Some(found)) if exp < found => (true, false),
1285                     (Some(exp), Some(found)) if exp > found => (false, true),
1286                     (None, Some(8 | 16)) => (false, true),
1287                     (Some(8 | 16), None) => (true, false),
1288                     (None, _) | (_, None) => (true, true),
1289                     _ => (false, false),
1290                 };
1291                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1292                 true
1293             }
1294             (&ty::Int(exp), &ty::Uint(found)) => {
1295                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1296                 {
1297                     (Some(exp), Some(found)) if found < exp => (false, true),
1298                     (None, Some(8)) => (false, true),
1299                     _ => (true, true),
1300                 };
1301                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1302                 true
1303             }
1304             (&ty::Uint(exp), &ty::Int(found)) => {
1305                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1306                 {
1307                     (Some(exp), Some(found)) if found > exp => (true, false),
1308                     (Some(8), None) => (true, false),
1309                     _ => (true, true),
1310                 };
1311                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1312                 true
1313             }
1314             (&ty::Float(ref exp), &ty::Float(ref found)) => {
1315                 if found.bit_width() < exp.bit_width() {
1316                     suggest_to_change_suffix_or_into(err, false, true);
1317                 } else if literal_is_ty_suffixed(expr) {
1318                     err.multipart_suggestion_verbose(
1319                         &lit_msg,
1320                         suffix_suggestion,
1321                         Applicability::MachineApplicable,
1322                     );
1323                 } else if can_cast {
1324                     // Missing try_into implementation for `f64` to `f32`
1325                     err.multipart_suggestion_verbose(
1326                         &format!("{cast_msg}, producing the closest possible value"),
1327                         cast_suggestion,
1328                         Applicability::MaybeIncorrect, // lossy conversion
1329                     );
1330                 }
1331                 true
1332             }
1333             (&ty::Uint(_) | &ty::Int(_), &ty::Float(_)) => {
1334                 if literal_is_ty_suffixed(expr) {
1335                     err.multipart_suggestion_verbose(
1336                         &lit_msg,
1337                         suffix_suggestion,
1338                         Applicability::MachineApplicable,
1339                     );
1340                 } else if can_cast {
1341                     // Missing try_into implementation for `{float}` to `{integer}`
1342                     err.multipart_suggestion_verbose(
1343                         &format!("{msg}, rounding the float towards zero"),
1344                         cast_suggestion,
1345                         Applicability::MaybeIncorrect, // lossy conversion
1346                     );
1347                 }
1348                 true
1349             }
1350             (&ty::Float(ref exp), &ty::Uint(ref found)) => {
1351                 // if `found` is `None` (meaning found is `usize`), don't suggest `.into()`
1352                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
1353                     err.multipart_suggestion_verbose(
1354                         &format!(
1355                             "{msg}, producing the floating point representation of the integer",
1356                         ),
1357                         into_suggestion,
1358                         Applicability::MachineApplicable,
1359                     );
1360                 } else if literal_is_ty_suffixed(expr) {
1361                     err.multipart_suggestion_verbose(
1362                         &lit_msg,
1363                         suffix_suggestion,
1364                         Applicability::MachineApplicable,
1365                     );
1366                 } else {
1367                     // Missing try_into implementation for `{integer}` to `{float}`
1368                     err.multipart_suggestion_verbose(
1369                         &format!(
1370                             "{cast_msg}, producing the floating point representation of the integer, \
1371                                  rounded if necessary",
1372                         ),
1373                         cast_suggestion,
1374                         Applicability::MaybeIncorrect, // lossy conversion
1375                     );
1376                 }
1377                 true
1378             }
1379             (&ty::Float(ref exp), &ty::Int(ref found)) => {
1380                 // if `found` is `None` (meaning found is `isize`), don't suggest `.into()`
1381                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
1382                     err.multipart_suggestion_verbose(
1383                         &format!(
1384                             "{}, producing the floating point representation of the integer",
1385                             &msg,
1386                         ),
1387                         into_suggestion,
1388                         Applicability::MachineApplicable,
1389                     );
1390                 } else if literal_is_ty_suffixed(expr) {
1391                     err.multipart_suggestion_verbose(
1392                         &lit_msg,
1393                         suffix_suggestion,
1394                         Applicability::MachineApplicable,
1395                     );
1396                 } else {
1397                     // Missing try_into implementation for `{integer}` to `{float}`
1398                     err.multipart_suggestion_verbose(
1399                         &format!(
1400                             "{}, producing the floating point representation of the integer, \
1401                                 rounded if necessary",
1402                             &msg,
1403                         ),
1404                         cast_suggestion,
1405                         Applicability::MaybeIncorrect, // lossy conversion
1406                     );
1407                 }
1408                 true
1409             }
1410             (
1411                 &ty::Uint(ty::UintTy::U32 | ty::UintTy::U64 | ty::UintTy::U128)
1412                 | &ty::Int(ty::IntTy::I32 | ty::IntTy::I64 | ty::IntTy::I128),
1413                 &ty::Char,
1414             ) => {
1415                 err.multipart_suggestion_verbose(
1416                     &format!("{cast_msg}, since a `char` always occupies 4 bytes"),
1417                     cast_suggestion,
1418                     Applicability::MachineApplicable,
1419                 );
1420                 true
1421             }
1422             _ => false,
1423         }
1424     }
1425 }