compiler/rustc_hir_typeck/src/demand.rs

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