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 mut methods =
 534             self.probe_for_return_type(span, probe::Mode::MethodCall, expected, checked_ty, hir_id);
 535         methods.retain(|m| {
 536             self.has_only_self_parameter(m)
 537                 && self
 538                     .tcx
 539                     // This special internal attribute is used to permit
 540                     // "identity-like" conversion methods to be suggested here.
 541                     //
 542                     // FIXME (#46459 and #46460): ideally
 543                     // `std::convert::Into::into` and `std::borrow:ToOwned` would
 544                     // also be `#[rustc_conversion_suggestion]`, if not for
 545                     // method-probing false-positives and -negatives (respectively).
 546                     //
 547                     // FIXME? Other potential candidate methods: `as_ref` and
 548                     // `as_mut`?
 549                     .has_attr(m.def_id, sym::rustc_conversion_suggestion)
 550         });
 551
 552         methods
 553     }
 554
 555     /// This function checks whether the method is not static and does not accept other parameters than `self`.
 556     fn has_only_self_parameter(&self, method: &AssocItem) -> bool {
 557         match method.kind {
 558             ty::AssocKind::Fn => {
 559                 method.fn_has_self_parameter
 560                     && self.tcx.fn_sig(method.def_id).inputs().skip_binder().len() == 1
 561             }
 562             _ => false,
 563         }
 564     }
 565
 566     /// Identify some cases where `as_ref()` would be appropriate and suggest it.
 567     ///
 568     /// Given the following code:
 569     /// ```compile_fail,E0308
 570     /// struct Foo;
 571     /// fn takes_ref(_: &Foo) {}
 572     /// let ref opt = Some(Foo);
 573     ///
 574     /// opt.map(|param| takes_ref(param));
 575     /// ```
 576     /// Suggest using `opt.as_ref().map(|param| takes_ref(param));` instead.
 577     ///
 578     /// It only checks for `Option` and `Result` and won't work with
 579     /// ```ignore (illustrative)
 580     /// opt.map(|param| { takes_ref(param) });
 581     /// ```
 582     fn can_use_as_ref(&self, expr: &hir::Expr<'_>) -> Option<(Span, &'static str, String)> {
 583         let hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) = expr.kind else {
 584             return None;
 585         };
 586
 587         let hir::def::Res::Local(local_id) = path.res else {
 588             return None;
 589         };
 590
 591         let local_parent = self.tcx.hir().get_parent_node(local_id);
 592         let Some(Node::Param(hir::Param { hir_id: param_hir_id, .. })) = self.tcx.hir().find(local_parent) else {
 593             return None;
 594         };
 595
 596         let param_parent = self.tcx.hir().get_parent_node(*param_hir_id);
 597         let Some(Node::Expr(hir::Expr {
 598             hir_id: expr_hir_id,
 599             kind: hir::ExprKind::Closure(hir::Closure { fn_decl: closure_fn_decl, .. }),
 600             ..
 601         })) = self.tcx.hir().find(param_parent) else {
 602             return None;
 603         };
 604
 605         let expr_parent = self.tcx.hir().get_parent_node(*expr_hir_id);
 606         let hir = self.tcx.hir().find(expr_parent);
 607         let closure_params_len = closure_fn_decl.inputs.len();
 608         let (
 609             Some(Node::Expr(hir::Expr {
 610                 kind: hir::ExprKind::MethodCall(method_path, receiver, ..),
 611                 ..
 612             })),
 613             1,
 614         ) = (hir, closure_params_len) else {
 615             return None;
 616         };
 617
 618         let self_ty = self.typeck_results.borrow().expr_ty(receiver);
 619         let name = method_path.ident.name;
 620         let is_as_ref_able = match self_ty.peel_refs().kind() {
 621             ty::Adt(def, _) => {
 622                 (self.tcx.is_diagnostic_item(sym::Option, def.did())
 623                     || self.tcx.is_diagnostic_item(sym::Result, def.did()))
 624                     && (name == sym::map || name == sym::and_then)
 625             }
 626             _ => false,
 627         };
 628         match (is_as_ref_able, self.sess().source_map().span_to_snippet(method_path.ident.span)) {
 629             (true, Ok(src)) => {
 630                 let suggestion = format!("as_ref().{}", src);
 631                 Some((method_path.ident.span, "consider using `as_ref` instead", suggestion))
 632             }
 633             _ => None,
 634         }
 635     }
 636
 637     pub(crate) fn maybe_get_struct_pattern_shorthand_field(
 638         &self,
 639         expr: &hir::Expr<'_>,
 640     ) -> Option<Symbol> {
 641         let hir = self.tcx.hir();
 642         let local = match expr {
 643             hir::Expr {
 644                 kind:
 645                     hir::ExprKind::Path(hir::QPath::Resolved(
 646                         None,
 647                         hir::Path {
 648                             res: hir::def::Res::Local(_),
 649                             segments: [hir::PathSegment { ident, .. }],
 650                             ..
 651                         },
 652                     )),
 653                 ..
 654             } => Some(ident),
 655             _ => None,
 656         }?;
 657
 658         match hir.find(hir.get_parent_node(expr.hir_id))? {
 659             Node::ExprField(field) => {
 660                 if field.ident.name == local.name && field.is_shorthand {
 661                     return Some(local.name);
 662                 }
 663             }
 664             _ => {}
 665         }
 666
 667         None
 668     }
 669
 670     /// If the given `HirId` corresponds to a block with a trailing expression, return that expression
 671     pub(crate) fn maybe_get_block_expr(
 672         &self,
 673         expr: &hir::Expr<'tcx>,
 674     ) -> Option<&'tcx hir::Expr<'tcx>> {
 675         match expr {
 676             hir::Expr { kind: hir::ExprKind::Block(block, ..), .. } => block.expr,
 677             _ => None,
 678         }
 679     }
 680
 681     /// Returns whether the given expression is an `else if`.
 682     pub(crate) fn is_else_if_block(&self, expr: &hir::Expr<'_>) -> bool {
 683         if let hir::ExprKind::If(..) = expr.kind {
 684             let parent_id = self.tcx.hir().get_parent_node(expr.hir_id);
 685             if let Some(Node::Expr(hir::Expr {
 686                 kind: hir::ExprKind::If(_, _, Some(else_expr)),
 687                 ..
 688             })) = self.tcx.hir().find(parent_id)
 689             {
 690                 return else_expr.hir_id == expr.hir_id;
 691             }
 692         }
 693         false
 694     }
 695
 696     /// This function is used to determine potential "simple" improvements or users' errors and
 697     /// provide them useful help. For example:
 698     ///
 699     /// ```compile_fail,E0308
 700     /// fn some_fn(s: &str) {}
 701     ///
 702     /// let x = "hey!".to_owned();
 703     /// some_fn(x); // error
 704     /// ```
 705     ///
 706     /// No need to find every potential function which could make a coercion to transform a
 707     /// `String` into a `&str` since a `&` would do the trick!
 708     ///
 709     /// In addition of this check, it also checks between references mutability state. If the
 710     /// expected is mutable but the provided isn't, maybe we could just say "Hey, try with
 711     /// `&mut`!".
 712     pub fn check_ref(
 713         &self,
 714         expr: &hir::Expr<'tcx>,
 715         checked_ty: Ty<'tcx>,
 716         expected: Ty<'tcx>,
 717     ) -> Option<(
 718         Span,
 719         String,
 720         String,
 721         Applicability,
 722         bool, /* verbose */
 723         bool, /* suggest `&` or `&mut` type annotation */
 724     )> {
 725         let sess = self.sess();
 726         let sp = expr.span;
 727
 728         // If the span is from an external macro, there's no suggestion we can make.
 729         if in_external_macro(sess, sp) {
 730             return None;
 731         }
 732
 733         let sm = sess.source_map();
 734
 735         let replace_prefix = |s: &str, old: &str, new: &str| {
 736             s.strip_prefix(old).map(|stripped| new.to_string() + stripped)
 737         };
 738
 739         // `ExprKind::DropTemps` is semantically irrelevant for these suggestions.
 740         let expr = expr.peel_drop_temps();
 741
 742         match (&expr.kind, expected.kind(), checked_ty.kind()) {
 743             (_, &ty::Ref(_, exp, _), &ty::Ref(_, check, _)) => match (exp.kind(), check.kind()) {
 744                 (&ty::Str, &ty::Array(arr, _) | &ty::Slice(arr)) if arr == self.tcx.types.u8 => {
 745                     if let hir::ExprKind::Lit(_) = expr.kind
 746                         && let Ok(src) = sm.span_to_snippet(sp)
 747                         && replace_prefix(&src, "b\"", "\"").is_some()
 748                     {
 749                                 let pos = sp.lo() + BytePos(1);
 750                                 return Some((
 751                                     sp.with_hi(pos),
 752                                     "consider removing the leading `b`".to_string(),
 753                                     String::new(),
 754                                     Applicability::MachineApplicable,
 755                                     true,
 756                                     false,
 757                                 ));
 758                             }
 759                         }
 760                 (&ty::Array(arr, _) | &ty::Slice(arr), &ty::Str) if arr == self.tcx.types.u8 => {
 761                     if let hir::ExprKind::Lit(_) = expr.kind
 762                         && let Ok(src) = sm.span_to_snippet(sp)
 763                         && replace_prefix(&src, "\"", "b\"").is_some()
 764                     {
 765                                 return Some((
 766                                     sp.shrink_to_lo(),
 767                                     "consider adding a leading `b`".to_string(),
 768                                     "b".to_string(),
 769                                     Applicability::MachineApplicable,
 770                                     true,
 771                                     false,
 772                                 ));
 773                     }
 774                 }
 775                 _ => {}
 776             },
 777             (_, &ty::Ref(_, _, mutability), _) => {
 778                 // Check if it can work when put into a ref. For example:
 779                 //
 780                 // ```
 781                 // fn bar(x: &mut i32) {}
 782                 //
 783                 // let x = 0u32;
 784                 // bar(&x); // error, expected &mut
 785                 // ```
 786                 let ref_ty = match mutability {
 787                     hir::Mutability::Mut => {
 788                         self.tcx.mk_mut_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 789                     }
 790                     hir::Mutability::Not => {
 791                         self.tcx.mk_imm_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 792                     }
 793                 };
 794                 if self.can_coerce(ref_ty, expected) {
 795                     let mut sugg_sp = sp;
 796                     if let hir::ExprKind::MethodCall(ref segment, receiver, args, _) = expr.kind {
 797                         let clone_trait =
 798                             self.tcx.require_lang_item(LangItem::Clone, Some(segment.ident.span));
 799                         if args.is_empty()
 800                             && self.typeck_results.borrow().type_dependent_def_id(expr.hir_id).map(
 801                                 |did| {
 802                                     let ai = self.tcx.associated_item(did);
 803                                     ai.trait_container(self.tcx) == Some(clone_trait)
 804                                 },
 805                             ) == Some(true)
 806                             && segment.ident.name == sym::clone
 807                         {
 808                             // If this expression had a clone call when suggesting borrowing
 809                             // we want to suggest removing it because it'd now be unnecessary.
 810                             sugg_sp = receiver.span;
 811                         }
 812                     }
 813                     if let Ok(src) = sm.span_to_snippet(sugg_sp) {
 814                         let needs_parens = match expr.kind {
 815                             // parenthesize if needed (Issue #46756)
 816                             hir::ExprKind::Cast(_, _) | hir::ExprKind::Binary(_, _, _) => true,
 817                             // parenthesize borrows of range literals (Issue #54505)
 818                             _ if is_range_literal(expr) => true,
 819                             _ => false,
 820                         };
 821
 822                         if let Some(sugg) = self.can_use_as_ref(expr) {
 823                             return Some((
 824                                 sugg.0,
 825                                 sugg.1.to_string(),
 826                                 sugg.2,
 827                                 Applicability::MachineApplicable,
 828                                 false,
 829                                 false,
 830                             ));
 831                         }
 832
 833                         let prefix = match self.maybe_get_struct_pattern_shorthand_field(expr) {
 834                             Some(ident) => format!("{ident}: "),
 835                             None => String::new(),
 836                         };
 837
 838                         if let Some(hir::Node::Expr(hir::Expr {
 839                             kind: hir::ExprKind::Assign(..),
 840                             ..
 841                         })) = self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
 842                         {
 843                             if mutability == hir::Mutability::Mut {
 844                                 // Suppressing this diagnostic, we'll properly print it in `check_expr_assign`
 845                                 return None;
 846                             }
 847                         }
 848
 849                         let sugg_expr = if needs_parens { format!("({src})") } else { src };
 850                         return Some(match mutability {
 851                             hir::Mutability::Mut => (
 852                                 sp,
 853                                 "consider mutably borrowing here".to_string(),
 854                                 format!("{prefix}&mut {sugg_expr}"),
 855                                 Applicability::MachineApplicable,
 856                                 false,
 857                                 false,
 858                             ),
 859                             hir::Mutability::Not => (
 860                                 sp,
 861                                 "consider borrowing here".to_string(),
 862                                 format!("{prefix}&{sugg_expr}"),
 863                                 Applicability::MachineApplicable,
 864                                 false,
 865                                 false,
 866                             ),
 867                         });
 868                     }
 869                 }
 870             }
 871             (
 872                 hir::ExprKind::AddrOf(hir::BorrowKind::Ref, _, ref expr),
 873                 _,
 874                 &ty::Ref(_, checked, _),
 875             ) if self.can_sub(self.param_env, checked, expected).is_ok() => {
 876                 // We have `&T`, check if what was expected was `T`. If so,
 877                 // we may want to suggest removing a `&`.
 878                 if sm.is_imported(expr.span) {
 879                     // Go through the spans from which this span was expanded,
 880                     // and find the one that's pointing inside `sp`.
 881                     //
 882                     // E.g. for `&format!("")`, where we want the span to the
 883                     // `format!()` invocation instead of its expansion.
 884                     if let Some(call_span) =
 885                         iter::successors(Some(expr.span), |s| s.parent_callsite())
 886                             .find(|&s| sp.contains(s))
 887                         && sm.is_span_accessible(call_span)
 888                     {
 889                         return Some((
 890                             sp.with_hi(call_span.lo()),
 891                             "consider removing the borrow".to_string(),
 892                             String::new(),
 893                             Applicability::MachineApplicable,
 894                             true,
 895                             true
 896                         ));
 897                     }
 898                     return None;
 899                 }
 900                 if sp.contains(expr.span)
 901                     && sm.is_span_accessible(expr.span)
 902                 {
 903                     return Some((
 904                         sp.with_hi(expr.span.lo()),
 905                         "consider removing the borrow".to_string(),
 906                         String::new(),
 907                         Applicability::MachineApplicable,
 908                         true,
 909                         true,
 910                     ));
 911                 }
 912             }
 913             (
 914                 _,
 915                 &ty::RawPtr(TypeAndMut { ty: ty_b, mutbl: mutbl_b }),
 916                 &ty::Ref(_, ty_a, mutbl_a),
 917             ) => {
 918                 if let Some(steps) = self.deref_steps(ty_a, ty_b)
 919                     // Only suggest valid if dereferencing needed.
 920                     && steps > 0
 921                     // The pointer type implements `Copy` trait so the suggestion is always valid.
 922                     && let Ok(src) = sm.span_to_snippet(sp)
 923                 {
 924                     let derefs = "*".repeat(steps);
 925                     if let Some((span, src, applicability)) = match mutbl_b {
 926                         hir::Mutability::Mut => {
 927                             let new_prefix = "&mut ".to_owned() + &derefs;
 928                             match mutbl_a {
 929                                 hir::Mutability::Mut => {
 930                                     replace_prefix(&src, "&mut ", &new_prefix).map(|_| {
 931                                         let pos = sp.lo() + BytePos(5);
 932                                         let sp = sp.with_lo(pos).with_hi(pos);
 933                                         (sp, derefs, Applicability::MachineApplicable)
 934                                     })
 935                                 }
 936                                 hir::Mutability::Not => {
 937                                     replace_prefix(&src, "&", &new_prefix).map(|_| {
 938                                         let pos = sp.lo() + BytePos(1);
 939                                         let sp = sp.with_lo(pos).with_hi(pos);
 940                                         (
 941                                             sp,
 942                                             format!("mut {derefs}"),
 943                                             Applicability::Unspecified,
 944                                         )
 945                                     })
 946                                 }
 947                             }
 948                         }
 949                         hir::Mutability::Not => {
 950                             let new_prefix = "&".to_owned() + &derefs;
 951                             match mutbl_a {
 952                                 hir::Mutability::Mut => {
 953                                     replace_prefix(&src, "&mut ", &new_prefix).map(|_| {
 954                                         let lo = sp.lo() + BytePos(1);
 955                                         let hi = sp.lo() + BytePos(5);
 956                                         let sp = sp.with_lo(lo).with_hi(hi);
 957                                         (sp, derefs, Applicability::MachineApplicable)
 958                                     })
 959                                 }
 960                                 hir::Mutability::Not => {
 961                                     replace_prefix(&src, "&", &new_prefix).map(|_| {
 962                                         let pos = sp.lo() + BytePos(1);
 963                                         let sp = sp.with_lo(pos).with_hi(pos);
 964                                         (sp, derefs, Applicability::MachineApplicable)
 965                                     })
 966                                 }
 967                             }
 968                         }
 969                     } {
 970                         return Some((
 971                             span,
 972                             "consider dereferencing".to_string(),
 973                             src,
 974                             applicability,
 975                             true,
 976                             false,
 977                         ));
 978                     }
 979                 }
 980             }
 981             _ if sp == expr.span => {
 982                 if let Some(mut steps) = self.deref_steps(checked_ty, expected) {
 983                     let mut expr = expr.peel_blocks();
 984                     let mut prefix_span = expr.span.shrink_to_lo();
 985                     let mut remove = String::new();
 986
 987                     // Try peeling off any existing `&` and `&mut` to reach our target type
 988                     while steps > 0 {
 989                         if let hir::ExprKind::AddrOf(_, mutbl, inner) = expr.kind {
 990                             // If the expression has `&`, removing it would fix the error
 991                             prefix_span = prefix_span.with_hi(inner.span.lo());
 992                             expr = inner;
 993                             remove += match mutbl {
 994                                 hir::Mutability::Not => "&",
 995                                 hir::Mutability::Mut => "&mut ",
 996                             };
 997                             steps -= 1;
 998                         } else {
 999                             break;
1000                         }
1001                     }
1002                     // If we've reached our target type with just removing `&`, then just print now.
1003                     if steps == 0 {
1004                         return Some((
1005                             prefix_span,
1006                             format!("consider removing the `{}`", remove.trim()),
1007                             String::new(),
1008                             // Do not remove `&&` to get to bool, because it might be something like
1009                             // { a } && b, which we have a separate fixup suggestion that is more
1010                             // likely correct...
1011                             if remove.trim() == "&&" && expected == self.tcx.types.bool {
1012                                 Applicability::MaybeIncorrect
1013                             } else {
1014                                 Applicability::MachineApplicable
1015                             },
1016                             true,
1017                             false,
1018                         ));
1019                     }
1020
1021                     // For this suggestion to make sense, the type would need to be `Copy`,
1022                     // or we have to be moving out of a `Box<T>`
1023                     if self.type_is_copy_modulo_regions(self.param_env, expected, sp)
1024                         // FIXME(compiler-errors): We can actually do this if the checked_ty is
1025                         // `steps` layers of boxes, not just one, but this is easier and most likely.
1026                         || (checked_ty.is_box() && steps == 1)
1027                     {
1028                         let deref_kind = if checked_ty.is_box() {
1029                             "unboxing the value"
1030                         } else if checked_ty.is_region_ptr() {
1031                             "dereferencing the borrow"
1032                         } else {
1033                             "dereferencing the type"
1034                         };
1035
1036                         // Suggest removing `&` if we have removed any, otherwise suggest just
1037                         // dereferencing the remaining number of steps.
1038                         let message = if remove.is_empty() {
1039                             format!("consider {deref_kind}")
1040                         } else {
1041                             format!(
1042                                 "consider removing the `{}` and {} instead",
1043                                 remove.trim(),
1044                                 deref_kind
1045                             )
1046                         };
1047
1048                         let prefix = match self.maybe_get_struct_pattern_shorthand_field(expr) {
1049                             Some(ident) => format!("{ident}: "),
1050                             None => String::new(),
1051                         };
1052
1053                         let (span, suggestion) = if self.is_else_if_block(expr) {
1054                             // Don't suggest nonsense like `else *if`
1055                             return None;
1056                         } else if let Some(expr) = self.maybe_get_block_expr(expr) {
1057                             // prefix should be empty here..
1058                             (expr.span.shrink_to_lo(), "*".to_string())
1059                         } else {
1060                             (prefix_span, format!("{}{}", prefix, "*".repeat(steps)))
1061                         };
1062
1063                         return Some((
1064                             span,
1065                             message,
1066                             suggestion,
1067                             Applicability::MachineApplicable,
1068                             true,
1069                             false,
1070                         ));
1071                     }
1072                 }
1073             }
1074             _ => {}
1075         }
1076         None
1077     }
1078
1079     pub fn check_for_cast(
1080         &self,
1081         err: &mut Diagnostic,
1082         expr: &hir::Expr<'_>,
1083         checked_ty: Ty<'tcx>,
1084         expected_ty: Ty<'tcx>,
1085         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
1086     ) -> bool {
1087         if self.tcx.sess.source_map().is_imported(expr.span) {
1088             // Ignore if span is from within a macro.
1089             return false;
1090         }
1091
1092         let Ok(src) = self.tcx.sess.source_map().span_to_snippet(expr.span) else {
1093             return false;
1094         };
1095
1096         // If casting this expression to a given numeric type would be appropriate in case of a type
1097         // mismatch.
1098         //
1099         // We want to minimize the amount of casting operations that are suggested, as it can be a
1100         // lossy operation with potentially bad side effects, so we only suggest when encountering
1101         // an expression that indicates that the original type couldn't be directly changed.
1102         //
1103         // For now, don't suggest casting with `as`.
1104         let can_cast = false;
1105
1106         let mut sugg = vec![];
1107
1108         if let Some(hir::Node::ExprField(field)) =
1109             self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
1110         {
1111             // `expr` is a literal field for a struct, only suggest if appropriate
1112             if field.is_shorthand {
1113                 // This is a field literal
1114                 sugg.push((field.ident.span.shrink_to_lo(), format!("{}: ", field.ident)));
1115             } else {
1116                 // Likely a field was meant, but this field wasn't found. Do not suggest anything.
1117                 return false;
1118             }
1119         };
1120
1121         if let hir::ExprKind::Call(path, args) = &expr.kind
1122             && let (hir::ExprKind::Path(hir::QPath::TypeRelative(base_ty, path_segment)), 1) =
1123                 (&path.kind, args.len())
1124             // `expr` is a conversion like `u32::from(val)`, do not suggest anything (#63697).
1125             && let (hir::TyKind::Path(hir::QPath::Resolved(None, base_ty_path)), sym::from) =
1126                 (&base_ty.kind, path_segment.ident.name)
1127         {
1128             if let Some(ident) = &base_ty_path.segments.iter().map(|s| s.ident).next() {
1129                 match ident.name {
1130                     sym::i128
1131                     | sym::i64
1132                     | sym::i32
1133                     | sym::i16
1134                     | sym::i8
1135                     | sym::u128
1136                     | sym::u64
1137                     | sym::u32
1138                     | sym::u16
1139                     | sym::u8
1140                     | sym::isize
1141                     | sym::usize
1142                         if base_ty_path.segments.len() == 1 =>
1143                     {
1144                         return false;
1145                     }
1146                     _ => {}
1147                 }
1148             }
1149         }
1150
1151         let msg = format!(
1152             "you can convert {} `{}` to {} `{}`",
1153             checked_ty.kind().article(),
1154             checked_ty,
1155             expected_ty.kind().article(),
1156             expected_ty,
1157         );
1158         let cast_msg = format!(
1159             "you can cast {} `{}` to {} `{}`",
1160             checked_ty.kind().article(),
1161             checked_ty,
1162             expected_ty.kind().article(),
1163             expected_ty,
1164         );
1165         let lit_msg = format!(
1166             "change the type of the numeric literal from `{checked_ty}` to `{expected_ty}`",
1167         );
1168
1169         let close_paren = if expr.precedence().order() < PREC_POSTFIX {
1170             sugg.push((expr.span.shrink_to_lo(), "(".to_string()));
1171             ")"
1172         } else {
1173             ""
1174         };
1175
1176         let mut cast_suggestion = sugg.clone();
1177         cast_suggestion.push((expr.span.shrink_to_hi(), format!("{close_paren} as {expected_ty}")));
1178         let mut into_suggestion = sugg.clone();
1179         into_suggestion.push((expr.span.shrink_to_hi(), format!("{close_paren}.into()")));
1180         let mut suffix_suggestion = sugg.clone();
1181         suffix_suggestion.push((
1182             if matches!(
1183                 (&expected_ty.kind(), &checked_ty.kind()),
1184                 (ty::Int(_) | ty::Uint(_), ty::Float(_))
1185             ) {
1186                 // Remove fractional part from literal, for example `42.0f32` into `42`
1187                 let src = src.trim_end_matches(&checked_ty.to_string());
1188                 let len = src.split('.').next().unwrap().len();
1189                 expr.span.with_lo(expr.span.lo() + BytePos(len as u32))
1190             } else {
1191                 let len = src.trim_end_matches(&checked_ty.to_string()).len();
1192                 expr.span.with_lo(expr.span.lo() + BytePos(len as u32))
1193             },
1194             if expr.precedence().order() < PREC_POSTFIX {
1195                 // Readd `)`
1196                 format!("{expected_ty})")
1197             } else {
1198                 expected_ty.to_string()
1199             },
1200         ));
1201         let literal_is_ty_suffixed = |expr: &hir::Expr<'_>| {
1202             if let hir::ExprKind::Lit(lit) = &expr.kind { lit.node.is_suffixed() } else { false }
1203         };
1204         let is_negative_int =
1205             |expr: &hir::Expr<'_>| matches!(expr.kind, hir::ExprKind::Unary(hir::UnOp::Neg, ..));
1206         let is_uint = |ty: Ty<'_>| matches!(ty.kind(), ty::Uint(..));
1207
1208         let in_const_context = self.tcx.hir().is_inside_const_context(expr.hir_id);
1209
1210         let suggest_fallible_into_or_lhs_from =
1211             |err: &mut Diagnostic, exp_to_found_is_fallible: bool| {
1212                 // If we know the expression the expected type is derived from, we might be able
1213                 // to suggest a widening conversion rather than a narrowing one (which may
1214                 // panic). For example, given x: u8 and y: u32, if we know the span of "x",
1215                 //   x > y
1216                 // can be given the suggestion "u32::from(x) > y" rather than
1217                 // "x > y.try_into().unwrap()".
1218                 let lhs_expr_and_src = expected_ty_expr.and_then(|expr| {
1219                     self.tcx
1220                         .sess
1221                         .source_map()
1222                         .span_to_snippet(expr.span)
1223                         .ok()
1224                         .map(|src| (expr, src))
1225                 });
1226                 let (msg, suggestion) = if let (Some((lhs_expr, lhs_src)), false) =
1227                     (lhs_expr_and_src, exp_to_found_is_fallible)
1228                 {
1229                     let msg = format!(
1230                         "you can convert `{lhs_src}` from `{expected_ty}` to `{checked_ty}`, matching the type of `{src}`",
1231                     );
1232                     let suggestion = vec![
1233                         (lhs_expr.span.shrink_to_lo(), format!("{checked_ty}::from(")),
1234                         (lhs_expr.span.shrink_to_hi(), ")".to_string()),
1235                     ];
1236                     (msg, suggestion)
1237                 } else {
1238                     let msg = format!("{msg} and panic if the converted value doesn't fit");
1239                     let mut suggestion = sugg.clone();
1240                     suggestion.push((
1241                         expr.span.shrink_to_hi(),
1242                         format!("{close_paren}.try_into().unwrap()"),
1243                     ));
1244                     (msg, suggestion)
1245                 };
1246                 err.multipart_suggestion_verbose(
1247                     &msg,
1248                     suggestion,
1249                     Applicability::MachineApplicable,
1250                 );
1251             };
1252
1253         let suggest_to_change_suffix_or_into =
1254             |err: &mut Diagnostic,
1255              found_to_exp_is_fallible: bool,
1256              exp_to_found_is_fallible: bool| {
1257                 let exp_is_lhs =
1258                     expected_ty_expr.map(|e| self.tcx.hir().is_lhs(e.hir_id)).unwrap_or(false);
1259
1260                 if exp_is_lhs {
1261                     return;
1262                 }
1263
1264                 let always_fallible = found_to_exp_is_fallible
1265                     && (exp_to_found_is_fallible || expected_ty_expr.is_none());
1266                 let msg = if literal_is_ty_suffixed(expr) {
1267                     &lit_msg
1268                 } else if always_fallible && (is_negative_int(expr) && is_uint(expected_ty)) {
1269                     // We now know that converting either the lhs or rhs is fallible. Before we
1270                     // suggest a fallible conversion, check if the value can never fit in the
1271                     // expected type.
1272                     let msg = format!("`{src}` cannot fit into type `{expected_ty}`");
1273                     err.note(&msg);
1274                     return;
1275                 } else if in_const_context {
1276                     // Do not recommend `into` or `try_into` in const contexts.
1277                     return;
1278                 } else if found_to_exp_is_fallible {
1279                     return suggest_fallible_into_or_lhs_from(err, exp_to_found_is_fallible);
1280                 } else {
1281                     &msg
1282                 };
1283                 let suggestion = if literal_is_ty_suffixed(expr) {
1284                     suffix_suggestion.clone()
1285                 } else {
1286                     into_suggestion.clone()
1287                 };
1288                 err.multipart_suggestion_verbose(msg, suggestion, Applicability::MachineApplicable);
1289             };
1290
1291         match (&expected_ty.kind(), &checked_ty.kind()) {
1292             (&ty::Int(ref exp), &ty::Int(ref found)) => {
1293                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1294                 {
1295                     (Some(exp), Some(found)) if exp < found => (true, false),
1296                     (Some(exp), Some(found)) if exp > found => (false, true),
1297                     (None, Some(8 | 16)) => (false, true),
1298                     (Some(8 | 16), None) => (true, false),
1299                     (None, _) | (_, None) => (true, true),
1300                     _ => (false, false),
1301                 };
1302                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1303                 true
1304             }
1305             (&ty::Uint(ref exp), &ty::Uint(ref found)) => {
1306                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1307                 {
1308                     (Some(exp), Some(found)) if exp < found => (true, false),
1309                     (Some(exp), Some(found)) if exp > found => (false, true),
1310                     (None, Some(8 | 16)) => (false, true),
1311                     (Some(8 | 16), None) => (true, false),
1312                     (None, _) | (_, None) => (true, true),
1313                     _ => (false, false),
1314                 };
1315                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1316                 true
1317             }
1318             (&ty::Int(exp), &ty::Uint(found)) => {
1319                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1320                 {
1321                     (Some(exp), Some(found)) if found < exp => (false, true),
1322                     (None, Some(8)) => (false, true),
1323                     _ => (true, true),
1324                 };
1325                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1326                 true
1327             }
1328             (&ty::Uint(exp), &ty::Int(found)) => {
1329                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1330                 {
1331                     (Some(exp), Some(found)) if found > exp => (true, false),
1332                     (Some(8), None) => (true, false),
1333                     _ => (true, true),
1334                 };
1335                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1336                 true
1337             }
1338             (&ty::Float(ref exp), &ty::Float(ref found)) => {
1339                 if found.bit_width() < exp.bit_width() {
1340                     suggest_to_change_suffix_or_into(err, false, true);
1341                 } else if literal_is_ty_suffixed(expr) {
1342                     err.multipart_suggestion_verbose(
1343                         &lit_msg,
1344                         suffix_suggestion,
1345                         Applicability::MachineApplicable,
1346                     );
1347                 } else if can_cast {
1348                     // Missing try_into implementation for `f64` to `f32`
1349                     err.multipart_suggestion_verbose(
1350                         &format!("{cast_msg}, producing the closest possible value"),
1351                         cast_suggestion,
1352                         Applicability::MaybeIncorrect, // lossy conversion
1353                     );
1354                 }
1355                 true
1356             }
1357             (&ty::Uint(_) | &ty::Int(_), &ty::Float(_)) => {
1358                 if literal_is_ty_suffixed(expr) {
1359                     err.multipart_suggestion_verbose(
1360                         &lit_msg,
1361                         suffix_suggestion,
1362                         Applicability::MachineApplicable,
1363                     );
1364                 } else if can_cast {
1365                     // Missing try_into implementation for `{float}` to `{integer}`
1366                     err.multipart_suggestion_verbose(
1367                         &format!("{msg}, rounding the float towards zero"),
1368                         cast_suggestion,
1369                         Applicability::MaybeIncorrect, // lossy conversion
1370                     );
1371                 }
1372                 true
1373             }
1374             (&ty::Float(ref exp), &ty::Uint(ref found)) => {
1375                 // if `found` is `None` (meaning found is `usize`), don't suggest `.into()`
1376                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
1377                     err.multipart_suggestion_verbose(
1378                         &format!(
1379                             "{msg}, producing the floating point representation of the integer",
1380                         ),
1381                         into_suggestion,
1382                         Applicability::MachineApplicable,
1383                     );
1384                 } else if literal_is_ty_suffixed(expr) {
1385                     err.multipart_suggestion_verbose(
1386                         &lit_msg,
1387                         suffix_suggestion,
1388                         Applicability::MachineApplicable,
1389                     );
1390                 } else {
1391                     // Missing try_into implementation for `{integer}` to `{float}`
1392                     err.multipart_suggestion_verbose(
1393                         &format!(
1394                             "{cast_msg}, producing the floating point representation of the integer, \
1395                                  rounded if necessary",
1396                         ),
1397                         cast_suggestion,
1398                         Applicability::MaybeIncorrect, // lossy conversion
1399                     );
1400                 }
1401                 true
1402             }
1403             (&ty::Float(ref exp), &ty::Int(ref found)) => {
1404                 // if `found` is `None` (meaning found is `isize`), don't suggest `.into()`
1405                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
1406                     err.multipart_suggestion_verbose(
1407                         &format!(
1408                             "{}, producing the floating point representation of the integer",
1409                             &msg,
1410                         ),
1411                         into_suggestion,
1412                         Applicability::MachineApplicable,
1413                     );
1414                 } else if literal_is_ty_suffixed(expr) {
1415                     err.multipart_suggestion_verbose(
1416                         &lit_msg,
1417                         suffix_suggestion,
1418                         Applicability::MachineApplicable,
1419                     );
1420                 } else {
1421                     // Missing try_into implementation for `{integer}` to `{float}`
1422                     err.multipart_suggestion_verbose(
1423                         &format!(
1424                             "{}, producing the floating point representation of the integer, \
1425                                 rounded if necessary",
1426                             &msg,
1427                         ),
1428                         cast_suggestion,
1429                         Applicability::MaybeIncorrect, // lossy conversion
1430                     );
1431                 }
1432                 true
1433             }
1434             (
1435                 &ty::Uint(ty::UintTy::U32 | ty::UintTy::U64 | ty::UintTy::U128)
1436                 | &ty::Int(ty::IntTy::I32 | ty::IntTy::I64 | ty::IntTy::I128),
1437                 &ty::Char,
1438             ) => {
1439                 err.multipart_suggestion_verbose(
1440                     &format!("{cast_msg}, since a `char` always occupies 4 bytes"),
1441                     cast_suggestion,
1442                     Applicability::MachineApplicable,
1443                 );
1444                 true
1445             }
1446             _ => false,
1447         }
1448     }
1449 }