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             || self.suggest_option_to_bool(err, expr, expr_ty, expected)
  47             || self.suggest_floating_point_literal(err, expr, expected);
  48
  49         self.note_type_is_not_clone(err, expected, expr_ty, expr);
  50         self.note_need_for_fn_pointer(err, expected, expr_ty);
  51         self.note_internal_mutation_in_method(err, expr, expected, expr_ty);
  52     }
  53
  54     // Requires that the two types unify, and prints an error message if
  55     // they don't.
  56     pub fn demand_suptype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  57         if let Some(mut e) = self.demand_suptype_diag(sp, expected, actual) {
  58             e.emit();
  59         }
  60     }
  61
  62     pub fn demand_suptype_diag(
  63         &self,
  64         sp: Span,
  65         expected: Ty<'tcx>,
  66         actual: Ty<'tcx>,
  67     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
  68         self.demand_suptype_with_origin(&self.misc(sp), expected, actual)
  69     }
  70
  71     #[instrument(skip(self), level = "debug")]
  72     pub fn demand_suptype_with_origin(
  73         &self,
  74         cause: &ObligationCause<'tcx>,
  75         expected: Ty<'tcx>,
  76         actual: Ty<'tcx>,
  77     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
  78         match self.at(cause, self.param_env).sup(expected, actual) {
  79             Ok(InferOk { obligations, value: () }) => {
  80                 self.register_predicates(obligations);
  81                 None
  82             }
  83             Err(e) => Some(self.err_ctxt().report_mismatched_types(&cause, expected, actual, e)),
  84         }
  85     }
  86
  87     pub fn demand_eqtype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  88         if let Some(mut err) = self.demand_eqtype_diag(sp, expected, actual) {
  89             err.emit();
  90         }
  91     }
  92
  93     pub fn demand_eqtype_diag(
  94         &self,
  95         sp: Span,
  96         expected: Ty<'tcx>,
  97         actual: Ty<'tcx>,
  98     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
  99         self.demand_eqtype_with_origin(&self.misc(sp), expected, actual)
 100     }
 101
 102     pub fn demand_eqtype_with_origin(
 103         &self,
 104         cause: &ObligationCause<'tcx>,
 105         expected: Ty<'tcx>,
 106         actual: Ty<'tcx>,
 107     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
 108         match self.at(cause, self.param_env).eq(expected, actual) {
 109             Ok(InferOk { obligations, value: () }) => {
 110                 self.register_predicates(obligations);
 111                 None
 112             }
 113             Err(e) => Some(self.err_ctxt().report_mismatched_types(cause, expected, actual, e)),
 114         }
 115     }
 116
 117     pub fn demand_coerce(
 118         &self,
 119         expr: &hir::Expr<'tcx>,
 120         checked_ty: Ty<'tcx>,
 121         expected: Ty<'tcx>,
 122         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 123         allow_two_phase: AllowTwoPhase,
 124     ) -> Ty<'tcx> {
 125         let (ty, err) =
 126             self.demand_coerce_diag(expr, checked_ty, expected, expected_ty_expr, allow_two_phase);
 127         if let Some(mut err) = err {
 128             err.emit();
 129         }
 130         ty
 131     }
 132
 133     /// Checks that the type of `expr` can be coerced to `expected`.
 134     ///
 135     /// N.B., this code relies on `self.diverges` to be accurate. In particular, assignments to `!`
 136     /// will be permitted if the diverges flag is currently "always".
 137     #[instrument(level = "debug", skip(self, expr, expected_ty_expr, allow_two_phase))]
 138     pub fn demand_coerce_diag(
 139         &self,
 140         expr: &hir::Expr<'tcx>,
 141         checked_ty: Ty<'tcx>,
 142         expected: Ty<'tcx>,
 143         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 144         allow_two_phase: AllowTwoPhase,
 145     ) -> (Ty<'tcx>, Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>>) {
 146         let expected = self.resolve_vars_with_obligations(expected);
 147
 148         let e = match self.try_coerce(expr, checked_ty, expected, allow_two_phase, None) {
 149             Ok(ty) => return (ty, None),
 150             Err(e) => e,
 151         };
 152
 153         self.set_tainted_by_errors();
 154         let expr = expr.peel_drop_temps();
 155         let cause = self.misc(expr.span);
 156         let expr_ty = self.resolve_vars_with_obligations(checked_ty);
 157         let mut err = self.err_ctxt().report_mismatched_types(&cause, expected, expr_ty, e.clone());
 158
 159         let is_insufficiently_polymorphic =
 160             matches!(e, TypeError::RegionsInsufficientlyPolymorphic(..));
 161
 162         // FIXME(#73154): For now, we do leak check when coercing function
 163         // pointers in typeck, instead of only during borrowck. This can lead
 164         // to these `RegionsInsufficientlyPolymorphic` errors that aren't helpful.
 165         if !is_insufficiently_polymorphic {
 166             self.emit_coerce_suggestions(
 167                 &mut err,
 168                 expr,
 169                 expr_ty,
 170                 expected,
 171                 expected_ty_expr,
 172                 Some(e),
 173             );
 174         }
 175
 176         (expected, Some(err))
 177     }
 178
 179     fn annotate_expected_due_to_let_ty(
 180         &self,
 181         err: &mut Diagnostic,
 182         expr: &hir::Expr<'_>,
 183         error: Option<TypeError<'_>>,
 184     ) {
 185         let parent = self.tcx.hir().get_parent_node(expr.hir_id);
 186         match (self.tcx.hir().find(parent), error) {
 187             (Some(hir::Node::Local(hir::Local { ty: Some(ty), init: Some(init), .. })), _)
 188                 if init.hir_id == expr.hir_id =>
 189             {
 190                 // Point at `let` assignment type.
 191                 err.span_label(ty.span, "expected due to this");
 192             }
 193             (
 194                 Some(hir::Node::Expr(hir::Expr {
 195                     kind: hir::ExprKind::Assign(lhs, rhs, _), ..
 196                 })),
 197                 Some(TypeError::Sorts(ExpectedFound { expected, .. })),
 198             ) if rhs.hir_id == expr.hir_id && !expected.is_closure() => {
 199                 // We ignore closures explicitly because we already point at them elsewhere.
 200                 // Point at the assigned-to binding.
 201                 let mut primary_span = lhs.span;
 202                 let mut secondary_span = lhs.span;
 203                 let mut post_message = "";
 204                 match lhs.kind {
 205                     hir::ExprKind::Path(hir::QPath::Resolved(
 206                         None,
 207                         hir::Path {
 208                             res:
 209                                 hir::def::Res::Def(
 210                                     hir::def::DefKind::Static(_) | hir::def::DefKind::Const,
 211                                     def_id,
 212                                 ),
 213                             ..
 214                         },
 215                     )) => {
 216                         if let Some(hir::Node::Item(hir::Item {
 217                             ident,
 218                             kind: hir::ItemKind::Static(ty, ..) | hir::ItemKind::Const(ty, ..),
 219                             ..
 220                         })) = self.tcx.hir().get_if_local(*def_id)
 221                         {
 222                             primary_span = ty.span;
 223                             secondary_span = ident.span;
 224                             post_message = " type";
 225                         }
 226                     }
 227                     hir::ExprKind::Path(hir::QPath::Resolved(
 228                         None,
 229                         hir::Path { res: hir::def::Res::Local(hir_id), .. },
 230                     )) => {
 231                         if let Some(hir::Node::Pat(pat)) = self.tcx.hir().find(*hir_id) {
 232                             let parent = self.tcx.hir().get_parent_node(pat.hir_id);
 233                             primary_span = pat.span;
 234                             secondary_span = pat.span;
 235                             match self.tcx.hir().find(parent) {
 236                                 Some(hir::Node::Local(hir::Local { ty: Some(ty), .. })) => {
 237                                     primary_span = ty.span;
 238                                     post_message = " type";
 239                                 }
 240                                 Some(hir::Node::Local(hir::Local { init: Some(init), .. })) => {
 241                                     primary_span = init.span;
 242                                     post_message = " value";
 243                                 }
 244                                 Some(hir::Node::Param(hir::Param { ty_span, .. })) => {
 245                                     primary_span = *ty_span;
 246                                     post_message = " parameter type";
 247                                 }
 248                                 _ => {}
 249                             }
 250                         }
 251                     }
 252                     _ => {}
 253                 }
 254
 255                 if primary_span != secondary_span
 256                     && self
 257                         .tcx
 258                         .sess
 259                         .source_map()
 260                         .is_multiline(secondary_span.shrink_to_hi().until(primary_span))
 261                 {
 262                     // We are pointing at the binding's type or initializer value, but it's pattern
 263                     // is in a different line, so we point at both.
 264                     err.span_label(secondary_span, "expected due to the type of this binding");
 265                     err.span_label(primary_span, &format!("expected due to this{post_message}"));
 266                 } else if post_message == "" {
 267                     // We are pointing at either the assignment lhs or the binding def pattern.
 268                     err.span_label(primary_span, "expected due to the type of this binding");
 269                 } else {
 270                     // We are pointing at the binding's type or initializer value.
 271                     err.span_label(primary_span, &format!("expected due to this{post_message}"));
 272                 }
 273
 274                 if !lhs.is_syntactic_place_expr() {
 275                     // We already emitted E0070 "invalid left-hand side of assignment", so we
 276                     // silence this.
 277                     err.downgrade_to_delayed_bug();
 278                 }
 279             }
 280             _ => {}
 281         }
 282     }
 283
 284     /// If the expected type is an enum (Issue #55250) with any variants whose
 285     /// sole field is of the found type, suggest such variants. (Issue #42764)
 286     fn suggest_compatible_variants(
 287         &self,
 288         err: &mut Diagnostic,
 289         expr: &hir::Expr<'_>,
 290         expected: Ty<'tcx>,
 291         expr_ty: Ty<'tcx>,
 292     ) -> bool {
 293         if let ty::Adt(expected_adt, substs) = expected.kind() {
 294             if let hir::ExprKind::Field(base, ident) = expr.kind {
 295                 let base_ty = self.typeck_results.borrow().expr_ty(base);
 296                 if self.can_eq(self.param_env, base_ty, expected).is_ok()
 297                     && let Some(base_span) = base.span.find_ancestor_inside(expr.span)
 298                 {
 299                     err.span_suggestion_verbose(
 300                         expr.span.with_lo(base_span.hi()),
 301                         format!("consider removing the tuple struct field `{ident}`"),
 302                         "",
 303                         Applicability::MaybeIncorrect,
 304                     );
 305                     return true;
 306                 }
 307             }
 308
 309             // If the expression is of type () and it's the return expression of a block,
 310             // we suggest adding a separate return expression instead.
 311             // (To avoid things like suggesting `Ok(while .. { .. })`.)
 312             if expr_ty.is_unit() {
 313                 let mut id = expr.hir_id;
 314                 let mut parent;
 315
 316                 // Unroll desugaring, to make sure this works for `for` loops etc.
 317                 loop {
 318                     parent = self.tcx.hir().get_parent_node(id);
 319                     if let Some(parent_span) = self.tcx.hir().opt_span(parent) {
 320                         if parent_span.find_ancestor_inside(expr.span).is_some() {
 321                             // The parent node is part of the same span, so is the result of the
 322                             // same expansion/desugaring and not the 'real' parent node.
 323                             id = parent;
 324                             continue;
 325                         }
 326                     }
 327                     break;
 328                 }
 329
 330                 if let Some(hir::Node::Block(&hir::Block {
 331                     span: block_span, expr: Some(e), ..
 332                 })) = self.tcx.hir().find(parent)
 333                 {
 334                     if e.hir_id == id {
 335                         if let Some(span) = expr.span.find_ancestor_inside(block_span) {
 336                             let return_suggestions = if self
 337                                 .tcx
 338                                 .is_diagnostic_item(sym::Result, expected_adt.did())
 339                             {
 340                                 vec!["Ok(())"]
 341                             } else if self.tcx.is_diagnostic_item(sym::Option, expected_adt.did()) {
 342                                 vec!["None", "Some(())"]
 343                             } else {
 344                                 return false;
 345                             };
 346                             if let Some(indent) =
 347                                 self.tcx.sess.source_map().indentation_before(span.shrink_to_lo())
 348                             {
 349                                 // Add a semicolon, except after `}`.
 350                                 let semicolon =
 351                                     match self.tcx.sess.source_map().span_to_snippet(span) {
 352                                         Ok(s) if s.ends_with('}') => "",
 353                                         _ => ";",
 354                                     };
 355                                 err.span_suggestions(
 356                                     span.shrink_to_hi(),
 357                                     "try adding an expression at the end of the block",
 358                                     return_suggestions
 359                                         .into_iter()
 360                                         .map(|r| format!("{semicolon}\n{indent}{r}")),
 361                                     Applicability::MaybeIncorrect,
 362                                 );
 363                             }
 364                             return true;
 365                         }
 366                     }
 367                 }
 368             }
 369
 370             let compatible_variants: Vec<(String, _, _, Option<String>)> = expected_adt
 371                 .variants()
 372                 .iter()
 373                 .filter(|variant| {
 374                     variant.fields.len() == 1
 375                 })
 376                 .filter_map(|variant| {
 377                     let sole_field = &variant.fields[0];
 378
 379                     let field_is_local = sole_field.did.is_local();
 380                     let field_is_accessible =
 381                         sole_field.vis.is_accessible_from(expr.hir_id.owner.def_id, self.tcx)
 382                         // Skip suggestions for unstable public fields (for example `Pin::pointer`)
 383                         && matches!(self.tcx.eval_stability(sole_field.did, None, expr.span, None), EvalResult::Allow | EvalResult::Unmarked);
 384
 385                     if !field_is_local && !field_is_accessible {
 386                         return None;
 387                     }
 388
 389                     let note_about_variant_field_privacy = (field_is_local && !field_is_accessible)
 390                         .then(|| format!(" (its field is private, but it's local to this crate and its privacy can be changed)"));
 391
 392                     let sole_field_ty = sole_field.ty(self.tcx, substs);
 393                     if self.can_coerce(expr_ty, sole_field_ty) {
 394                         let variant_path =
 395                             with_no_trimmed_paths!(self.tcx.def_path_str(variant.def_id));
 396                         // FIXME #56861: DRYer prelude filtering
 397                         if let Some(path) = variant_path.strip_prefix("std::prelude::")
 398                             && let Some((_, path)) = path.split_once("::")
 399                         {
 400                             return Some((path.to_string(), variant.ctor_kind, sole_field.name, note_about_variant_field_privacy));
 401                         }
 402                         Some((variant_path, variant.ctor_kind, sole_field.name, note_about_variant_field_privacy))
 403                     } else {
 404                         None
 405                     }
 406                 })
 407                 .collect();
 408
 409             let suggestions_for = |variant: &_, ctor, field_name| {
 410                 let prefix = match self.maybe_get_struct_pattern_shorthand_field(expr) {
 411                     Some(ident) => format!("{ident}: "),
 412                     None => String::new(),
 413                 };
 414
 415                 let (open, close) = match ctor {
 416                     hir::def::CtorKind::Fn => ("(".to_owned(), ")"),
 417                     hir::def::CtorKind::Fictive => (format!(" {{ {field_name}: "), " }"),
 418
 419                     // unit variants don't have fields
 420                     hir::def::CtorKind::Const => unreachable!(),
 421                 };
 422
 423                 // Suggest constructor as deep into the block tree as possible.
 424                 // This fixes https://github.com/rust-lang/rust/issues/101065,
 425                 // and also just helps make the most minimal suggestions.
 426                 let mut expr = expr;
 427                 while let hir::ExprKind::Block(block, _) = &expr.kind
 428                     && let Some(expr_) = &block.expr
 429                 {
 430                     expr = expr_
 431                 }
 432
 433                 vec![
 434                     (expr.span.shrink_to_lo(), format!("{prefix}{variant}{open}")),
 435                     (expr.span.shrink_to_hi(), close.to_owned()),
 436                 ]
 437             };
 438
 439             match &compatible_variants[..] {
 440                 [] => { /* No variants to format */ }
 441                 [(variant, ctor_kind, field_name, note)] => {
 442                     // Just a single matching variant.
 443                     err.multipart_suggestion_verbose(
 444                         &format!(
 445                             "try wrapping the expression in `{variant}`{note}",
 446                             note = note.as_deref().unwrap_or("")
 447                         ),
 448                         suggestions_for(&**variant, *ctor_kind, *field_name),
 449                         Applicability::MaybeIncorrect,
 450                     );
 451                     return true;
 452                 }
 453                 _ => {
 454                     // More than one matching variant.
 455                     err.multipart_suggestions(
 456                         &format!(
 457                             "try wrapping the expression in a variant of `{}`",
 458                             self.tcx.def_path_str(expected_adt.did())
 459                         ),
 460                         compatible_variants.into_iter().map(
 461                             |(variant, ctor_kind, field_name, _)| {
 462                                 suggestions_for(&variant, ctor_kind, field_name)
 463                             },
 464                         ),
 465                         Applicability::MaybeIncorrect,
 466                     );
 467                     return true;
 468                 }
 469             }
 470         }
 471
 472         false
 473     }
 474
 475     fn suggest_non_zero_new_unwrap(
 476         &self,
 477         err: &mut Diagnostic,
 478         expr: &hir::Expr<'_>,
 479         expected: Ty<'tcx>,
 480         expr_ty: Ty<'tcx>,
 481     ) -> bool {
 482         let tcx = self.tcx;
 483         let (adt, unwrap) = match expected.kind() {
 484             // In case Option<NonZero*> is wanted, but * is provided, suggest calling new
 485             ty::Adt(adt, substs) if tcx.is_diagnostic_item(sym::Option, adt.did()) => {
 486                 // Unwrap option
 487                 let ty::Adt(adt, _) = substs.type_at(0).kind() else { return false; };
 488
 489                 (adt, "")
 490             }
 491             // In case NonZero* is wanted, but * is provided also add `.unwrap()` to satisfy types
 492             ty::Adt(adt, _) => (adt, ".unwrap()"),
 493             _ => return false,
 494         };
 495
 496         let map = [
 497             (sym::NonZeroU8, tcx.types.u8),
 498             (sym::NonZeroU16, tcx.types.u16),
 499             (sym::NonZeroU32, tcx.types.u32),
 500             (sym::NonZeroU64, tcx.types.u64),
 501             (sym::NonZeroU128, tcx.types.u128),
 502             (sym::NonZeroI8, tcx.types.i8),
 503             (sym::NonZeroI16, tcx.types.i16),
 504             (sym::NonZeroI32, tcx.types.i32),
 505             (sym::NonZeroI64, tcx.types.i64),
 506             (sym::NonZeroI128, tcx.types.i128),
 507         ];
 508
 509         let Some((s, _)) = map
 510             .iter()
 511             .find(|&&(s, t)| self.tcx.is_diagnostic_item(s, adt.did()) && self.can_coerce(expr_ty, t))
 512             else { return false; };
 513
 514         let path = self.tcx.def_path_str(adt.non_enum_variant().def_id);
 515
 516         err.multipart_suggestion(
 517             format!("consider calling `{s}::new`"),
 518             vec![
 519                 (expr.span.shrink_to_lo(), format!("{path}::new(")),
 520                 (expr.span.shrink_to_hi(), format!("){unwrap}")),
 521             ],
 522             Applicability::MaybeIncorrect,
 523         );
 524
 525         true
 526     }
 527
 528     pub fn get_conversion_methods(
 529         &self,
 530         span: Span,
 531         expected: Ty<'tcx>,
 532         checked_ty: Ty<'tcx>,
 533         hir_id: hir::HirId,
 534     ) -> Vec<AssocItem> {
 535         let methods = self.probe_for_return_type(
 536             span,
 537             probe::Mode::MethodCall,
 538             expected,
 539             checked_ty,
 540             hir_id,
 541             |m| {
 542                 self.has_only_self_parameter(m)
 543                     && self
 544                         .tcx
 545                         // This special internal attribute is used to permit
 546                         // "identity-like" conversion methods to be suggested here.
 547                         //
 548                         // FIXME (#46459 and #46460): ideally
 549                         // `std::convert::Into::into` and `std::borrow:ToOwned` would
 550                         // also be `#[rustc_conversion_suggestion]`, if not for
 551                         // method-probing false-positives and -negatives (respectively).
 552                         //
 553                         // FIXME? Other potential candidate methods: `as_ref` and
 554                         // `as_mut`?
 555                         .has_attr(m.def_id, sym::rustc_conversion_suggestion)
 556             },
 557         );
 558
 559         methods
 560     }
 561
 562     /// This function checks whether the method is not static and does not accept other parameters than `self`.
 563     fn has_only_self_parameter(&self, method: &AssocItem) -> bool {
 564         match method.kind {
 565             ty::AssocKind::Fn => {
 566                 method.fn_has_self_parameter
 567                     && self.tcx.fn_sig(method.def_id).inputs().skip_binder().len() == 1
 568             }
 569             _ => false,
 570         }
 571     }
 572
 573     /// Identify some cases where `as_ref()` would be appropriate and suggest it.
 574     ///
 575     /// Given the following code:
 576     /// ```compile_fail,E0308
 577     /// struct Foo;
 578     /// fn takes_ref(_: &Foo) {}
 579     /// let ref opt = Some(Foo);
 580     ///
 581     /// opt.map(|param| takes_ref(param));
 582     /// ```
 583     /// Suggest using `opt.as_ref().map(|param| takes_ref(param));` instead.
 584     ///
 585     /// It only checks for `Option` and `Result` and won't work with
 586     /// ```ignore (illustrative)
 587     /// opt.map(|param| { takes_ref(param) });
 588     /// ```
 589     fn can_use_as_ref(&self, expr: &hir::Expr<'_>) -> Option<(Span, &'static str, String)> {
 590         let hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) = expr.kind else {
 591             return None;
 592         };
 593
 594         let hir::def::Res::Local(local_id) = path.res else {
 595             return None;
 596         };
 597
 598         let local_parent = self.tcx.hir().get_parent_node(local_id);
 599         let Some(Node::Param(hir::Param { hir_id: param_hir_id, .. })) = self.tcx.hir().find(local_parent) else {
 600             return None;
 601         };
 602
 603         let param_parent = self.tcx.hir().get_parent_node(*param_hir_id);
 604         let Some(Node::Expr(hir::Expr {
 605             hir_id: expr_hir_id,
 606             kind: hir::ExprKind::Closure(hir::Closure { fn_decl: closure_fn_decl, .. }),
 607             ..
 608         })) = self.tcx.hir().find(param_parent) else {
 609             return None;
 610         };
 611
 612         let expr_parent = self.tcx.hir().get_parent_node(*expr_hir_id);
 613         let hir = self.tcx.hir().find(expr_parent);
 614         let closure_params_len = closure_fn_decl.inputs.len();
 615         let (
 616             Some(Node::Expr(hir::Expr {
 617                 kind: hir::ExprKind::MethodCall(method_path, receiver, ..),
 618                 ..
 619             })),
 620             1,
 621         ) = (hir, closure_params_len) else {
 622             return None;
 623         };
 624
 625         let self_ty = self.typeck_results.borrow().expr_ty(receiver);
 626         let name = method_path.ident.name;
 627         let is_as_ref_able = match self_ty.peel_refs().kind() {
 628             ty::Adt(def, _) => {
 629                 (self.tcx.is_diagnostic_item(sym::Option, def.did())
 630                     || self.tcx.is_diagnostic_item(sym::Result, def.did()))
 631                     && (name == sym::map || name == sym::and_then)
 632             }
 633             _ => false,
 634         };
 635         match (is_as_ref_able, self.sess().source_map().span_to_snippet(method_path.ident.span)) {
 636             (true, Ok(src)) => {
 637                 let suggestion = format!("as_ref().{}", src);
 638                 Some((method_path.ident.span, "consider using `as_ref` instead", suggestion))
 639             }
 640             _ => None,
 641         }
 642     }
 643
 644     pub(crate) fn maybe_get_struct_pattern_shorthand_field(
 645         &self,
 646         expr: &hir::Expr<'_>,
 647     ) -> Option<Symbol> {
 648         let hir = self.tcx.hir();
 649         let local = match expr {
 650             hir::Expr {
 651                 kind:
 652                     hir::ExprKind::Path(hir::QPath::Resolved(
 653                         None,
 654                         hir::Path {
 655                             res: hir::def::Res::Local(_),
 656                             segments: [hir::PathSegment { ident, .. }],
 657                             ..
 658                         },
 659                     )),
 660                 ..
 661             } => Some(ident),
 662             _ => None,
 663         }?;
 664
 665         match hir.find(hir.get_parent_node(expr.hir_id))? {
 666             Node::ExprField(field) => {
 667                 if field.ident.name == local.name && field.is_shorthand {
 668                     return Some(local.name);
 669                 }
 670             }
 671             _ => {}
 672         }
 673
 674         None
 675     }
 676
 677     /// If the given `HirId` corresponds to a block with a trailing expression, return that expression
 678     pub(crate) fn maybe_get_block_expr(
 679         &self,
 680         expr: &hir::Expr<'tcx>,
 681     ) -> Option<&'tcx hir::Expr<'tcx>> {
 682         match expr {
 683             hir::Expr { kind: hir::ExprKind::Block(block, ..), .. } => block.expr,
 684             _ => None,
 685         }
 686     }
 687
 688     /// Returns whether the given expression is an `else if`.
 689     pub(crate) fn is_else_if_block(&self, expr: &hir::Expr<'_>) -> bool {
 690         if let hir::ExprKind::If(..) = expr.kind {
 691             let parent_id = self.tcx.hir().get_parent_node(expr.hir_id);
 692             if let Some(Node::Expr(hir::Expr {
 693                 kind: hir::ExprKind::If(_, _, Some(else_expr)),
 694                 ..
 695             })) = self.tcx.hir().find(parent_id)
 696             {
 697                 return else_expr.hir_id == expr.hir_id;
 698             }
 699         }
 700         false
 701     }
 702
 703     /// This function is used to determine potential "simple" improvements or users' errors and
 704     /// provide them useful help. For example:
 705     ///
 706     /// ```compile_fail,E0308
 707     /// fn some_fn(s: &str) {}
 708     ///
 709     /// let x = "hey!".to_owned();
 710     /// some_fn(x); // error
 711     /// ```
 712     ///
 713     /// No need to find every potential function which could make a coercion to transform a
 714     /// `String` into a `&str` since a `&` would do the trick!
 715     ///
 716     /// In addition of this check, it also checks between references mutability state. If the
 717     /// expected is mutable but the provided isn't, maybe we could just say "Hey, try with
 718     /// `&mut`!".
 719     pub fn check_ref(
 720         &self,
 721         expr: &hir::Expr<'tcx>,
 722         checked_ty: Ty<'tcx>,
 723         expected: Ty<'tcx>,
 724     ) -> Option<(
 725         Span,
 726         String,
 727         String,
 728         Applicability,
 729         bool, /* verbose */
 730         bool, /* suggest `&` or `&mut` type annotation */
 731     )> {
 732         let sess = self.sess();
 733         let sp = expr.span;
 734
 735         // If the span is from an external macro, there's no suggestion we can make.
 736         if in_external_macro(sess, sp) {
 737             return None;
 738         }
 739
 740         let sm = sess.source_map();
 741
 742         let replace_prefix = |s: &str, old: &str, new: &str| {
 743             s.strip_prefix(old).map(|stripped| new.to_string() + stripped)
 744         };
 745
 746         // `ExprKind::DropTemps` is semantically irrelevant for these suggestions.
 747         let expr = expr.peel_drop_temps();
 748
 749         match (&expr.kind, expected.kind(), checked_ty.kind()) {
 750             (_, &ty::Ref(_, exp, _), &ty::Ref(_, check, _)) => match (exp.kind(), check.kind()) {
 751                 (&ty::Str, &ty::Array(arr, _) | &ty::Slice(arr)) if arr == self.tcx.types.u8 => {
 752                     if let hir::ExprKind::Lit(_) = expr.kind
 753                         && let Ok(src) = sm.span_to_snippet(sp)
 754                         && replace_prefix(&src, "b\"", "\"").is_some()
 755                     {
 756                                 let pos = sp.lo() + BytePos(1);
 757                                 return Some((
 758                                     sp.with_hi(pos),
 759                                     "consider removing the leading `b`".to_string(),
 760                                     String::new(),
 761                                     Applicability::MachineApplicable,
 762                                     true,
 763                                     false,
 764                                 ));
 765                             }
 766                         }
 767                 (&ty::Array(arr, _) | &ty::Slice(arr), &ty::Str) if arr == self.tcx.types.u8 => {
 768                     if let hir::ExprKind::Lit(_) = expr.kind
 769                         && let Ok(src) = sm.span_to_snippet(sp)
 770                         && replace_prefix(&src, "\"", "b\"").is_some()
 771                     {
 772                                 return Some((
 773                                     sp.shrink_to_lo(),
 774                                     "consider adding a leading `b`".to_string(),
 775                                     "b".to_string(),
 776                                     Applicability::MachineApplicable,
 777                                     true,
 778                                     false,
 779                                 ));
 780                     }
 781                 }
 782                 _ => {}
 783             },
 784             (_, &ty::Ref(_, _, mutability), _) => {
 785                 // Check if it can work when put into a ref. For example:
 786                 //
 787                 // ```
 788                 // fn bar(x: &mut i32) {}
 789                 //
 790                 // let x = 0u32;
 791                 // bar(&x); // error, expected &mut
 792                 // ```
 793                 let ref_ty = match mutability {
 794                     hir::Mutability::Mut => {
 795                         self.tcx.mk_mut_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 796                     }
 797                     hir::Mutability::Not => {
 798                         self.tcx.mk_imm_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 799                     }
 800                 };
 801                 if self.can_coerce(ref_ty, expected) {
 802                     let mut sugg_sp = sp;
 803                     if let hir::ExprKind::MethodCall(ref segment, receiver, args, _) = expr.kind {
 804                         let clone_trait =
 805                             self.tcx.require_lang_item(LangItem::Clone, Some(segment.ident.span));
 806                         if args.is_empty()
 807                             && self.typeck_results.borrow().type_dependent_def_id(expr.hir_id).map(
 808                                 |did| {
 809                                     let ai = self.tcx.associated_item(did);
 810                                     ai.trait_container(self.tcx) == Some(clone_trait)
 811                                 },
 812                             ) == Some(true)
 813                             && segment.ident.name == sym::clone
 814                         {
 815                             // If this expression had a clone call when suggesting borrowing
 816                             // we want to suggest removing it because it'd now be unnecessary.
 817                             sugg_sp = receiver.span;
 818                         }
 819                     }
 820                     if let Ok(src) = sm.span_to_snippet(sugg_sp) {
 821                         let needs_parens = match expr.kind {
 822                             // parenthesize if needed (Issue #46756)
 823                             hir::ExprKind::Cast(_, _) | hir::ExprKind::Binary(_, _, _) => true,
 824                             // parenthesize borrows of range literals (Issue #54505)
 825                             _ if is_range_literal(expr) => true,
 826                             _ => false,
 827                         };
 828
 829                         if let Some(sugg) = self.can_use_as_ref(expr) {
 830                             return Some((
 831                                 sugg.0,
 832                                 sugg.1.to_string(),
 833                                 sugg.2,
 834                                 Applicability::MachineApplicable,
 835                                 false,
 836                                 false,
 837                             ));
 838                         }
 839
 840                         let prefix = match self.maybe_get_struct_pattern_shorthand_field(expr) {
 841                             Some(ident) => format!("{ident}: "),
 842                             None => String::new(),
 843                         };
 844
 845                         if let Some(hir::Node::Expr(hir::Expr {
 846                             kind: hir::ExprKind::Assign(..),
 847                             ..
 848                         })) = self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
 849                         {
 850                             if mutability == hir::Mutability::Mut {
 851                                 // Suppressing this diagnostic, we'll properly print it in `check_expr_assign`
 852                                 return None;
 853                             }
 854                         }
 855
 856                         let sugg_expr = if needs_parens { format!("({src})") } else { src };
 857                         return Some(match mutability {
 858                             hir::Mutability::Mut => (
 859                                 sp,
 860                                 "consider mutably borrowing here".to_string(),
 861                                 format!("{prefix}&mut {sugg_expr}"),
 862                                 Applicability::MachineApplicable,
 863                                 false,
 864                                 false,
 865                             ),
 866                             hir::Mutability::Not => (
 867                                 sp,
 868                                 "consider borrowing here".to_string(),
 869                                 format!("{prefix}&{sugg_expr}"),
 870                                 Applicability::MachineApplicable,
 871                                 false,
 872                                 false,
 873                             ),
 874                         });
 875                     }
 876                 }
 877             }
 878             (
 879                 hir::ExprKind::AddrOf(hir::BorrowKind::Ref, _, ref expr),
 880                 _,
 881                 &ty::Ref(_, checked, _),
 882             ) if self.can_sub(self.param_env, checked, expected).is_ok() => {
 883                 // We have `&T`, check if what was expected was `T`. If so,
 884                 // we may want to suggest removing a `&`.
 885                 if sm.is_imported(expr.span) {
 886                     // Go through the spans from which this span was expanded,
 887                     // and find the one that's pointing inside `sp`.
 888                     //
 889                     // E.g. for `&format!("")`, where we want the span to the
 890                     // `format!()` invocation instead of its expansion.
 891                     if let Some(call_span) =
 892                         iter::successors(Some(expr.span), |s| s.parent_callsite())
 893                             .find(|&s| sp.contains(s))
 894                         && sm.is_span_accessible(call_span)
 895                     {
 896                         return Some((
 897                             sp.with_hi(call_span.lo()),
 898                             "consider removing the borrow".to_string(),
 899                             String::new(),
 900                             Applicability::MachineApplicable,
 901                             true,
 902                             true
 903                         ));
 904                     }
 905                     return None;
 906                 }
 907                 if sp.contains(expr.span)
 908                     && sm.is_span_accessible(expr.span)
 909                 {
 910                     return Some((
 911                         sp.with_hi(expr.span.lo()),
 912                         "consider removing the borrow".to_string(),
 913                         String::new(),
 914                         Applicability::MachineApplicable,
 915                         true,
 916                         true,
 917                     ));
 918                 }
 919             }
 920             (
 921                 _,
 922                 &ty::RawPtr(TypeAndMut { ty: ty_b, mutbl: mutbl_b }),
 923                 &ty::Ref(_, ty_a, mutbl_a),
 924             ) => {
 925                 if let Some(steps) = self.deref_steps(ty_a, ty_b)
 926                     // Only suggest valid if dereferencing needed.
 927                     && steps > 0
 928                     // The pointer type implements `Copy` trait so the suggestion is always valid.
 929                     && let Ok(src) = sm.span_to_snippet(sp)
 930                 {
 931                     let derefs = "*".repeat(steps);
 932                     if let Some((span, src, applicability)) = match mutbl_b {
 933                         hir::Mutability::Mut => {
 934                             let new_prefix = "&mut ".to_owned() + &derefs;
 935                             match mutbl_a {
 936                                 hir::Mutability::Mut => {
 937                                     replace_prefix(&src, "&mut ", &new_prefix).map(|_| {
 938                                         let pos = sp.lo() + BytePos(5);
 939                                         let sp = sp.with_lo(pos).with_hi(pos);
 940                                         (sp, derefs, Applicability::MachineApplicable)
 941                                     })
 942                                 }
 943                                 hir::Mutability::Not => {
 944                                     replace_prefix(&src, "&", &new_prefix).map(|_| {
 945                                         let pos = sp.lo() + BytePos(1);
 946                                         let sp = sp.with_lo(pos).with_hi(pos);
 947                                         (
 948                                             sp,
 949                                             format!("mut {derefs}"),
 950                                             Applicability::Unspecified,
 951                                         )
 952                                     })
 953                                 }
 954                             }
 955                         }
 956                         hir::Mutability::Not => {
 957                             let new_prefix = "&".to_owned() + &derefs;
 958                             match mutbl_a {
 959                                 hir::Mutability::Mut => {
 960                                     replace_prefix(&src, "&mut ", &new_prefix).map(|_| {
 961                                         let lo = sp.lo() + BytePos(1);
 962                                         let hi = sp.lo() + BytePos(5);
 963                                         let sp = sp.with_lo(lo).with_hi(hi);
 964                                         (sp, derefs, Applicability::MachineApplicable)
 965                                     })
 966                                 }
 967                                 hir::Mutability::Not => {
 968                                     replace_prefix(&src, "&", &new_prefix).map(|_| {
 969                                         let pos = sp.lo() + BytePos(1);
 970                                         let sp = sp.with_lo(pos).with_hi(pos);
 971                                         (sp, derefs, Applicability::MachineApplicable)
 972                                     })
 973                                 }
 974                             }
 975                         }
 976                     } {
 977                         return Some((
 978                             span,
 979                             "consider dereferencing".to_string(),
 980                             src,
 981                             applicability,
 982                             true,
 983                             false,
 984                         ));
 985                     }
 986                 }
 987             }
 988             _ if sp == expr.span => {
 989                 if let Some(mut steps) = self.deref_steps(checked_ty, expected) {
 990                     let mut expr = expr.peel_blocks();
 991                     let mut prefix_span = expr.span.shrink_to_lo();
 992                     let mut remove = String::new();
 993
 994                     // Try peeling off any existing `&` and `&mut` to reach our target type
 995                     while steps > 0 {
 996                         if let hir::ExprKind::AddrOf(_, mutbl, inner) = expr.kind {
 997                             // If the expression has `&`, removing it would fix the error
 998                             prefix_span = prefix_span.with_hi(inner.span.lo());
 999                             expr = inner;
1000                             remove += match mutbl {
1001                                 hir::Mutability::Not => "&",
1002                                 hir::Mutability::Mut => "&mut ",
1003                             };
1004                             steps -= 1;
1005                         } else {
1006                             break;
1007                         }
1008                     }
1009                     // If we've reached our target type with just removing `&`, then just print now.
1010                     if steps == 0 {
1011                         return Some((
1012                             prefix_span,
1013                             format!("consider removing the `{}`", remove.trim()),
1014                             String::new(),
1015                             // Do not remove `&&` to get to bool, because it might be something like
1016                             // { a } && b, which we have a separate fixup suggestion that is more
1017                             // likely correct...
1018                             if remove.trim() == "&&" && expected == self.tcx.types.bool {
1019                                 Applicability::MaybeIncorrect
1020                             } else {
1021                                 Applicability::MachineApplicable
1022                             },
1023                             true,
1024                             false,
1025                         ));
1026                     }
1027
1028                     // For this suggestion to make sense, the type would need to be `Copy`,
1029                     // or we have to be moving out of a `Box<T>`
1030                     if self.type_is_copy_modulo_regions(self.param_env, expected, sp)
1031                         // FIXME(compiler-errors): We can actually do this if the checked_ty is
1032                         // `steps` layers of boxes, not just one, but this is easier and most likely.
1033                         || (checked_ty.is_box() && steps == 1)
1034                     {
1035                         let deref_kind = if checked_ty.is_box() {
1036                             "unboxing the value"
1037                         } else if checked_ty.is_region_ptr() {
1038                             "dereferencing the borrow"
1039                         } else {
1040                             "dereferencing the type"
1041                         };
1042
1043                         // Suggest removing `&` if we have removed any, otherwise suggest just
1044                         // dereferencing the remaining number of steps.
1045                         let message = if remove.is_empty() {
1046                             format!("consider {deref_kind}")
1047                         } else {
1048                             format!(
1049                                 "consider removing the `{}` and {} instead",
1050                                 remove.trim(),
1051                                 deref_kind
1052                             )
1053                         };
1054
1055                         let prefix = match self.maybe_get_struct_pattern_shorthand_field(expr) {
1056                             Some(ident) => format!("{ident}: "),
1057                             None => String::new(),
1058                         };
1059
1060                         let (span, suggestion) = if self.is_else_if_block(expr) {
1061                             // Don't suggest nonsense like `else *if`
1062                             return None;
1063                         } else if let Some(expr) = self.maybe_get_block_expr(expr) {
1064                             // prefix should be empty here..
1065                             (expr.span.shrink_to_lo(), "*".to_string())
1066                         } else {
1067                             (prefix_span, format!("{}{}", prefix, "*".repeat(steps)))
1068                         };
1069
1070                         return Some((
1071                             span,
1072                             message,
1073                             suggestion,
1074                             Applicability::MachineApplicable,
1075                             true,
1076                             false,
1077                         ));
1078                     }
1079                 }
1080             }
1081             _ => {}
1082         }
1083         None
1084     }
1085
1086     pub fn check_for_cast(
1087         &self,
1088         err: &mut Diagnostic,
1089         expr: &hir::Expr<'_>,
1090         checked_ty: Ty<'tcx>,
1091         expected_ty: Ty<'tcx>,
1092         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
1093     ) -> bool {
1094         if self.tcx.sess.source_map().is_imported(expr.span) {
1095             // Ignore if span is from within a macro.
1096             return false;
1097         }
1098
1099         let Ok(src) = self.tcx.sess.source_map().span_to_snippet(expr.span) else {
1100             return false;
1101         };
1102
1103         // If casting this expression to a given numeric type would be appropriate in case of a type
1104         // mismatch.
1105         //
1106         // We want to minimize the amount of casting operations that are suggested, as it can be a
1107         // lossy operation with potentially bad side effects, so we only suggest when encountering
1108         // an expression that indicates that the original type couldn't be directly changed.
1109         //
1110         // For now, don't suggest casting with `as`.
1111         let can_cast = false;
1112
1113         let mut sugg = vec![];
1114
1115         if let Some(hir::Node::ExprField(field)) =
1116             self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
1117         {
1118             // `expr` is a literal field for a struct, only suggest if appropriate
1119             if field.is_shorthand {
1120                 // This is a field literal
1121                 sugg.push((field.ident.span.shrink_to_lo(), format!("{}: ", field.ident)));
1122             } else {
1123                 // Likely a field was meant, but this field wasn't found. Do not suggest anything.
1124                 return false;
1125             }
1126         };
1127
1128         if let hir::ExprKind::Call(path, args) = &expr.kind
1129             && let (hir::ExprKind::Path(hir::QPath::TypeRelative(base_ty, path_segment)), 1) =
1130                 (&path.kind, args.len())
1131             // `expr` is a conversion like `u32::from(val)`, do not suggest anything (#63697).
1132             && let (hir::TyKind::Path(hir::QPath::Resolved(None, base_ty_path)), sym::from) =
1133                 (&base_ty.kind, path_segment.ident.name)
1134         {
1135             if let Some(ident) = &base_ty_path.segments.iter().map(|s| s.ident).next() {
1136                 match ident.name {
1137                     sym::i128
1138                     | sym::i64
1139                     | sym::i32
1140                     | sym::i16
1141                     | sym::i8
1142                     | sym::u128
1143                     | sym::u64
1144                     | sym::u32
1145                     | sym::u16
1146                     | sym::u8
1147                     | sym::isize
1148                     | sym::usize
1149                         if base_ty_path.segments.len() == 1 =>
1150                     {
1151                         return false;
1152                     }
1153                     _ => {}
1154                 }
1155             }
1156         }
1157
1158         let msg = format!(
1159             "you can convert {} `{}` to {} `{}`",
1160             checked_ty.kind().article(),
1161             checked_ty,
1162             expected_ty.kind().article(),
1163             expected_ty,
1164         );
1165         let cast_msg = format!(
1166             "you can cast {} `{}` to {} `{}`",
1167             checked_ty.kind().article(),
1168             checked_ty,
1169             expected_ty.kind().article(),
1170             expected_ty,
1171         );
1172         let lit_msg = format!(
1173             "change the type of the numeric literal from `{checked_ty}` to `{expected_ty}`",
1174         );
1175
1176         let close_paren = if expr.precedence().order() < PREC_POSTFIX {
1177             sugg.push((expr.span.shrink_to_lo(), "(".to_string()));
1178             ")"
1179         } else {
1180             ""
1181         };
1182
1183         let mut cast_suggestion = sugg.clone();
1184         cast_suggestion.push((expr.span.shrink_to_hi(), format!("{close_paren} as {expected_ty}")));
1185         let mut into_suggestion = sugg.clone();
1186         into_suggestion.push((expr.span.shrink_to_hi(), format!("{close_paren}.into()")));
1187         let mut suffix_suggestion = sugg.clone();
1188         suffix_suggestion.push((
1189             if matches!(
1190                 (&expected_ty.kind(), &checked_ty.kind()),
1191                 (ty::Int(_) | ty::Uint(_), ty::Float(_))
1192             ) {
1193                 // Remove fractional part from literal, for example `42.0f32` into `42`
1194                 let src = src.trim_end_matches(&checked_ty.to_string());
1195                 let len = src.split('.').next().unwrap().len();
1196                 expr.span.with_lo(expr.span.lo() + BytePos(len as u32))
1197             } else {
1198                 let len = src.trim_end_matches(&checked_ty.to_string()).len();
1199                 expr.span.with_lo(expr.span.lo() + BytePos(len as u32))
1200             },
1201             if expr.precedence().order() < PREC_POSTFIX {
1202                 // Readd `)`
1203                 format!("{expected_ty})")
1204             } else {
1205                 expected_ty.to_string()
1206             },
1207         ));
1208         let literal_is_ty_suffixed = |expr: &hir::Expr<'_>| {
1209             if let hir::ExprKind::Lit(lit) = &expr.kind { lit.node.is_suffixed() } else { false }
1210         };
1211         let is_negative_int =
1212             |expr: &hir::Expr<'_>| matches!(expr.kind, hir::ExprKind::Unary(hir::UnOp::Neg, ..));
1213         let is_uint = |ty: Ty<'_>| matches!(ty.kind(), ty::Uint(..));
1214
1215         let in_const_context = self.tcx.hir().is_inside_const_context(expr.hir_id);
1216
1217         let suggest_fallible_into_or_lhs_from =
1218             |err: &mut Diagnostic, exp_to_found_is_fallible: bool| {
1219                 // If we know the expression the expected type is derived from, we might be able
1220                 // to suggest a widening conversion rather than a narrowing one (which may
1221                 // panic). For example, given x: u8 and y: u32, if we know the span of "x",
1222                 //   x > y
1223                 // can be given the suggestion "u32::from(x) > y" rather than
1224                 // "x > y.try_into().unwrap()".
1225                 let lhs_expr_and_src = expected_ty_expr.and_then(|expr| {
1226                     self.tcx
1227                         .sess
1228                         .source_map()
1229                         .span_to_snippet(expr.span)
1230                         .ok()
1231                         .map(|src| (expr, src))
1232                 });
1233                 let (msg, suggestion) = if let (Some((lhs_expr, lhs_src)), false) =
1234                     (lhs_expr_and_src, exp_to_found_is_fallible)
1235                 {
1236                     let msg = format!(
1237                         "you can convert `{lhs_src}` from `{expected_ty}` to `{checked_ty}`, matching the type of `{src}`",
1238                     );
1239                     let suggestion = vec![
1240                         (lhs_expr.span.shrink_to_lo(), format!("{checked_ty}::from(")),
1241                         (lhs_expr.span.shrink_to_hi(), ")".to_string()),
1242                     ];
1243                     (msg, suggestion)
1244                 } else {
1245                     let msg = format!("{msg} and panic if the converted value doesn't fit");
1246                     let mut suggestion = sugg.clone();
1247                     suggestion.push((
1248                         expr.span.shrink_to_hi(),
1249                         format!("{close_paren}.try_into().unwrap()"),
1250                     ));
1251                     (msg, suggestion)
1252                 };
1253                 err.multipart_suggestion_verbose(
1254                     &msg,
1255                     suggestion,
1256                     Applicability::MachineApplicable,
1257                 );
1258             };
1259
1260         let suggest_to_change_suffix_or_into =
1261             |err: &mut Diagnostic,
1262              found_to_exp_is_fallible: bool,
1263              exp_to_found_is_fallible: bool| {
1264                 let exp_is_lhs =
1265                     expected_ty_expr.map(|e| self.tcx.hir().is_lhs(e.hir_id)).unwrap_or(false);
1266
1267                 if exp_is_lhs {
1268                     return;
1269                 }
1270
1271                 let always_fallible = found_to_exp_is_fallible
1272                     && (exp_to_found_is_fallible || expected_ty_expr.is_none());
1273                 let msg = if literal_is_ty_suffixed(expr) {
1274                     &lit_msg
1275                 } else if always_fallible && (is_negative_int(expr) && is_uint(expected_ty)) {
1276                     // We now know that converting either the lhs or rhs is fallible. Before we
1277                     // suggest a fallible conversion, check if the value can never fit in the
1278                     // expected type.
1279                     let msg = format!("`{src}` cannot fit into type `{expected_ty}`");
1280                     err.note(&msg);
1281                     return;
1282                 } else if in_const_context {
1283                     // Do not recommend `into` or `try_into` in const contexts.
1284                     return;
1285                 } else if found_to_exp_is_fallible {
1286                     return suggest_fallible_into_or_lhs_from(err, exp_to_found_is_fallible);
1287                 } else {
1288                     &msg
1289                 };
1290                 let suggestion = if literal_is_ty_suffixed(expr) {
1291                     suffix_suggestion.clone()
1292                 } else {
1293                     into_suggestion.clone()
1294                 };
1295                 err.multipart_suggestion_verbose(msg, suggestion, Applicability::MachineApplicable);
1296             };
1297
1298         match (&expected_ty.kind(), &checked_ty.kind()) {
1299             (&ty::Int(ref exp), &ty::Int(ref found)) => {
1300                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1301                 {
1302                     (Some(exp), Some(found)) if exp < found => (true, false),
1303                     (Some(exp), Some(found)) if exp > found => (false, true),
1304                     (None, Some(8 | 16)) => (false, true),
1305                     (Some(8 | 16), None) => (true, false),
1306                     (None, _) | (_, None) => (true, true),
1307                     _ => (false, false),
1308                 };
1309                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1310                 true
1311             }
1312             (&ty::Uint(ref exp), &ty::Uint(ref found)) => {
1313                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1314                 {
1315                     (Some(exp), Some(found)) if exp < found => (true, false),
1316                     (Some(exp), Some(found)) if exp > found => (false, true),
1317                     (None, Some(8 | 16)) => (false, true),
1318                     (Some(8 | 16), None) => (true, false),
1319                     (None, _) | (_, None) => (true, true),
1320                     _ => (false, false),
1321                 };
1322                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1323                 true
1324             }
1325             (&ty::Int(exp), &ty::Uint(found)) => {
1326                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1327                 {
1328                     (Some(exp), Some(found)) if found < exp => (false, true),
1329                     (None, Some(8)) => (false, true),
1330                     _ => (true, true),
1331                 };
1332                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1333                 true
1334             }
1335             (&ty::Uint(exp), &ty::Int(found)) => {
1336                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1337                 {
1338                     (Some(exp), Some(found)) if found > exp => (true, false),
1339                     (Some(8), None) => (true, false),
1340                     _ => (true, true),
1341                 };
1342                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1343                 true
1344             }
1345             (&ty::Float(ref exp), &ty::Float(ref found)) => {
1346                 if found.bit_width() < exp.bit_width() {
1347                     suggest_to_change_suffix_or_into(err, false, true);
1348                 } else if literal_is_ty_suffixed(expr) {
1349                     err.multipart_suggestion_verbose(
1350                         &lit_msg,
1351                         suffix_suggestion,
1352                         Applicability::MachineApplicable,
1353                     );
1354                 } else if can_cast {
1355                     // Missing try_into implementation for `f64` to `f32`
1356                     err.multipart_suggestion_verbose(
1357                         &format!("{cast_msg}, producing the closest possible value"),
1358                         cast_suggestion,
1359                         Applicability::MaybeIncorrect, // lossy conversion
1360                     );
1361                 }
1362                 true
1363             }
1364             (&ty::Uint(_) | &ty::Int(_), &ty::Float(_)) => {
1365                 if literal_is_ty_suffixed(expr) {
1366                     err.multipart_suggestion_verbose(
1367                         &lit_msg,
1368                         suffix_suggestion,
1369                         Applicability::MachineApplicable,
1370                     );
1371                 } else if can_cast {
1372                     // Missing try_into implementation for `{float}` to `{integer}`
1373                     err.multipart_suggestion_verbose(
1374                         &format!("{msg}, rounding the float towards zero"),
1375                         cast_suggestion,
1376                         Applicability::MaybeIncorrect, // lossy conversion
1377                     );
1378                 }
1379                 true
1380             }
1381             (&ty::Float(ref exp), &ty::Uint(ref found)) => {
1382                 // if `found` is `None` (meaning found is `usize`), don't suggest `.into()`
1383                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
1384                     err.multipart_suggestion_verbose(
1385                         &format!(
1386                             "{msg}, producing the floating point representation of the integer",
1387                         ),
1388                         into_suggestion,
1389                         Applicability::MachineApplicable,
1390                     );
1391                 } else if literal_is_ty_suffixed(expr) {
1392                     err.multipart_suggestion_verbose(
1393                         &lit_msg,
1394                         suffix_suggestion,
1395                         Applicability::MachineApplicable,
1396                     );
1397                 } else {
1398                     // Missing try_into implementation for `{integer}` to `{float}`
1399                     err.multipart_suggestion_verbose(
1400                         &format!(
1401                             "{cast_msg}, producing the floating point representation of the integer, \
1402                                  rounded if necessary",
1403                         ),
1404                         cast_suggestion,
1405                         Applicability::MaybeIncorrect, // lossy conversion
1406                     );
1407                 }
1408                 true
1409             }
1410             (&ty::Float(ref exp), &ty::Int(ref found)) => {
1411                 // if `found` is `None` (meaning found is `isize`), don't suggest `.into()`
1412                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
1413                     err.multipart_suggestion_verbose(
1414                         &format!(
1415                             "{}, producing the floating point representation of the integer",
1416                             &msg,
1417                         ),
1418                         into_suggestion,
1419                         Applicability::MachineApplicable,
1420                     );
1421                 } else if literal_is_ty_suffixed(expr) {
1422                     err.multipart_suggestion_verbose(
1423                         &lit_msg,
1424                         suffix_suggestion,
1425                         Applicability::MachineApplicable,
1426                     );
1427                 } else {
1428                     // Missing try_into implementation for `{integer}` to `{float}`
1429                     err.multipart_suggestion_verbose(
1430                         &format!(
1431                             "{}, producing the floating point representation of the integer, \
1432                                 rounded if necessary",
1433                             &msg,
1434                         ),
1435                         cast_suggestion,
1436                         Applicability::MaybeIncorrect, // lossy conversion
1437                     );
1438                 }
1439                 true
1440             }
1441             (
1442                 &ty::Uint(ty::UintTy::U32 | ty::UintTy::U64 | ty::UintTy::U128)
1443                 | &ty::Int(ty::IntTy::I32 | ty::IntTy::I64 | ty::IntTy::I128),
1444                 &ty::Char,
1445             ) => {
1446                 err.multipart_suggestion_verbose(
1447                     &format!("{cast_msg}, since a `char` always occupies 4 bytes"),
1448                     cast_suggestion,
1449                     Applicability::MachineApplicable,
1450                 );
1451                 true
1452             }
1453             _ => false,
1454         }
1455     }
1456 }