compiler/rustc_typeck/src/check/demand.rs

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