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