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