compiler/rustc_hir_typeck/src/demand.rs

   1 use crate::FnCtxt;
   2 use rustc_ast::util::parser::PREC_POSTFIX;
   3 use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
   4 use rustc_hir as hir;
   5 use rustc_hir::lang_items::LangItem;
   6 use rustc_hir::{is_range_literal, Node};
   7 use rustc_infer::infer::InferOk;
   8 use rustc_middle::lint::in_external_macro;
   9 use rustc_middle::middle::stability::EvalResult;
  10 use rustc_middle::ty::adjustment::AllowTwoPhase;
  11 use rustc_middle::ty::error::{ExpectedFound, TypeError};
  12 use rustc_middle::ty::print::with_no_trimmed_paths;
  13 use rustc_middle::ty::{self, Article, AssocItem, Ty, TypeAndMut};
  14 use rustc_span::symbol::{sym, Symbol};
  15 use rustc_span::{BytePos, Span};
  16 use rustc_trait_selection::infer::InferCtxtExt as _;
  17 use rustc_trait_selection::traits::ObligationCause;
  18
  19 use super::method::probe;
  20
  21 use std::iter;
  22
  23 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  24     pub fn emit_coerce_suggestions(
  25         &self,
  26         err: &mut Diagnostic,
  27         expr: &hir::Expr<'tcx>,
  28         expr_ty: Ty<'tcx>,
  29         expected: Ty<'tcx>,
  30         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
  31         error: Option<TypeError<'tcx>>,
  32     ) {
  33         if expr_ty == expected {
  34             return;
  35         }
  36
  37         self.annotate_expected_due_to_let_ty(err, expr, error);
  38
  39         // Use `||` to give these suggestions a precedence
  40         let _ = self.suggest_missing_parentheses(err, expr)
  41             || self.suggest_deref_ref_or_into(err, expr, expected, expr_ty, expected_ty_expr)
  42             || self.suggest_compatible_variants(err, expr, expected, expr_ty)
  43             || self.suggest_non_zero_new_unwrap(err, expr, expected, expr_ty)
  44             || self.suggest_calling_boxed_future_when_appropriate(err, expr, expected, expr_ty)
  45             || self.suggest_no_capture_closure(err, expected, expr_ty)
  46             || self.suggest_boxing_when_appropriate(err, expr, expected, expr_ty)
  47             || self.suggest_block_to_brackets_peeling_refs(err, expr, expr_ty, expected)
  48             || self.suggest_copied_or_cloned(err, expr, expr_ty, expected)
  49             || self.suggest_into(err, expr, expr_ty, expected)
  50             || self.suggest_option_to_bool(err, expr, expr_ty, expected)
  51             || self.suggest_floating_point_literal(err, expr, expected);
  52
  53         self.note_type_is_not_clone(err, expected, expr_ty, expr);
  54         self.note_need_for_fn_pointer(err, expected, expr_ty);
  55         self.note_internal_mutation_in_method(err, expr, expected, expr_ty);
  56     }
  57
  58     // Requires that the two types unify, and prints an error message if
  59     // they don't.
  60     pub fn demand_suptype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  61         if let Some(mut e) = self.demand_suptype_diag(sp, expected, actual) {
  62             e.emit();
  63         }
  64     }
  65
  66     pub fn demand_suptype_diag(
  67         &self,
  68         sp: Span,
  69         expected: Ty<'tcx>,
  70         actual: Ty<'tcx>,
  71     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
  72         self.demand_suptype_with_origin(&self.misc(sp), expected, actual)
  73     }
  74
  75     #[instrument(skip(self), level = "debug")]
  76     pub fn demand_suptype_with_origin(
  77         &self,
  78         cause: &ObligationCause<'tcx>,
  79         expected: Ty<'tcx>,
  80         actual: Ty<'tcx>,
  81     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
  82         match self.at(cause, self.param_env).sup(expected, actual) {
  83             Ok(InferOk { obligations, value: () }) => {
  84                 self.register_predicates(obligations);
  85                 None
  86             }
  87             Err(e) => Some(self.err_ctxt().report_mismatched_types(&cause, expected, actual, e)),
  88         }
  89     }
  90
  91     pub fn demand_eqtype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  92         if let Some(mut err) = self.demand_eqtype_diag(sp, expected, actual) {
  93             err.emit();
  94         }
  95     }
  96
  97     pub fn demand_eqtype_diag(
  98         &self,
  99         sp: Span,
 100         expected: Ty<'tcx>,
 101         actual: Ty<'tcx>,
 102     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
 103         self.demand_eqtype_with_origin(&self.misc(sp), expected, actual)
 104     }
 105
 106     pub fn demand_eqtype_with_origin(
 107         &self,
 108         cause: &ObligationCause<'tcx>,
 109         expected: Ty<'tcx>,
 110         actual: Ty<'tcx>,
 111     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
 112         match self.at(cause, self.param_env).eq(expected, actual) {
 113             Ok(InferOk { obligations, value: () }) => {
 114                 self.register_predicates(obligations);
 115                 None
 116             }
 117             Err(e) => Some(self.err_ctxt().report_mismatched_types(cause, expected, actual, e)),
 118         }
 119     }
 120
 121     pub fn demand_coerce(
 122         &self,
 123         expr: &hir::Expr<'tcx>,
 124         checked_ty: Ty<'tcx>,
 125         expected: Ty<'tcx>,
 126         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 127         allow_two_phase: AllowTwoPhase,
 128     ) -> Ty<'tcx> {
 129         let (ty, err) =
 130             self.demand_coerce_diag(expr, checked_ty, expected, expected_ty_expr, allow_two_phase);
 131         if let Some(mut err) = err {
 132             err.emit();
 133         }
 134         ty
 135     }
 136
 137     /// Checks that the type of `expr` can be coerced to `expected`.
 138     ///
 139     /// N.B., this code relies on `self.diverges` to be accurate. In particular, assignments to `!`
 140     /// will be permitted if the diverges flag is currently "always".
 141     #[instrument(level = "debug", skip(self, expr, expected_ty_expr, allow_two_phase))]
 142     pub fn demand_coerce_diag(
 143         &self,
 144         expr: &hir::Expr<'tcx>,
 145         checked_ty: Ty<'tcx>,
 146         expected: Ty<'tcx>,
 147         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 148         allow_two_phase: AllowTwoPhase,
 149     ) -> (Ty<'tcx>, Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>>) {
 150         let expected = self.resolve_vars_with_obligations(expected);
 151
 152         let e = match self.try_coerce(expr, checked_ty, expected, allow_two_phase, None) {
 153             Ok(ty) => return (ty, None),
 154             Err(e) => e,
 155         };
 156
 157         self.set_tainted_by_errors();
 158         let expr = expr.peel_drop_temps();
 159         let cause = self.misc(expr.span);
 160         let expr_ty = self.resolve_vars_with_obligations(checked_ty);
 161         let mut err = self.err_ctxt().report_mismatched_types(&cause, expected, expr_ty, e.clone());
 162
 163         let is_insufficiently_polymorphic =
 164             matches!(e, TypeError::RegionsInsufficientlyPolymorphic(..));
 165
 166         // FIXME(#73154): For now, we do leak check when coercing function
 167         // pointers in typeck, instead of only during borrowck. This can lead
 168         // to these `RegionsInsufficientlyPolymorphic` errors that aren't helpful.
 169         if !is_insufficiently_polymorphic {
 170             self.emit_coerce_suggestions(
 171                 &mut err,
 172                 expr,
 173                 expr_ty,
 174                 expected,
 175                 expected_ty_expr,
 176                 Some(e),
 177             );
 178         }
 179
 180         (expected, Some(err))
 181     }
 182
 183     fn annotate_expected_due_to_let_ty(
 184         &self,
 185         err: &mut Diagnostic,
 186         expr: &hir::Expr<'_>,
 187         error: Option<TypeError<'_>>,
 188     ) {
 189         let parent = self.tcx.hir().get_parent_node(expr.hir_id);
 190         match (self.tcx.hir().find(parent), error) {
 191             (Some(hir::Node::Local(hir::Local { ty: Some(ty), init: Some(init), .. })), _)
 192                 if init.hir_id == expr.hir_id =>
 193             {
 194                 // Point at `let` assignment type.
 195                 err.span_label(ty.span, "expected due to this");
 196             }
 197             (
 198                 Some(hir::Node::Expr(hir::Expr {
 199                     kind: hir::ExprKind::Assign(lhs, rhs, _), ..
 200                 })),
 201                 Some(TypeError::Sorts(ExpectedFound { expected, .. })),
 202             ) if rhs.hir_id == expr.hir_id && !expected.is_closure() => {
 203                 // We ignore closures explicitly because we already point at them elsewhere.
 204                 // Point at the assigned-to binding.
 205                 let mut primary_span = lhs.span;
 206                 let mut secondary_span = lhs.span;
 207                 let mut post_message = "";
 208                 match lhs.kind {
 209                     hir::ExprKind::Path(hir::QPath::Resolved(
 210                         None,
 211                         hir::Path {
 212                             res:
 213                                 hir::def::Res::Def(
 214                                     hir::def::DefKind::Static(_) | hir::def::DefKind::Const,
 215                                     def_id,
 216                                 ),
 217                             ..
 218                         },
 219                     )) => {
 220                         if let Some(hir::Node::Item(hir::Item {
 221                             ident,
 222                             kind: hir::ItemKind::Static(ty, ..) | hir::ItemKind::Const(ty, ..),
 223                             ..
 224                         })) = self.tcx.hir().get_if_local(*def_id)
 225                         {
 226                             primary_span = ty.span;
 227                             secondary_span = ident.span;
 228                             post_message = " type";
 229                         }
 230                     }
 231                     hir::ExprKind::Path(hir::QPath::Resolved(
 232                         None,
 233                         hir::Path { res: hir::def::Res::Local(hir_id), .. },
 234                     )) => {
 235                         if let Some(hir::Node::Pat(pat)) = self.tcx.hir().find(*hir_id) {
 236                             let parent = self.tcx.hir().get_parent_node(pat.hir_id);
 237                             primary_span = pat.span;
 238                             secondary_span = pat.span;
 239                             match self.tcx.hir().find(parent) {
 240                                 Some(hir::Node::Local(hir::Local { ty: Some(ty), .. })) => {
 241                                     primary_span = ty.span;
 242                                     post_message = " type";
 243                                 }
 244                                 Some(hir::Node::Local(hir::Local { init: Some(init), .. })) => {
 245                                     primary_span = init.span;
 246                                     post_message = " value";
 247                                 }
 248                                 Some(hir::Node::Param(hir::Param { ty_span, .. })) => {
 249                                     primary_span = *ty_span;
 250                                     post_message = " parameter type";
 251                                 }
 252                                 _ => {}
 253                             }
 254                         }
 255                     }
 256                     _ => {}
 257                 }
 258
 259                 if primary_span != secondary_span
 260                     && self
 261                         .tcx
 262                         .sess
 263                         .source_map()
 264                         .is_multiline(secondary_span.shrink_to_hi().until(primary_span))
 265                 {
 266                     // We are pointing at the binding's type or initializer value, but it's pattern
 267                     // is in a different line, so we point at both.
 268                     err.span_label(secondary_span, "expected due to the type of this binding");
 269                     err.span_label(primary_span, &format!("expected due to this{post_message}"));
 270                 } else if post_message == "" {
 271                     // We are pointing at either the assignment lhs or the binding def pattern.
 272                     err.span_label(primary_span, "expected due to the type of this binding");
 273                 } else {
 274                     // We are pointing at the binding's type or initializer value.
 275                     err.span_label(primary_span, &format!("expected due to this{post_message}"));
 276                 }
 277
 278                 if !lhs.is_syntactic_place_expr() {
 279                     // We already emitted E0070 "invalid left-hand side of assignment", so we
 280                     // silence this.
 281                     err.downgrade_to_delayed_bug();
 282                 }
 283             }
 284             _ => {}
 285         }
 286     }
 287
 288     /// If the expected type is an enum (Issue #55250) with any variants whose
 289     /// sole field is of the found type, suggest such variants. (Issue #42764)
 290     fn suggest_compatible_variants(
 291         &self,
 292         err: &mut Diagnostic,
 293         expr: &hir::Expr<'_>,
 294         expected: Ty<'tcx>,
 295         expr_ty: Ty<'tcx>,
 296     ) -> bool {
 297         if let ty::Adt(expected_adt, substs) = expected.kind() {
 298             if let hir::ExprKind::Field(base, ident) = expr.kind {
 299                 let base_ty = self.typeck_results.borrow().expr_ty(base);
 300                 if self.can_eq(self.param_env, base_ty, expected).is_ok()
 301                     && let Some(base_span) = base.span.find_ancestor_inside(expr.span)
 302                 {
 303                     err.span_suggestion_verbose(
 304                         expr.span.with_lo(base_span.hi()),
 305                         format!("consider removing the tuple struct field `{ident}`"),
 306                         "",
 307                         Applicability::MaybeIncorrect,
 308                     );
 309                     return true;
 310                 }
 311             }
 312
 313             // If the expression is of type () and it's the return expression of a block,
 314             // we suggest adding a separate return expression instead.
 315             // (To avoid things like suggesting `Ok(while .. { .. })`.)
 316             if expr_ty.is_unit() {
 317                 let mut id = expr.hir_id;
 318                 let mut parent;
 319
 320                 // Unroll desugaring, to make sure this works for `for` loops etc.
 321                 loop {
 322                     parent = self.tcx.hir().get_parent_node(id);
 323                     if let Some(parent_span) = self.tcx.hir().opt_span(parent) {
 324                         if parent_span.find_ancestor_inside(expr.span).is_some() {
 325                             // The parent node is part of the same span, so is the result of the
 326                             // same expansion/desugaring and not the 'real' parent node.
 327                             id = parent;
 328                             continue;
 329                         }
 330                     }
 331                     break;
 332                 }
 333
 334                 if let Some(hir::Node::Block(&hir::Block {
 335                     span: block_span, expr: Some(e), ..
 336                 })) = self.tcx.hir().find(parent)
 337                 {
 338                     if e.hir_id == id {
 339                         if let Some(span) = expr.span.find_ancestor_inside(block_span) {
 340                             let return_suggestions = if self
 341                                 .tcx
 342                                 .is_diagnostic_item(sym::Result, expected_adt.did())
 343                             {
 344                                 vec!["Ok(())"]
 345                             } else if self.tcx.is_diagnostic_item(sym::Option, expected_adt.did()) {
 346                                 vec!["None", "Some(())"]
 347                             } else {
 348                                 return false;
 349                             };
 350                             if let Some(indent) =
 351                                 self.tcx.sess.source_map().indentation_before(span.shrink_to_lo())
 352                             {
 353                                 // Add a semicolon, except after `}`.
 354                                 let semicolon =
 355                                     match self.tcx.sess.source_map().span_to_snippet(span) {
 356                                         Ok(s) if s.ends_with('}') => "",
 357                                         _ => ";",
 358                                     };
 359                                 err.span_suggestions(
 360                                     span.shrink_to_hi(),
 361                                     "try adding an expression at the end of the block",
 362                                     return_suggestions
 363                                         .into_iter()
 364                                         .map(|r| format!("{semicolon}\n{indent}{r}")),
 365                                     Applicability::MaybeIncorrect,
 366                                 );
 367                             }
 368                             return true;
 369                         }
 370                     }
 371                 }
 372             }
 373
 374             let compatible_variants: Vec<(String, _, _, Option<String>)> = expected_adt
 375                 .variants()
 376                 .iter()
 377                 .filter(|variant| {
 378                     variant.fields.len() == 1
 379                 })
 380                 .filter_map(|variant| {
 381                     let sole_field = &variant.fields[0];
 382
 383                     let field_is_local = sole_field.did.is_local();
 384                     let field_is_accessible =
 385                         sole_field.vis.is_accessible_from(expr.hir_id.owner.def_id, self.tcx)
 386                         // Skip suggestions for unstable public fields (for example `Pin::pointer`)
 387                         && matches!(self.tcx.eval_stability(sole_field.did, None, expr.span, None), EvalResult::Allow | EvalResult::Unmarked);
 388
 389                     if !field_is_local && !field_is_accessible {
 390                         return None;
 391                     }
 392
 393                     let note_about_variant_field_privacy = (field_is_local && !field_is_accessible)
 394                         .then(|| format!(" (its field is private, but it's local to this crate and its privacy can be changed)"));
 395
 396                     let sole_field_ty = sole_field.ty(self.tcx, substs);
 397                     if self.can_coerce(expr_ty, sole_field_ty) {
 398                         let variant_path =
 399                             with_no_trimmed_paths!(self.tcx.def_path_str(variant.def_id));
 400                         // FIXME #56861: DRYer prelude filtering
 401                         if let Some(path) = variant_path.strip_prefix("std::prelude::")
 402                             && let Some((_, path)) = path.split_once("::")
 403                         {
 404                             return Some((path.to_string(), variant.ctor_kind, sole_field.name, note_about_variant_field_privacy));
 405                         }
 406                         Some((variant_path, variant.ctor_kind, sole_field.name, note_about_variant_field_privacy))
 407                     } else {
 408                         None
 409                     }
 410                 })
 411                 .collect();
 412
 413             let suggestions_for = |variant: &_, ctor, field_name| {
 414                 let prefix = match self.maybe_get_struct_pattern_shorthand_field(expr) {
 415                     Some(ident) => format!("{ident}: "),
 416                     None => String::new(),
 417                 };
 418
 419                 let (open, close) = match ctor {
 420                     hir::def::CtorKind::Fn => ("(".to_owned(), ")"),
 421                     hir::def::CtorKind::Fictive => (format!(" {{ {field_name}: "), " }"),
 422
 423                     // unit variants don't have fields
 424                     hir::def::CtorKind::Const => unreachable!(),
 425                 };
 426
 427                 // Suggest constructor as deep into the block tree as possible.
 428                 // This fixes https://github.com/rust-lang/rust/issues/101065,
 429                 // and also just helps make the most minimal suggestions.
 430                 let mut expr = expr;
 431                 while let hir::ExprKind::Block(block, _) = &expr.kind
 432                     && let Some(expr_) = &block.expr
 433                 {
 434                     expr = expr_
 435                 }
 436
 437                 vec![
 438                     (expr.span.shrink_to_lo(), format!("{prefix}{variant}{open}")),
 439                     (expr.span.shrink_to_hi(), close.to_owned()),
 440                 ]
 441             };
 442
 443             match &compatible_variants[..] {
 444                 [] => { /* No variants to format */ }
 445                 [(variant, ctor_kind, field_name, note)] => {
 446                     // Just a single matching variant.
 447                     err.multipart_suggestion_verbose(
 448                         &format!(
 449                             "try wrapping the expression in `{variant}`{note}",
 450                             note = note.as_deref().unwrap_or("")
 451                         ),
 452                         suggestions_for(&**variant, *ctor_kind, *field_name),
 453                         Applicability::MaybeIncorrect,
 454                     );
 455                     return true;
 456                 }
 457                 _ => {
 458                     // More than one matching variant.
 459                     err.multipart_suggestions(
 460                         &format!(
 461                             "try wrapping the expression in a variant of `{}`",
 462                             self.tcx.def_path_str(expected_adt.did())
 463                         ),
 464                         compatible_variants.into_iter().map(
 465                             |(variant, ctor_kind, field_name, _)| {
 466                                 suggestions_for(&variant, ctor_kind, field_name)
 467                             },
 468                         ),
 469                         Applicability::MaybeIncorrect,
 470                     );
 471                     return true;
 472                 }
 473             }
 474         }
 475
 476         false
 477     }
 478
 479     fn suggest_non_zero_new_unwrap(
 480         &self,
 481         err: &mut Diagnostic,
 482         expr: &hir::Expr<'_>,
 483         expected: Ty<'tcx>,
 484         expr_ty: Ty<'tcx>,
 485     ) -> bool {
 486         let tcx = self.tcx;
 487         let (adt, unwrap) = match expected.kind() {
 488             // In case Option<NonZero*> is wanted, but * is provided, suggest calling new
 489             ty::Adt(adt, substs) if tcx.is_diagnostic_item(sym::Option, adt.did()) => {
 490                 // Unwrap option
 491                 let ty::Adt(adt, _) = substs.type_at(0).kind() else { return false; };
 492
 493                 (adt, "")
 494             }
 495             // In case NonZero* is wanted, but * is provided also add `.unwrap()` to satisfy types
 496             ty::Adt(adt, _) => (adt, ".unwrap()"),
 497             _ => return false,
 498         };
 499
 500         let map = [
 501             (sym::NonZeroU8, tcx.types.u8),
 502             (sym::NonZeroU16, tcx.types.u16),
 503             (sym::NonZeroU32, tcx.types.u32),
 504             (sym::NonZeroU64, tcx.types.u64),
 505             (sym::NonZeroU128, tcx.types.u128),
 506             (sym::NonZeroI8, tcx.types.i8),
 507             (sym::NonZeroI16, tcx.types.i16),
 508             (sym::NonZeroI32, tcx.types.i32),
 509             (sym::NonZeroI64, tcx.types.i64),
 510             (sym::NonZeroI128, tcx.types.i128),
 511         ];
 512
 513         let Some((s, _)) = map
 514             .iter()
 515             .find(|&&(s, t)| self.tcx.is_diagnostic_item(s, adt.did()) && self.can_coerce(expr_ty, t))
 516             else { return false; };
 517
 518         let path = self.tcx.def_path_str(adt.non_enum_variant().def_id);
 519
 520         err.multipart_suggestion(
 521             format!("consider calling `{s}::new`"),
 522             vec![
 523                 (expr.span.shrink_to_lo(), format!("{path}::new(")),
 524                 (expr.span.shrink_to_hi(), format!("){unwrap}")),
 525             ],
 526             Applicability::MaybeIncorrect,
 527         );
 528
 529         true
 530     }
 531
 532     pub fn get_conversion_methods(
 533         &self,
 534         span: Span,
 535         expected: Ty<'tcx>,
 536         checked_ty: Ty<'tcx>,
 537         hir_id: hir::HirId,
 538     ) -> Vec<AssocItem> {
 539         let methods = self.probe_for_return_type(
 540             span,
 541             probe::Mode::MethodCall,
 542             expected,
 543             checked_ty,
 544             hir_id,
 545             |m| {
 546                 self.has_only_self_parameter(m)
 547                     && self
 548                         .tcx
 549                         // This special internal attribute is used to permit
 550                         // "identity-like" conversion methods to be suggested here.
 551                         //
 552                         // FIXME (#46459 and #46460): ideally
 553                         // `std::convert::Into::into` and `std::borrow:ToOwned` would
 554                         // also be `#[rustc_conversion_suggestion]`, if not for
 555                         // method-probing false-positives and -negatives (respectively).
 556                         //
 557                         // FIXME? Other potential candidate methods: `as_ref` and
 558                         // `as_mut`?
 559                         .has_attr(m.def_id, sym::rustc_conversion_suggestion)
 560             },
 561         );
 562
 563         methods
 564     }
 565
 566     /// This function checks whether the method is not static and does not accept other parameters than `self`.
 567     fn has_only_self_parameter(&self, method: &AssocItem) -> bool {
 568         match method.kind {
 569             ty::AssocKind::Fn => {
 570                 method.fn_has_self_parameter
 571                     && self.tcx.fn_sig(method.def_id).inputs().skip_binder().len() == 1
 572             }
 573             _ => false,
 574         }
 575     }
 576
 577     /// Identify some cases where `as_ref()` would be appropriate and suggest it.
 578     ///
 579     /// Given the following code:
 580     /// ```compile_fail,E0308
 581     /// struct Foo;
 582     /// fn takes_ref(_: &Foo) {}
 583     /// let ref opt = Some(Foo);
 584     ///
 585     /// opt.map(|param| takes_ref(param));
 586     /// ```
 587     /// Suggest using `opt.as_ref().map(|param| takes_ref(param));` instead.
 588     ///
 589     /// It only checks for `Option` and `Result` and won't work with
 590     /// ```ignore (illustrative)
 591     /// opt.map(|param| { takes_ref(param) });
 592     /// ```
 593     fn can_use_as_ref(&self, expr: &hir::Expr<'_>) -> Option<(Span, &'static str, String)> {
 594         let hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) = expr.kind else {
 595             return None;
 596         };
 597
 598         let hir::def::Res::Local(local_id) = path.res else {
 599             return None;
 600         };
 601
 602         let local_parent = self.tcx.hir().get_parent_node(local_id);
 603         let Some(Node::Param(hir::Param { hir_id: param_hir_id, .. })) = self.tcx.hir().find(local_parent) else {
 604             return None;
 605         };
 606
 607         let param_parent = self.tcx.hir().get_parent_node(*param_hir_id);
 608         let Some(Node::Expr(hir::Expr {
 609             hir_id: expr_hir_id,
 610             kind: hir::ExprKind::Closure(hir::Closure { fn_decl: closure_fn_decl, .. }),
 611             ..
 612         })) = self.tcx.hir().find(param_parent) else {
 613             return None;
 614         };
 615
 616         let expr_parent = self.tcx.hir().get_parent_node(*expr_hir_id);
 617         let hir = self.tcx.hir().find(expr_parent);
 618         let closure_params_len = closure_fn_decl.inputs.len();
 619         let (
 620             Some(Node::Expr(hir::Expr {
 621                 kind: hir::ExprKind::MethodCall(method_path, receiver, ..),
 622                 ..
 623             })),
 624             1,
 625         ) = (hir, closure_params_len) else {
 626             return None;
 627         };
 628
 629         let self_ty = self.typeck_results.borrow().expr_ty(receiver);
 630         let name = method_path.ident.name;
 631         let is_as_ref_able = match self_ty.peel_refs().kind() {
 632             ty::Adt(def, _) => {
 633                 (self.tcx.is_diagnostic_item(sym::Option, def.did())
 634                     || self.tcx.is_diagnostic_item(sym::Result, def.did()))
 635                     && (name == sym::map || name == sym::and_then)
 636             }
 637             _ => false,
 638         };
 639         match (is_as_ref_able, self.sess().source_map().span_to_snippet(method_path.ident.span)) {
 640             (true, Ok(src)) => {
 641                 let suggestion = format!("as_ref().{}", src);
 642                 Some((method_path.ident.span, "consider using `as_ref` instead", suggestion))
 643             }
 644             _ => None,
 645         }
 646     }
 647
 648     pub(crate) fn maybe_get_struct_pattern_shorthand_field(
 649         &self,
 650         expr: &hir::Expr<'_>,
 651     ) -> Option<Symbol> {
 652         let hir = self.tcx.hir();
 653         let local = match expr {
 654             hir::Expr {
 655                 kind:
 656                     hir::ExprKind::Path(hir::QPath::Resolved(
 657                         None,
 658                         hir::Path {
 659                             res: hir::def::Res::Local(_),
 660                             segments: [hir::PathSegment { ident, .. }],
 661                             ..
 662                         },
 663                     )),
 664                 ..
 665             } => Some(ident),
 666             _ => None,
 667         }?;
 668
 669         match hir.find(hir.get_parent_node(expr.hir_id))? {
 670             Node::ExprField(field) => {
 671                 if field.ident.name == local.name && field.is_shorthand {
 672                     return Some(local.name);
 673                 }
 674             }
 675             _ => {}
 676         }
 677
 678         None
 679     }
 680
 681     /// If the given `HirId` corresponds to a block with a trailing expression, return that expression
 682     pub(crate) fn maybe_get_block_expr(
 683         &self,
 684         expr: &hir::Expr<'tcx>,
 685     ) -> Option<&'tcx hir::Expr<'tcx>> {
 686         match expr {
 687             hir::Expr { kind: hir::ExprKind::Block(block, ..), .. } => block.expr,
 688             _ => None,
 689         }
 690     }
 691
 692     /// Returns whether the given expression is an `else if`.
 693     pub(crate) fn is_else_if_block(&self, expr: &hir::Expr<'_>) -> bool {
 694         if let hir::ExprKind::If(..) = expr.kind {
 695             let parent_id = self.tcx.hir().get_parent_node(expr.hir_id);
 696             if let Some(Node::Expr(hir::Expr {
 697                 kind: hir::ExprKind::If(_, _, Some(else_expr)),
 698                 ..
 699             })) = self.tcx.hir().find(parent_id)
 700             {
 701                 return else_expr.hir_id == expr.hir_id;
 702             }
 703         }
 704         false
 705     }
 706
 707     /// This function is used to determine potential "simple" improvements or users' errors and
 708     /// provide them useful help. For example:
 709     ///
 710     /// ```compile_fail,E0308
 711     /// fn some_fn(s: &str) {}
 712     ///
 713     /// let x = "hey!".to_owned();
 714     /// some_fn(x); // error
 715     /// ```
 716     ///
 717     /// No need to find every potential function which could make a coercion to transform a
 718     /// `String` into a `&str` since a `&` would do the trick!
 719     ///
 720     /// In addition of this check, it also checks between references mutability state. If the
 721     /// expected is mutable but the provided isn't, maybe we could just say "Hey, try with
 722     /// `&mut`!".
 723     pub fn check_ref(
 724         &self,
 725         expr: &hir::Expr<'tcx>,
 726         checked_ty: Ty<'tcx>,
 727         expected: Ty<'tcx>,
 728     ) -> Option<(
 729         Span,
 730         String,
 731         String,
 732         Applicability,
 733         bool, /* verbose */
 734         bool, /* suggest `&` or `&mut` type annotation */
 735     )> {
 736         let sess = self.sess();
 737         let sp = expr.span;
 738
 739         // If the span is from an external macro, there's no suggestion we can make.
 740         if in_external_macro(sess, sp) {
 741             return None;
 742         }
 743
 744         let sm = sess.source_map();
 745
 746         let replace_prefix = |s: &str, old: &str, new: &str| {
 747             s.strip_prefix(old).map(|stripped| new.to_string() + stripped)
 748         };
 749
 750         // `ExprKind::DropTemps` is semantically irrelevant for these suggestions.
 751         let expr = expr.peel_drop_temps();
 752
 753         match (&expr.kind, expected.kind(), checked_ty.kind()) {
 754             (_, &ty::Ref(_, exp, _), &ty::Ref(_, check, _)) => match (exp.kind(), check.kind()) {
 755                 (&ty::Str, &ty::Array(arr, _) | &ty::Slice(arr)) if arr == self.tcx.types.u8 => {
 756                     if let hir::ExprKind::Lit(_) = expr.kind
 757                         && let Ok(src) = sm.span_to_snippet(sp)
 758                         && replace_prefix(&src, "b\"", "\"").is_some()
 759                     {
 760                                 let pos = sp.lo() + BytePos(1);
 761                                 return Some((
 762                                     sp.with_hi(pos),
 763                                     "consider removing the leading `b`".to_string(),
 764                                     String::new(),
 765                                     Applicability::MachineApplicable,
 766                                     true,
 767                                     false,
 768                                 ));
 769                             }
 770                         }
 771                 (&ty::Array(arr, _) | &ty::Slice(arr), &ty::Str) if arr == self.tcx.types.u8 => {
 772                     if let hir::ExprKind::Lit(_) = expr.kind
 773                         && let Ok(src) = sm.span_to_snippet(sp)
 774                         && replace_prefix(&src, "\"", "b\"").is_some()
 775                     {
 776                                 return Some((
 777                                     sp.shrink_to_lo(),
 778                                     "consider adding a leading `b`".to_string(),
 779                                     "b".to_string(),
 780                                     Applicability::MachineApplicable,
 781                                     true,
 782                                     false,
 783                                 ));
 784                     }
 785                 }
 786                 _ => {}
 787             },
 788             (_, &ty::Ref(_, _, mutability), _) => {
 789                 // Check if it can work when put into a ref. For example:
 790                 //
 791                 // ```
 792                 // fn bar(x: &mut i32) {}
 793                 //
 794                 // let x = 0u32;
 795                 // bar(&x); // error, expected &mut
 796                 // ```
 797                 let ref_ty = match mutability {
 798                     hir::Mutability::Mut => {
 799                         self.tcx.mk_mut_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 800                     }
 801                     hir::Mutability::Not => {
 802                         self.tcx.mk_imm_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 803                     }
 804                 };
 805                 if self.can_coerce(ref_ty, expected) {
 806                     let mut sugg_sp = sp;
 807                     if let hir::ExprKind::MethodCall(ref segment, receiver, args, _) = expr.kind {
 808                         let clone_trait =
 809                             self.tcx.require_lang_item(LangItem::Clone, Some(segment.ident.span));
 810                         if args.is_empty()
 811                             && self.typeck_results.borrow().type_dependent_def_id(expr.hir_id).map(
 812                                 |did| {
 813                                     let ai = self.tcx.associated_item(did);
 814                                     ai.trait_container(self.tcx) == Some(clone_trait)
 815                                 },
 816                             ) == Some(true)
 817                             && segment.ident.name == sym::clone
 818                         {
 819                             // If this expression had a clone call when suggesting borrowing
 820                             // we want to suggest removing it because it'd now be unnecessary.
 821                             sugg_sp = receiver.span;
 822                         }
 823                     }
 824                     if let Ok(src) = sm.span_to_snippet(sugg_sp) {
 825                         let needs_parens = match expr.kind {
 826                             // parenthesize if needed (Issue #46756)
 827                             hir::ExprKind::Cast(_, _) | hir::ExprKind::Binary(_, _, _) => true,
 828                             // parenthesize borrows of range literals (Issue #54505)
 829                             _ if is_range_literal(expr) => true,
 830                             _ => false,
 831                         };
 832
 833                         if let Some(sugg) = self.can_use_as_ref(expr) {
 834                             return Some((
 835                                 sugg.0,
 836                                 sugg.1.to_string(),
 837                                 sugg.2,
 838                                 Applicability::MachineApplicable,
 839                                 false,
 840                                 false,
 841                             ));
 842                         }
 843
 844                         let prefix = match self.maybe_get_struct_pattern_shorthand_field(expr) {
 845                             Some(ident) => format!("{ident}: "),
 846                             None => String::new(),
 847                         };
 848
 849                         if let Some(hir::Node::Expr(hir::Expr {
 850                             kind: hir::ExprKind::Assign(..),
 851                             ..
 852                         })) = self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
 853                         {
 854                             if mutability == hir::Mutability::Mut {
 855                                 // Suppressing this diagnostic, we'll properly print it in `check_expr_assign`
 856                                 return None;
 857                             }
 858                         }
 859
 860                         let sugg_expr = if needs_parens { format!("({src})") } else { src };
 861                         return Some(match mutability {
 862                             hir::Mutability::Mut => (
 863                                 sp,
 864                                 "consider mutably borrowing here".to_string(),
 865                                 format!("{prefix}&mut {sugg_expr}"),
 866                                 Applicability::MachineApplicable,
 867                                 false,
 868                                 false,
 869                             ),
 870                             hir::Mutability::Not => (
 871                                 sp,
 872                                 "consider borrowing here".to_string(),
 873                                 format!("{prefix}&{sugg_expr}"),
 874                                 Applicability::MachineApplicable,
 875                                 false,
 876                                 false,
 877                             ),
 878                         });
 879                     }
 880                 }
 881             }
 882             (
 883                 hir::ExprKind::AddrOf(hir::BorrowKind::Ref, _, ref expr),
 884                 _,
 885                 &ty::Ref(_, checked, _),
 886             ) if self.can_sub(self.param_env, checked, expected).is_ok() => {
 887                 // We have `&T`, check if what was expected was `T`. If so,
 888                 // we may want to suggest removing a `&`.
 889                 if sm.is_imported(expr.span) {
 890                     // Go through the spans from which this span was expanded,
 891                     // and find the one that's pointing inside `sp`.
 892                     //
 893                     // E.g. for `&format!("")`, where we want the span to the
 894                     // `format!()` invocation instead of its expansion.
 895                     if let Some(call_span) =
 896                         iter::successors(Some(expr.span), |s| s.parent_callsite())
 897                             .find(|&s| sp.contains(s))
 898                         && sm.is_span_accessible(call_span)
 899                     {
 900                         return Some((
 901                             sp.with_hi(call_span.lo()),
 902                             "consider removing the borrow".to_string(),
 903                             String::new(),
 904                             Applicability::MachineApplicable,
 905                             true,
 906                             true
 907                         ));
 908                     }
 909                     return None;
 910                 }
 911                 if sp.contains(expr.span)
 912                     && sm.is_span_accessible(expr.span)
 913                 {
 914                     return Some((
 915                         sp.with_hi(expr.span.lo()),
 916                         "consider removing the borrow".to_string(),
 917                         String::new(),
 918                         Applicability::MachineApplicable,
 919                         true,
 920                         true,
 921                     ));
 922                 }
 923             }
 924             (
 925                 _,
 926                 &ty::RawPtr(TypeAndMut { ty: ty_b, mutbl: mutbl_b }),
 927                 &ty::Ref(_, ty_a, mutbl_a),
 928             ) => {
 929                 if let Some(steps) = self.deref_steps(ty_a, ty_b)
 930                     // Only suggest valid if dereferencing needed.
 931                     && steps > 0
 932                     // The pointer type implements `Copy` trait so the suggestion is always valid.
 933                     && let Ok(src) = sm.span_to_snippet(sp)
 934                 {
 935                     let derefs = "*".repeat(steps);
 936                     if let Some((span, src, applicability)) = match mutbl_b {
 937                         hir::Mutability::Mut => {
 938                             let new_prefix = "&mut ".to_owned() + &derefs;
 939                             match mutbl_a {
 940                                 hir::Mutability::Mut => {
 941                                     replace_prefix(&src, "&mut ", &new_prefix).map(|_| {
 942                                         let pos = sp.lo() + BytePos(5);
 943                                         let sp = sp.with_lo(pos).with_hi(pos);
 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                                         (
 952                                             sp,
 953                                             format!("mut {derefs}"),
 954                                             Applicability::Unspecified,
 955                                         )
 956                                     })
 957                                 }
 958                             }
 959                         }
 960                         hir::Mutability::Not => {
 961                             let new_prefix = "&".to_owned() + &derefs;
 962                             match mutbl_a {
 963                                 hir::Mutability::Mut => {
 964                                     replace_prefix(&src, "&mut ", &new_prefix).map(|_| {
 965                                         let lo = sp.lo() + BytePos(1);
 966                                         let hi = sp.lo() + BytePos(5);
 967                                         let sp = sp.with_lo(lo).with_hi(hi);
 968                                         (sp, derefs, Applicability::MachineApplicable)
 969                                     })
 970                                 }
 971                                 hir::Mutability::Not => {
 972                                     replace_prefix(&src, "&", &new_prefix).map(|_| {
 973                                         let pos = sp.lo() + BytePos(1);
 974                                         let sp = sp.with_lo(pos).with_hi(pos);
 975                                         (sp, derefs, Applicability::MachineApplicable)
 976                                     })
 977                                 }
 978                             }
 979                         }
 980                     } {
 981                         return Some((
 982                             span,
 983                             "consider dereferencing".to_string(),
 984                             src,
 985                             applicability,
 986                             true,
 987                             false,
 988                         ));
 989                     }
 990                 }
 991             }
 992             _ if sp == expr.span => {
 993                 if let Some(mut steps) = self.deref_steps(checked_ty, expected) {
 994                     let mut expr = expr.peel_blocks();
 995                     let mut prefix_span = expr.span.shrink_to_lo();
 996                     let mut remove = String::new();
 997
 998                     // Try peeling off any existing `&` and `&mut` to reach our target type
 999                     while steps > 0 {
1000                         if let hir::ExprKind::AddrOf(_, mutbl, inner) = expr.kind {
1001                             // If the expression has `&`, removing it would fix the error
1002                             prefix_span = prefix_span.with_hi(inner.span.lo());
1003                             expr = inner;
1004                             remove += match mutbl {
1005                                 hir::Mutability::Not => "&",
1006                                 hir::Mutability::Mut => "&mut ",
1007                             };
1008                             steps -= 1;
1009                         } else {
1010                             break;
1011                         }
1012                     }
1013                     // If we've reached our target type with just removing `&`, then just print now.
1014                     if steps == 0 {
1015                         return Some((
1016                             prefix_span,
1017                             format!("consider removing the `{}`", remove.trim()),
1018                             String::new(),
1019                             // Do not remove `&&` to get to bool, because it might be something like
1020                             // { a } && b, which we have a separate fixup suggestion that is more
1021                             // likely correct...
1022                             if remove.trim() == "&&" && expected == self.tcx.types.bool {
1023                                 Applicability::MaybeIncorrect
1024                             } else {
1025                                 Applicability::MachineApplicable
1026                             },
1027                             true,
1028                             false,
1029                         ));
1030                     }
1031
1032                     // For this suggestion to make sense, the type would need to be `Copy`,
1033                     // or we have to be moving out of a `Box<T>`
1034                     if self.type_is_copy_modulo_regions(self.param_env, expected, sp)
1035                         // FIXME(compiler-errors): We can actually do this if the checked_ty is
1036                         // `steps` layers of boxes, not just one, but this is easier and most likely.
1037                         || (checked_ty.is_box() && steps == 1)
1038                     {
1039                         let deref_kind = if checked_ty.is_box() {
1040                             "unboxing the value"
1041                         } else if checked_ty.is_region_ptr() {
1042                             "dereferencing the borrow"
1043                         } else {
1044                             "dereferencing the type"
1045                         };
1046
1047                         // Suggest removing `&` if we have removed any, otherwise suggest just
1048                         // dereferencing the remaining number of steps.
1049                         let message = if remove.is_empty() {
1050                             format!("consider {deref_kind}")
1051                         } else {
1052                             format!(
1053                                 "consider removing the `{}` and {} instead",
1054                                 remove.trim(),
1055                                 deref_kind
1056                             )
1057                         };
1058
1059                         let prefix = match self.maybe_get_struct_pattern_shorthand_field(expr) {
1060                             Some(ident) => format!("{ident}: "),
1061                             None => String::new(),
1062                         };
1063
1064                         let (span, suggestion) = if self.is_else_if_block(expr) {
1065                             // Don't suggest nonsense like `else *if`
1066                             return None;
1067                         } else if let Some(expr) = self.maybe_get_block_expr(expr) {
1068                             // prefix should be empty here..
1069                             (expr.span.shrink_to_lo(), "*".to_string())
1070                         } else {
1071                             (prefix_span, format!("{}{}", prefix, "*".repeat(steps)))
1072                         };
1073
1074                         return Some((
1075                             span,
1076                             message,
1077                             suggestion,
1078                             Applicability::MachineApplicable,
1079                             true,
1080                             false,
1081                         ));
1082                     }
1083                 }
1084             }
1085             _ => {}
1086         }
1087         None
1088     }
1089
1090     pub fn check_for_cast(
1091         &self,
1092         err: &mut Diagnostic,
1093         expr: &hir::Expr<'_>,
1094         checked_ty: Ty<'tcx>,
1095         expected_ty: Ty<'tcx>,
1096         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
1097     ) -> bool {
1098         if self.tcx.sess.source_map().is_imported(expr.span) {
1099             // Ignore if span is from within a macro.
1100             return false;
1101         }
1102
1103         let Ok(src) = self.tcx.sess.source_map().span_to_snippet(expr.span) else {
1104             return false;
1105         };
1106
1107         // If casting this expression to a given numeric type would be appropriate in case of a type
1108         // mismatch.
1109         //
1110         // We want to minimize the amount of casting operations that are suggested, as it can be a
1111         // lossy operation with potentially bad side effects, so we only suggest when encountering
1112         // an expression that indicates that the original type couldn't be directly changed.
1113         //
1114         // For now, don't suggest casting with `as`.
1115         let can_cast = false;
1116
1117         let mut sugg = vec![];
1118
1119         if let Some(hir::Node::ExprField(field)) =
1120             self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
1121         {
1122             // `expr` is a literal field for a struct, only suggest if appropriate
1123             if field.is_shorthand {
1124                 // This is a field literal
1125                 sugg.push((field.ident.span.shrink_to_lo(), format!("{}: ", field.ident)));
1126             } else {
1127                 // Likely a field was meant, but this field wasn't found. Do not suggest anything.
1128                 return false;
1129             }
1130         };
1131
1132         if let hir::ExprKind::Call(path, args) = &expr.kind
1133             && let (hir::ExprKind::Path(hir::QPath::TypeRelative(base_ty, path_segment)), 1) =
1134                 (&path.kind, args.len())
1135             // `expr` is a conversion like `u32::from(val)`, do not suggest anything (#63697).
1136             && let (hir::TyKind::Path(hir::QPath::Resolved(None, base_ty_path)), sym::from) =
1137                 (&base_ty.kind, path_segment.ident.name)
1138         {
1139             if let Some(ident) = &base_ty_path.segments.iter().map(|s| s.ident).next() {
1140                 match ident.name {
1141                     sym::i128
1142                     | sym::i64
1143                     | sym::i32
1144                     | sym::i16
1145                     | sym::i8
1146                     | sym::u128
1147                     | sym::u64
1148                     | sym::u32
1149                     | sym::u16
1150                     | sym::u8
1151                     | sym::isize
1152                     | sym::usize
1153                         if base_ty_path.segments.len() == 1 =>
1154                     {
1155                         return false;
1156                     }
1157                     _ => {}
1158                 }
1159             }
1160         }
1161
1162         let msg = format!(
1163             "you can convert {} `{}` to {} `{}`",
1164             checked_ty.kind().article(),
1165             checked_ty,
1166             expected_ty.kind().article(),
1167             expected_ty,
1168         );
1169         let cast_msg = format!(
1170             "you can cast {} `{}` to {} `{}`",
1171             checked_ty.kind().article(),
1172             checked_ty,
1173             expected_ty.kind().article(),
1174             expected_ty,
1175         );
1176         let lit_msg = format!(
1177             "change the type of the numeric literal from `{checked_ty}` to `{expected_ty}`",
1178         );
1179
1180         let close_paren = if expr.precedence().order() < PREC_POSTFIX {
1181             sugg.push((expr.span.shrink_to_lo(), "(".to_string()));
1182             ")"
1183         } else {
1184             ""
1185         };
1186
1187         let mut cast_suggestion = sugg.clone();
1188         cast_suggestion.push((expr.span.shrink_to_hi(), format!("{close_paren} as {expected_ty}")));
1189         let mut into_suggestion = sugg.clone();
1190         into_suggestion.push((expr.span.shrink_to_hi(), format!("{close_paren}.into()")));
1191         let mut suffix_suggestion = sugg.clone();
1192         suffix_suggestion.push((
1193             if matches!(
1194                 (&expected_ty.kind(), &checked_ty.kind()),
1195                 (ty::Int(_) | ty::Uint(_), ty::Float(_))
1196             ) {
1197                 // Remove fractional part from literal, for example `42.0f32` into `42`
1198                 let src = src.trim_end_matches(&checked_ty.to_string());
1199                 let len = src.split('.').next().unwrap().len();
1200                 expr.span.with_lo(expr.span.lo() + BytePos(len as u32))
1201             } else {
1202                 let len = src.trim_end_matches(&checked_ty.to_string()).len();
1203                 expr.span.with_lo(expr.span.lo() + BytePos(len as u32))
1204             },
1205             if expr.precedence().order() < PREC_POSTFIX {
1206                 // Readd `)`
1207                 format!("{expected_ty})")
1208             } else {
1209                 expected_ty.to_string()
1210             },
1211         ));
1212         let literal_is_ty_suffixed = |expr: &hir::Expr<'_>| {
1213             if let hir::ExprKind::Lit(lit) = &expr.kind { lit.node.is_suffixed() } else { false }
1214         };
1215         let is_negative_int =
1216             |expr: &hir::Expr<'_>| matches!(expr.kind, hir::ExprKind::Unary(hir::UnOp::Neg, ..));
1217         let is_uint = |ty: Ty<'_>| matches!(ty.kind(), ty::Uint(..));
1218
1219         let in_const_context = self.tcx.hir().is_inside_const_context(expr.hir_id);
1220
1221         let suggest_fallible_into_or_lhs_from =
1222             |err: &mut Diagnostic, exp_to_found_is_fallible: bool| {
1223                 // If we know the expression the expected type is derived from, we might be able
1224                 // to suggest a widening conversion rather than a narrowing one (which may
1225                 // panic). For example, given x: u8 and y: u32, if we know the span of "x",
1226                 //   x > y
1227                 // can be given the suggestion "u32::from(x) > y" rather than
1228                 // "x > y.try_into().unwrap()".
1229                 let lhs_expr_and_src = expected_ty_expr.and_then(|expr| {
1230                     self.tcx
1231                         .sess
1232                         .source_map()
1233                         .span_to_snippet(expr.span)
1234                         .ok()
1235                         .map(|src| (expr, src))
1236                 });
1237                 let (msg, suggestion) = if let (Some((lhs_expr, lhs_src)), false) =
1238                     (lhs_expr_and_src, exp_to_found_is_fallible)
1239                 {
1240                     let msg = format!(
1241                         "you can convert `{lhs_src}` from `{expected_ty}` to `{checked_ty}`, matching the type of `{src}`",
1242                     );
1243                     let suggestion = vec![
1244                         (lhs_expr.span.shrink_to_lo(), format!("{checked_ty}::from(")),
1245                         (lhs_expr.span.shrink_to_hi(), ")".to_string()),
1246                     ];
1247                     (msg, suggestion)
1248                 } else {
1249                     let msg = format!("{msg} and panic if the converted value doesn't fit");
1250                     let mut suggestion = sugg.clone();
1251                     suggestion.push((
1252                         expr.span.shrink_to_hi(),
1253                         format!("{close_paren}.try_into().unwrap()"),
1254                     ));
1255                     (msg, suggestion)
1256                 };
1257                 err.multipart_suggestion_verbose(
1258                     &msg,
1259                     suggestion,
1260                     Applicability::MachineApplicable,
1261                 );
1262             };
1263
1264         let suggest_to_change_suffix_or_into =
1265             |err: &mut Diagnostic,
1266              found_to_exp_is_fallible: bool,
1267              exp_to_found_is_fallible: bool| {
1268                 let exp_is_lhs =
1269                     expected_ty_expr.map(|e| self.tcx.hir().is_lhs(e.hir_id)).unwrap_or(false);
1270
1271                 if exp_is_lhs {
1272                     return;
1273                 }
1274
1275                 let always_fallible = found_to_exp_is_fallible
1276                     && (exp_to_found_is_fallible || expected_ty_expr.is_none());
1277                 let msg = if literal_is_ty_suffixed(expr) {
1278                     &lit_msg
1279                 } else if always_fallible && (is_negative_int(expr) && is_uint(expected_ty)) {
1280                     // We now know that converting either the lhs or rhs is fallible. Before we
1281                     // suggest a fallible conversion, check if the value can never fit in the
1282                     // expected type.
1283                     let msg = format!("`{src}` cannot fit into type `{expected_ty}`");
1284                     err.note(&msg);
1285                     return;
1286                 } else if in_const_context {
1287                     // Do not recommend `into` or `try_into` in const contexts.
1288                     return;
1289                 } else if found_to_exp_is_fallible {
1290                     return suggest_fallible_into_or_lhs_from(err, exp_to_found_is_fallible);
1291                 } else {
1292                     &msg
1293                 };
1294                 let suggestion = if literal_is_ty_suffixed(expr) {
1295                     suffix_suggestion.clone()
1296                 } else {
1297                     into_suggestion.clone()
1298                 };
1299                 err.multipart_suggestion_verbose(msg, suggestion, Applicability::MachineApplicable);
1300             };
1301
1302         match (&expected_ty.kind(), &checked_ty.kind()) {
1303             (&ty::Int(ref exp), &ty::Int(ref found)) => {
1304                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1305                 {
1306                     (Some(exp), Some(found)) if exp < found => (true, false),
1307                     (Some(exp), Some(found)) if exp > found => (false, true),
1308                     (None, Some(8 | 16)) => (false, true),
1309                     (Some(8 | 16), None) => (true, false),
1310                     (None, _) | (_, None) => (true, true),
1311                     _ => (false, false),
1312                 };
1313                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1314                 true
1315             }
1316             (&ty::Uint(ref exp), &ty::Uint(ref found)) => {
1317                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1318                 {
1319                     (Some(exp), Some(found)) if exp < found => (true, false),
1320                     (Some(exp), Some(found)) if exp > found => (false, true),
1321                     (None, Some(8 | 16)) => (false, true),
1322                     (Some(8 | 16), None) => (true, false),
1323                     (None, _) | (_, None) => (true, true),
1324                     _ => (false, false),
1325                 };
1326                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1327                 true
1328             }
1329             (&ty::Int(exp), &ty::Uint(found)) => {
1330                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1331                 {
1332                     (Some(exp), Some(found)) if found < exp => (false, true),
1333                     (None, Some(8)) => (false, true),
1334                     _ => (true, true),
1335                 };
1336                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1337                 true
1338             }
1339             (&ty::Uint(exp), &ty::Int(found)) => {
1340                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1341                 {
1342                     (Some(exp), Some(found)) if found > exp => (true, false),
1343                     (Some(8), None) => (true, false),
1344                     _ => (true, true),
1345                 };
1346                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1347                 true
1348             }
1349             (&ty::Float(ref exp), &ty::Float(ref found)) => {
1350                 if found.bit_width() < exp.bit_width() {
1351                     suggest_to_change_suffix_or_into(err, false, true);
1352                 } else if literal_is_ty_suffixed(expr) {
1353                     err.multipart_suggestion_verbose(
1354                         &lit_msg,
1355                         suffix_suggestion,
1356                         Applicability::MachineApplicable,
1357                     );
1358                 } else if can_cast {
1359                     // Missing try_into implementation for `f64` to `f32`
1360                     err.multipart_suggestion_verbose(
1361                         &format!("{cast_msg}, producing the closest possible value"),
1362                         cast_suggestion,
1363                         Applicability::MaybeIncorrect, // lossy conversion
1364                     );
1365                 }
1366                 true
1367             }
1368             (&ty::Uint(_) | &ty::Int(_), &ty::Float(_)) => {
1369                 if literal_is_ty_suffixed(expr) {
1370                     err.multipart_suggestion_verbose(
1371                         &lit_msg,
1372                         suffix_suggestion,
1373                         Applicability::MachineApplicable,
1374                     );
1375                 } else if can_cast {
1376                     // Missing try_into implementation for `{float}` to `{integer}`
1377                     err.multipart_suggestion_verbose(
1378                         &format!("{msg}, rounding the float towards zero"),
1379                         cast_suggestion,
1380                         Applicability::MaybeIncorrect, // lossy conversion
1381                     );
1382                 }
1383                 true
1384             }
1385             (&ty::Float(ref exp), &ty::Uint(ref found)) => {
1386                 // if `found` is `None` (meaning found is `usize`), don't suggest `.into()`
1387                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
1388                     err.multipart_suggestion_verbose(
1389                         &format!(
1390                             "{msg}, producing the floating point representation of the integer",
1391                         ),
1392                         into_suggestion,
1393                         Applicability::MachineApplicable,
1394                     );
1395                 } else if literal_is_ty_suffixed(expr) {
1396                     err.multipart_suggestion_verbose(
1397                         &lit_msg,
1398                         suffix_suggestion,
1399                         Applicability::MachineApplicable,
1400                     );
1401                 } else {
1402                     // Missing try_into implementation for `{integer}` to `{float}`
1403                     err.multipart_suggestion_verbose(
1404                         &format!(
1405                             "{cast_msg}, producing the floating point representation of the integer, \
1406                                  rounded if necessary",
1407                         ),
1408                         cast_suggestion,
1409                         Applicability::MaybeIncorrect, // lossy conversion
1410                     );
1411                 }
1412                 true
1413             }
1414             (&ty::Float(ref exp), &ty::Int(ref found)) => {
1415                 // if `found` is `None` (meaning found is `isize`), don't suggest `.into()`
1416                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
1417                     err.multipart_suggestion_verbose(
1418                         &format!(
1419                             "{}, producing the floating point representation of the integer",
1420                             &msg,
1421                         ),
1422                         into_suggestion,
1423                         Applicability::MachineApplicable,
1424                     );
1425                 } else if literal_is_ty_suffixed(expr) {
1426                     err.multipart_suggestion_verbose(
1427                         &lit_msg,
1428                         suffix_suggestion,
1429                         Applicability::MachineApplicable,
1430                     );
1431                 } else {
1432                     // Missing try_into implementation for `{integer}` to `{float}`
1433                     err.multipart_suggestion_verbose(
1434                         &format!(
1435                             "{}, producing the floating point representation of the integer, \
1436                                 rounded if necessary",
1437                             &msg,
1438                         ),
1439                         cast_suggestion,
1440                         Applicability::MaybeIncorrect, // lossy conversion
1441                     );
1442                 }
1443                 true
1444             }
1445             (
1446                 &ty::Uint(ty::UintTy::U32 | ty::UintTy::U64 | ty::UintTy::U128)
1447                 | &ty::Int(ty::IntTy::I32 | ty::IntTy::I64 | ty::IntTy::I128),
1448                 &ty::Char,
1449             ) => {
1450                 err.multipart_suggestion_verbose(
1451                     &format!("{cast_msg}, since a `char` always occupies 4 bytes"),
1452                     cast_suggestion,
1453                     Applicability::MachineApplicable,
1454                 );
1455                 true
1456             }
1457             _ => false,
1458         }
1459     }
1460 }