compiler/rustc_typeck/src/check/demand.rs

   1 use crate::check::FnCtxt;
   2 use rustc_infer::infer::InferOk;
   3 use rustc_trait_selection::infer::InferCtxtExt as _;
   4 use rustc_trait_selection::traits::ObligationCause;
   5
   6 use rustc_ast::util::parser::PREC_POSTFIX;
   7 use rustc_errors::{Applicability, DiagnosticBuilder};
   8 use rustc_hir as hir;
   9 use rustc_hir::lang_items::LangItem;
  10 use rustc_hir::{is_range_literal, Node};
  11 use rustc_middle::ty::adjustment::AllowTwoPhase;
  12 use rustc_middle::ty::{self, AssocItem, Ty, TypeAndMut};
  13 use rustc_span::symbol::sym;
  14 use rustc_span::Span;
  15
  16 use super::method::probe;
  17
  18 use std::fmt;
  19
  20 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  21     pub fn emit_coerce_suggestions(
  22         &self,
  23         err: &mut DiagnosticBuilder<'_>,
  24         expr: &hir::Expr<'_>,
  25         expr_ty: Ty<'tcx>,
  26         expected: Ty<'tcx>,
  27         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
  28     ) {
  29         self.annotate_expected_due_to_let_ty(err, expr);
  30         self.suggest_compatible_variants(err, expr, expected, expr_ty);
  31         self.suggest_deref_ref_or_into(err, expr, expected, expr_ty, expected_ty_expr);
  32         if self.suggest_calling_boxed_future_when_appropriate(err, expr, expected, expr_ty) {
  33             return;
  34         }
  35         self.suggest_boxing_when_appropriate(err, expr, expected, expr_ty);
  36         self.suggest_missing_parentheses(err, expr);
  37         self.note_need_for_fn_pointer(err, expected, expr_ty);
  38         self.note_internal_mutation_in_method(err, expr, expected, expr_ty);
  39     }
  40
  41     // Requires that the two types unify, and prints an error message if
  42     // they don't.
  43     pub fn demand_suptype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  44         if let Some(mut e) = self.demand_suptype_diag(sp, expected, actual) {
  45             e.emit();
  46         }
  47     }
  48
  49     pub fn demand_suptype_diag(
  50         &self,
  51         sp: Span,
  52         expected: Ty<'tcx>,
  53         actual: Ty<'tcx>,
  54     ) -> Option<DiagnosticBuilder<'tcx>> {
  55         self.demand_suptype_with_origin(&self.misc(sp), expected, actual)
  56     }
  57
  58     pub fn demand_suptype_with_origin(
  59         &self,
  60         cause: &ObligationCause<'tcx>,
  61         expected: Ty<'tcx>,
  62         actual: Ty<'tcx>,
  63     ) -> Option<DiagnosticBuilder<'tcx>> {
  64         match self.at(cause, self.param_env).sup(expected, actual) {
  65             Ok(InferOk { obligations, value: () }) => {
  66                 self.register_predicates(obligations);
  67                 None
  68             }
  69             Err(e) => Some(self.report_mismatched_types(&cause, expected, actual, e)),
  70         }
  71     }
  72
  73     pub fn demand_eqtype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  74         if let Some(mut err) = self.demand_eqtype_diag(sp, expected, actual) {
  75             err.emit();
  76         }
  77     }
  78
  79     pub fn demand_eqtype_diag(
  80         &self,
  81         sp: Span,
  82         expected: Ty<'tcx>,
  83         actual: Ty<'tcx>,
  84     ) -> Option<DiagnosticBuilder<'tcx>> {
  85         self.demand_eqtype_with_origin(&self.misc(sp), expected, actual)
  86     }
  87
  88     pub fn demand_eqtype_with_origin(
  89         &self,
  90         cause: &ObligationCause<'tcx>,
  91         expected: Ty<'tcx>,
  92         actual: Ty<'tcx>,
  93     ) -> Option<DiagnosticBuilder<'tcx>> {
  94         match self.at(cause, self.param_env).eq(expected, actual) {
  95             Ok(InferOk { obligations, value: () }) => {
  96                 self.register_predicates(obligations);
  97                 None
  98             }
  99             Err(e) => Some(self.report_mismatched_types(cause, expected, actual, e)),
 100         }
 101     }
 102
 103     pub fn demand_coerce(
 104         &self,
 105         expr: &hir::Expr<'_>,
 106         checked_ty: Ty<'tcx>,
 107         expected: Ty<'tcx>,
 108         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 109         allow_two_phase: AllowTwoPhase,
 110     ) -> Ty<'tcx> {
 111         let (ty, err) =
 112             self.demand_coerce_diag(expr, checked_ty, expected, expected_ty_expr, allow_two_phase);
 113         if let Some(mut err) = err {
 114             err.emit();
 115         }
 116         ty
 117     }
 118
 119     /// Checks that the type of `expr` can be coerced to `expected`.
 120     ///
 121     /// N.B., this code relies on `self.diverges` to be accurate. In particular, assignments to `!`
 122     /// will be permitted if the diverges flag is currently "always".
 123     pub fn demand_coerce_diag(
 124         &self,
 125         expr: &hir::Expr<'_>,
 126         checked_ty: Ty<'tcx>,
 127         expected: Ty<'tcx>,
 128         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 129         allow_two_phase: AllowTwoPhase,
 130     ) -> (Ty<'tcx>, Option<DiagnosticBuilder<'tcx>>) {
 131         let expected = self.resolve_vars_with_obligations(expected);
 132
 133         let e = match self.try_coerce(expr, checked_ty, expected, allow_two_phase) {
 134             Ok(ty) => return (ty, None),
 135             Err(e) => e,
 136         };
 137
 138         let expr = expr.peel_drop_temps();
 139         let cause = self.misc(expr.span);
 140         let expr_ty = self.resolve_vars_with_obligations(checked_ty);
 141         let mut err = self.report_mismatched_types(&cause, expected, expr_ty, e);
 142
 143         if self.is_assign_to_bool(expr, expected) {
 144             // Error reported in `check_assign` so avoid emitting error again.
 145             err.delay_as_bug();
 146             return (expected, None);
 147         }
 148
 149         self.emit_coerce_suggestions(&mut err, expr, expr_ty, expected, expected_ty_expr);
 150
 151         (expected, Some(err))
 152     }
 153
 154     fn annotate_expected_due_to_let_ty(
 155         &self,
 156         err: &mut DiagnosticBuilder<'_>,
 157         expr: &hir::Expr<'_>,
 158     ) {
 159         let parent = self.tcx.hir().get_parent_node(expr.hir_id);
 160         if let Some(hir::Node::Local(hir::Local { ty: Some(ty), init: Some(init), .. })) =
 161             self.tcx.hir().find(parent)
 162         {
 163             if init.hir_id == expr.hir_id {
 164                 // Point at `let` assignment type.
 165                 err.span_label(ty.span, "expected due to this");
 166             }
 167         }
 168     }
 169
 170     /// Returns whether the expected type is `bool` and the expression is `x = y`.
 171     pub fn is_assign_to_bool(&self, expr: &hir::Expr<'_>, expected: Ty<'tcx>) -> bool {
 172         if let hir::ExprKind::Assign(..) = expr.kind {
 173             return expected == self.tcx.types.bool;
 174         }
 175         false
 176     }
 177
 178     /// If the expected type is an enum (Issue #55250) with any variants whose
 179     /// sole field is of the found type, suggest such variants. (Issue #42764)
 180     fn suggest_compatible_variants(
 181         &self,
 182         err: &mut DiagnosticBuilder<'_>,
 183         expr: &hir::Expr<'_>,
 184         expected: Ty<'tcx>,
 185         expr_ty: Ty<'tcx>,
 186     ) {
 187         if let ty::Adt(expected_adt, substs) = expected.kind() {
 188             if !expected_adt.is_enum() {
 189                 return;
 190             }
 191
 192             let mut compatible_variants = expected_adt
 193                 .variants
 194                 .iter()
 195                 .filter(|variant| variant.fields.len() == 1)
 196                 .filter_map(|variant| {
 197                     let sole_field = &variant.fields[0];
 198                     let sole_field_ty = sole_field.ty(self.tcx, substs);
 199                     if self.can_coerce(expr_ty, sole_field_ty) {
 200                         let variant_path = self.tcx.def_path_str(variant.def_id);
 201                         // FIXME #56861: DRYer prelude filtering
 202                         Some(variant_path.trim_start_matches("std::prelude::v1::").to_string())
 203                     } else {
 204                         None
 205                     }
 206                 })
 207                 .peekable();
 208
 209             if compatible_variants.peek().is_some() {
 210                 if let Ok(expr_text) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
 211                     let suggestions = compatible_variants.map(|v| format!("{}({})", v, expr_text));
 212                     let msg = "try using a variant of the expected enum";
 213                     err.span_suggestions(
 214                         expr.span,
 215                         msg,
 216                         suggestions,
 217                         Applicability::MaybeIncorrect,
 218                     );
 219                 }
 220             }
 221         }
 222     }
 223
 224     pub fn get_conversion_methods(
 225         &self,
 226         span: Span,
 227         expected: Ty<'tcx>,
 228         checked_ty: Ty<'tcx>,
 229         hir_id: hir::HirId,
 230     ) -> Vec<AssocItem> {
 231         let mut methods =
 232             self.probe_for_return_type(span, probe::Mode::MethodCall, expected, checked_ty, hir_id);
 233         methods.retain(|m| {
 234             self.has_only_self_parameter(m)
 235                 && self
 236                     .tcx
 237                     .get_attrs(m.def_id)
 238                     .iter()
 239                     // This special internal attribute is used to permit
 240                     // "identity-like" conversion methods to be suggested here.
 241                     //
 242                     // FIXME (#46459 and #46460): ideally
 243                     // `std::convert::Into::into` and `std::borrow:ToOwned` would
 244                     // also be `#[rustc_conversion_suggestion]`, if not for
 245                     // method-probing false-positives and -negatives (respectively).
 246                     //
 247                     // FIXME? Other potential candidate methods: `as_ref` and
 248                     // `as_mut`?
 249                     .any(|a| self.sess().check_name(a, sym::rustc_conversion_suggestion))
 250         });
 251
 252         methods
 253     }
 254
 255     /// This function checks whether the method is not static and does not accept other parameters than `self`.
 256     fn has_only_self_parameter(&self, method: &AssocItem) -> bool {
 257         match method.kind {
 258             ty::AssocKind::Fn => {
 259                 method.fn_has_self_parameter
 260                     && self.tcx.fn_sig(method.def_id).inputs().skip_binder().len() == 1
 261             }
 262             _ => false,
 263         }
 264     }
 265
 266     /// Identify some cases where `as_ref()` would be appropriate and suggest it.
 267     ///
 268     /// Given the following code:
 269     /// ```
 270     /// struct Foo;
 271     /// fn takes_ref(_: &Foo) {}
 272     /// let ref opt = Some(Foo);
 273     ///
 274     /// opt.map(|param| takes_ref(param));
 275     /// ```
 276     /// Suggest using `opt.as_ref().map(|param| takes_ref(param));` instead.
 277     ///
 278     /// It only checks for `Option` and `Result` and won't work with
 279     /// ```
 280     /// opt.map(|param| { takes_ref(param) });
 281     /// ```
 282     fn can_use_as_ref(&self, expr: &hir::Expr<'_>) -> Option<(Span, &'static str, String)> {
 283         let path = match expr.kind {
 284             hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => path,
 285             _ => return None,
 286         };
 287
 288         let local_id = match path.res {
 289             hir::def::Res::Local(id) => id,
 290             _ => return None,
 291         };
 292
 293         let local_parent = self.tcx.hir().get_parent_node(local_id);
 294         let param_hir_id = match self.tcx.hir().find(local_parent) {
 295             Some(Node::Param(hir::Param { hir_id, .. })) => hir_id,
 296             _ => return None,
 297         };
 298
 299         let param_parent = self.tcx.hir().get_parent_node(*param_hir_id);
 300         let (expr_hir_id, closure_fn_decl) = match self.tcx.hir().find(param_parent) {
 301             Some(Node::Expr(hir::Expr {
 302                 hir_id,
 303                 kind: hir::ExprKind::Closure(_, decl, ..),
 304                 ..
 305             })) => (hir_id, decl),
 306             _ => return None,
 307         };
 308
 309         let expr_parent = self.tcx.hir().get_parent_node(*expr_hir_id);
 310         let hir = self.tcx.hir().find(expr_parent);
 311         let closure_params_len = closure_fn_decl.inputs.len();
 312         let (method_path, method_span, method_expr) = match (hir, closure_params_len) {
 313             (
 314                 Some(Node::Expr(hir::Expr {
 315                     kind: hir::ExprKind::MethodCall(path, span, expr, _),
 316                     ..
 317                 })),
 318                 1,
 319             ) => (path, span, expr),
 320             _ => return None,
 321         };
 322
 323         let self_ty = self.typeck_results.borrow().node_type(method_expr[0].hir_id);
 324         let self_ty = format!("{:?}", self_ty);
 325         let name = method_path.ident.name;
 326         let is_as_ref_able = (self_ty.starts_with("&std::option::Option")
 327             || self_ty.starts_with("&std::result::Result")
 328             || self_ty.starts_with("std::option::Option")
 329             || self_ty.starts_with("std::result::Result"))
 330             && (name == sym::map || name == sym::and_then);
 331         match (is_as_ref_able, self.sess().source_map().span_to_snippet(*method_span)) {
 332             (true, Ok(src)) => {
 333                 let suggestion = format!("as_ref().{}", src);
 334                 Some((*method_span, "consider using `as_ref` instead", suggestion))
 335             }
 336             _ => None,
 337         }
 338     }
 339
 340     crate fn is_hir_id_from_struct_pattern_shorthand_field(
 341         &self,
 342         hir_id: hir::HirId,
 343         sp: Span,
 344     ) -> bool {
 345         let sm = self.sess().source_map();
 346         let parent_id = self.tcx.hir().get_parent_node(hir_id);
 347         if let Some(parent) = self.tcx.hir().find(parent_id) {
 348             // Account for fields
 349             if let Node::Expr(hir::Expr { kind: hir::ExprKind::Struct(_, fields, ..), .. }) = parent
 350             {
 351                 if let Ok(src) = sm.span_to_snippet(sp) {
 352                     for field in *fields {
 353                         if field.ident.as_str() == src && field.is_shorthand {
 354                             return true;
 355                         }
 356                     }
 357                 }
 358             }
 359         }
 360         false
 361     }
 362
 363     fn replace_prefix(&self, s: &str, old: &str, new: &str) -> Option<String> {
 364         s.strip_prefix(old).map(|stripped| new.to_string() + stripped)
 365     }
 366
 367     /// This function is used to determine potential "simple" improvements or users' errors and
 368     /// provide them useful help. For example:
 369     ///
 370     /// ```
 371     /// fn some_fn(s: &str) {}
 372     ///
 373     /// let x = "hey!".to_owned();
 374     /// some_fn(x); // error
 375     /// ```
 376     ///
 377     /// No need to find every potential function which could make a coercion to transform a
 378     /// `String` into a `&str` since a `&` would do the trick!
 379     ///
 380     /// In addition of this check, it also checks between references mutability state. If the
 381     /// expected is mutable but the provided isn't, maybe we could just say "Hey, try with
 382     /// `&mut`!".
 383     pub fn check_ref(
 384         &self,
 385         expr: &hir::Expr<'_>,
 386         checked_ty: Ty<'tcx>,
 387         expected: Ty<'tcx>,
 388     ) -> Option<(Span, &'static str, String, Applicability)> {
 389         let sm = self.sess().source_map();
 390         let sp = expr.span;
 391         if sm.is_imported(sp) {
 392             // Ignore if span is from within a macro #41858, #58298. We previously used the macro
 393             // call span, but that breaks down when the type error comes from multiple calls down.
 394             return None;
 395         }
 396
 397         let is_struct_pat_shorthand_field =
 398             self.is_hir_id_from_struct_pattern_shorthand_field(expr.hir_id, sp);
 399
 400         // If the span is from a macro, then it's hard to extract the text
 401         // and make a good suggestion, so don't bother.
 402         let is_macro = sp.from_expansion() && sp.desugaring_kind().is_none();
 403
 404         // `ExprKind::DropTemps` is semantically irrelevant for these suggestions.
 405         let expr = expr.peel_drop_temps();
 406
 407         match (&expr.kind, expected.kind(), checked_ty.kind()) {
 408             (_, &ty::Ref(_, exp, _), &ty::Ref(_, check, _)) => match (exp.kind(), check.kind()) {
 409                 (&ty::Str, &ty::Array(arr, _) | &ty::Slice(arr)) if arr == self.tcx.types.u8 => {
 410                     if let hir::ExprKind::Lit(_) = expr.kind {
 411                         if let Ok(src) = sm.span_to_snippet(sp) {
 412                             if let Some(src) = self.replace_prefix(&src, "b\"", "\"") {
 413                                 return Some((
 414                                     sp,
 415                                     "consider removing the leading `b`",
 416                                     src,
 417                                     Applicability::MachineApplicable,
 418                                 ));
 419                             }
 420                         }
 421                     }
 422                 }
 423                 (&ty::Array(arr, _) | &ty::Slice(arr), &ty::Str) if arr == self.tcx.types.u8 => {
 424                     if let hir::ExprKind::Lit(_) = expr.kind {
 425                         if let Ok(src) = sm.span_to_snippet(sp) {
 426                             if let Some(src) = self.replace_prefix(&src, "\"", "b\"") {
 427                                 return Some((
 428                                     sp,
 429                                     "consider adding a leading `b`",
 430                                     src,
 431                                     Applicability::MachineApplicable,
 432                                 ));
 433                             }
 434                         }
 435                     }
 436                 }
 437                 _ => {}
 438             },
 439             (_, &ty::Ref(_, _, mutability), _) => {
 440                 // Check if it can work when put into a ref. For example:
 441                 //
 442                 // ```
 443                 // fn bar(x: &mut i32) {}
 444                 //
 445                 // let x = 0u32;
 446                 // bar(&x); // error, expected &mut
 447                 // ```
 448                 let ref_ty = match mutability {
 449                     hir::Mutability::Mut => {
 450                         self.tcx.mk_mut_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 451                     }
 452                     hir::Mutability::Not => {
 453                         self.tcx.mk_imm_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 454                     }
 455                 };
 456                 if self.can_coerce(ref_ty, expected) {
 457                     let mut sugg_sp = sp;
 458                     if let hir::ExprKind::MethodCall(ref segment, sp, ref args, _) = expr.kind {
 459                         let clone_trait = self.tcx.require_lang_item(LangItem::Clone, Some(sp));
 460                         if let ([arg], Some(true), sym::clone) = (
 461                             &args[..],
 462                             self.typeck_results.borrow().type_dependent_def_id(expr.hir_id).map(
 463                                 |did| {
 464                                     let ai = self.tcx.associated_item(did);
 465                                     ai.container == ty::TraitContainer(clone_trait)
 466                                 },
 467                             ),
 468                             segment.ident.name,
 469                         ) {
 470                             // If this expression had a clone call when suggesting borrowing
 471                             // we want to suggest removing it because it'd now be unnecessary.
 472                             sugg_sp = arg.span;
 473                         }
 474                     }
 475                     if let Ok(src) = sm.span_to_snippet(sugg_sp) {
 476                         let needs_parens = match expr.kind {
 477                             // parenthesize if needed (Issue #46756)
 478                             hir::ExprKind::Cast(_, _) | hir::ExprKind::Binary(_, _, _) => true,
 479                             // parenthesize borrows of range literals (Issue #54505)
 480                             _ if is_range_literal(expr) => true,
 481                             _ => false,
 482                         };
 483                         let sugg_expr = if needs_parens { format!("({})", src) } else { src };
 484
 485                         if let Some(sugg) = self.can_use_as_ref(expr) {
 486                             return Some((
 487                                 sugg.0,
 488                                 sugg.1,
 489                                 sugg.2,
 490                                 Applicability::MachineApplicable,
 491                             ));
 492                         }
 493                         let field_name = if is_struct_pat_shorthand_field {
 494                             format!("{}: ", sugg_expr)
 495                         } else {
 496                             String::new()
 497                         };
 498                         if let Some(hir::Node::Expr(hir::Expr {
 499                             kind: hir::ExprKind::Assign(left_expr, ..),
 500                             ..
 501                         })) = self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
 502                         {
 503                             if mutability == hir::Mutability::Mut {
 504                                 // Found the following case:
 505                                 // fn foo(opt: &mut Option<String>){ opt = None }
 506                                 //                                   ---   ^^^^
 507                                 //                                   |     |
 508                                 //    consider dereferencing here: `*opt`  |
 509                                 // expected mutable reference, found enum `Option`
 510                                 if let Ok(src) = sm.span_to_snippet(left_expr.span) {
 511                                     return Some((
 512                                         left_expr.span,
 513                                         "consider dereferencing here to assign to the mutable \
 514                                          borrowed piece of memory",
 515                                         format!("*{}", src),
 516                                         Applicability::MachineApplicable,
 517                                     ));
 518                                 }
 519                             }
 520                         }
 521
 522                         return Some(match mutability {
 523                             hir::Mutability::Mut => (
 524                                 sp,
 525                                 "consider mutably borrowing here",
 526                                 format!("{}&mut {}", field_name, sugg_expr),
 527                                 Applicability::MachineApplicable,
 528                             ),
 529                             hir::Mutability::Not => (
 530                                 sp,
 531                                 "consider borrowing here",
 532                                 format!("{}&{}", field_name, sugg_expr),
 533                                 Applicability::MachineApplicable,
 534                             ),
 535                         });
 536                     }
 537                 }
 538             }
 539             (
 540                 hir::ExprKind::AddrOf(hir::BorrowKind::Ref, _, ref expr),
 541                 _,
 542                 &ty::Ref(_, checked, _),
 543             ) if {
 544                 self.infcx.can_sub(self.param_env, checked, &expected).is_ok() && !is_macro
 545             } =>
 546             {
 547                 // We have `&T`, check if what was expected was `T`. If so,
 548                 // we may want to suggest removing a `&`.
 549                 if sm.is_imported(expr.span) {
 550                     if let Ok(src) = sm.span_to_snippet(sp) {
 551                         if let Some(src) = src.strip_prefix('&') {
 552                             return Some((
 553                                 sp,
 554                                 "consider removing the borrow",
 555                                 src.to_string(),
 556                                 Applicability::MachineApplicable,
 557                             ));
 558                         }
 559                     }
 560                     return None;
 561                 }
 562                 if let Ok(code) = sm.span_to_snippet(expr.span) {
 563                     return Some((
 564                         sp,
 565                         "consider removing the borrow",
 566                         code,
 567                         Applicability::MachineApplicable,
 568                     ));
 569                 }
 570             }
 571             (
 572                 _,
 573                 &ty::RawPtr(TypeAndMut { ty: ty_b, mutbl: mutbl_b }),
 574                 &ty::Ref(_, ty_a, mutbl_a),
 575             ) => {
 576                 if let Some(steps) = self.deref_steps(ty_a, ty_b) {
 577                     // Only suggest valid if dereferencing needed.
 578                     if steps > 0 {
 579                         // The pointer type implements `Copy` trait so the suggestion is always valid.
 580                         if let Ok(src) = sm.span_to_snippet(sp) {
 581                             let derefs = &"*".repeat(steps);
 582                             if let Some((src, applicability)) = match mutbl_b {
 583                                 hir::Mutability::Mut => {
 584                                     let new_prefix = "&mut ".to_owned() + derefs;
 585                                     match mutbl_a {
 586                                         hir::Mutability::Mut => self
 587                                             .replace_prefix(&src, "&mut ", &new_prefix)
 588                                             .map(|s| (s, Applicability::MachineApplicable)),
 589                                         hir::Mutability::Not => self
 590                                             .replace_prefix(&src, "&", &new_prefix)
 591                                             .map(|s| (s, Applicability::Unspecified)),
 592                                     }
 593                                 }
 594                                 hir::Mutability::Not => {
 595                                     let new_prefix = "&".to_owned() + derefs;
 596                                     match mutbl_a {
 597                                         hir::Mutability::Mut => self
 598                                             .replace_prefix(&src, "&mut ", &new_prefix)
 599                                             .map(|s| (s, Applicability::MachineApplicable)),
 600                                         hir::Mutability::Not => self
 601                                             .replace_prefix(&src, "&", &new_prefix)
 602                                             .map(|s| (s, Applicability::MachineApplicable)),
 603                                     }
 604                                 }
 605                             } {
 606                                 return Some((sp, "consider dereferencing", src, applicability));
 607                             }
 608                         }
 609                     }
 610                 }
 611             }
 612             _ if sp == expr.span && !is_macro => {
 613                 if let Some(steps) = self.deref_steps(checked_ty, expected) {
 614                     if steps == 1 {
 615                         // For a suggestion to make sense, the type would need to be `Copy`.
 616                         if self.infcx.type_is_copy_modulo_regions(self.param_env, expected, sp) {
 617                             if let Ok(code) = sm.span_to_snippet(sp) {
 618                                 let message = if checked_ty.is_region_ptr() {
 619                                     "consider dereferencing the borrow"
 620                                 } else {
 621                                     "consider dereferencing the type"
 622                                 };
 623                                 let suggestion = if is_struct_pat_shorthand_field {
 624                                     format!("{}: *{}", code, code)
 625                                 } else {
 626                                     format!("*{}", code)
 627                                 };
 628                                 return Some((
 629                                     sp,
 630                                     message,
 631                                     suggestion,
 632                                     Applicability::MachineApplicable,
 633                                 ));
 634                             }
 635                         }
 636                     }
 637                 }
 638             }
 639             _ => {}
 640         }
 641         None
 642     }
 643
 644     pub fn check_for_cast(
 645         &self,
 646         err: &mut DiagnosticBuilder<'_>,
 647         expr: &hir::Expr<'_>,
 648         checked_ty: Ty<'tcx>,
 649         expected_ty: Ty<'tcx>,
 650         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 651     ) -> bool {
 652         if self.tcx.sess.source_map().is_imported(expr.span) {
 653             // Ignore if span is from within a macro.
 654             return false;
 655         }
 656
 657         let src = if let Ok(src) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
 658             src
 659         } else {
 660             return false;
 661         };
 662
 663         // If casting this expression to a given numeric type would be appropriate in case of a type
 664         // mismatch.
 665         //
 666         // We want to minimize the amount of casting operations that are suggested, as it can be a
 667         // lossy operation with potentially bad side effects, so we only suggest when encountering
 668         // an expression that indicates that the original type couldn't be directly changed.
 669         //
 670         // For now, don't suggest casting with `as`.
 671         let can_cast = false;
 672
 673         let prefix = if let Some(hir::Node::Expr(hir::Expr {
 674             kind: hir::ExprKind::Struct(_, fields, _),
 675             ..
 676         })) = self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
 677         {
 678             // `expr` is a literal field for a struct, only suggest if appropriate
 679             match (*fields)
 680                 .iter()
 681                 .find(|field| field.expr.hir_id == expr.hir_id && field.is_shorthand)
 682             {
 683                 // This is a field literal
 684                 Some(field) => format!("{}: ", field.ident),
 685                 // Likely a field was meant, but this field wasn't found. Do not suggest anything.
 686                 None => return false,
 687             }
 688         } else {
 689             String::new()
 690         };
 691
 692         if let hir::ExprKind::Call(path, args) = &expr.kind {
 693             if let (hir::ExprKind::Path(hir::QPath::TypeRelative(base_ty, path_segment)), 1) =
 694                 (&path.kind, args.len())
 695             {
 696                 // `expr` is a conversion like `u32::from(val)`, do not suggest anything (#63697).
 697                 if let (hir::TyKind::Path(hir::QPath::Resolved(None, base_ty_path)), sym::from) =
 698                     (&base_ty.kind, path_segment.ident.name)
 699                 {
 700                     if let Some(ident) = &base_ty_path.segments.iter().map(|s| s.ident).next() {
 701                         match ident.name {
 702                             sym::i128
 703                             | sym::i64
 704                             | sym::i32
 705                             | sym::i16
 706                             | sym::i8
 707                             | sym::u128
 708                             | sym::u64
 709                             | sym::u32
 710                             | sym::u16
 711                             | sym::u8
 712                             | sym::isize
 713                             | sym::usize
 714                                 if base_ty_path.segments.len() == 1 =>
 715                             {
 716                                 return false;
 717                             }
 718                             _ => {}
 719                         }
 720                     }
 721                 }
 722             }
 723         }
 724
 725         let msg = format!(
 726             "you can convert {} `{}` to {} `{}`",
 727             checked_ty.kind().article(),
 728             checked_ty,
 729             expected_ty.kind().article(),
 730             expected_ty,
 731         );
 732         let cast_msg = format!(
 733             "you can cast {} `{}` to {} `{}`",
 734             checked_ty.kind().article(),
 735             checked_ty,
 736             expected_ty.kind().article(),
 737             expected_ty,
 738         );
 739         let lit_msg = format!(
 740             "change the type of the numeric literal from `{}` to `{}`",
 741             checked_ty, expected_ty,
 742         );
 743
 744         let with_opt_paren: fn(&dyn fmt::Display) -> String =
 745             if expr.precedence().order() < PREC_POSTFIX {
 746                 |s| format!("({})", s)
 747             } else {
 748                 |s| s.to_string()
 749             };
 750
 751         let cast_suggestion = format!("{}{} as {}", prefix, with_opt_paren(&src), expected_ty);
 752         let into_suggestion = format!("{}{}.into()", prefix, with_opt_paren(&src));
 753         let suffix_suggestion = with_opt_paren(&format_args!(
 754             "{}{}",
 755             if matches!(
 756                 (&expected_ty.kind(), &checked_ty.kind()),
 757                 (ty::Int(_) | ty::Uint(_), ty::Float(_))
 758             ) {
 759                 // Remove fractional part from literal, for example `42.0f32` into `42`
 760                 let src = src.trim_end_matches(&checked_ty.to_string());
 761                 src.split('.').next().unwrap()
 762             } else {
 763                 src.trim_end_matches(&checked_ty.to_string())
 764             },
 765             expected_ty,
 766         ));
 767         let literal_is_ty_suffixed = |expr: &hir::Expr<'_>| {
 768             if let hir::ExprKind::Lit(lit) = &expr.kind { lit.node.is_suffixed() } else { false }
 769         };
 770         let is_negative_int =
 771             |expr: &hir::Expr<'_>| matches!(expr.kind, hir::ExprKind::Unary(hir::UnOp::UnNeg, ..));
 772         let is_uint = |ty: Ty<'_>| matches!(ty.kind(), ty::Uint(..));
 773
 774         let in_const_context = self.tcx.hir().is_inside_const_context(expr.hir_id);
 775
 776         let suggest_fallible_into_or_lhs_from =
 777             |err: &mut DiagnosticBuilder<'_>, exp_to_found_is_fallible: bool| {
 778                 // If we know the expression the expected type is derived from, we might be able
 779                 // to suggest a widening conversion rather than a narrowing one (which may
 780                 // panic). For example, given x: u8 and y: u32, if we know the span of "x",
 781                 //   x > y
 782                 // can be given the suggestion "u32::from(x) > y" rather than
 783                 // "x > y.try_into().unwrap()".
 784                 let lhs_expr_and_src = expected_ty_expr.and_then(|expr| {
 785                     self.tcx
 786                         .sess
 787                         .source_map()
 788                         .span_to_snippet(expr.span)
 789                         .ok()
 790                         .map(|src| (expr, src))
 791                 });
 792                 let (span, msg, suggestion) = if let (Some((lhs_expr, lhs_src)), false) =
 793                     (lhs_expr_and_src, exp_to_found_is_fallible)
 794                 {
 795                     let msg = format!(
 796                         "you can convert `{}` from `{}` to `{}`, matching the type of `{}`",
 797                         lhs_src, expected_ty, checked_ty, src
 798                     );
 799                     let suggestion = format!("{}::from({})", checked_ty, lhs_src);
 800                     (lhs_expr.span, msg, suggestion)
 801                 } else {
 802                     let msg = format!("{} and panic if the converted value doesn't fit", msg);
 803                     let suggestion =
 804                         format!("{}{}.try_into().unwrap()", prefix, with_opt_paren(&src));
 805                     (expr.span, msg, suggestion)
 806                 };
 807                 err.span_suggestion(span, &msg, suggestion, Applicability::MachineApplicable);
 808             };
 809
 810         let suggest_to_change_suffix_or_into =
 811             |err: &mut DiagnosticBuilder<'_>,
 812              found_to_exp_is_fallible: bool,
 813              exp_to_found_is_fallible: bool| {
 814                 let always_fallible = found_to_exp_is_fallible
 815                     && (exp_to_found_is_fallible || expected_ty_expr.is_none());
 816                 let msg = if literal_is_ty_suffixed(expr) {
 817                     &lit_msg
 818                 } else if always_fallible && (is_negative_int(expr) && is_uint(expected_ty)) {
 819                     // We now know that converting either the lhs or rhs is fallible. Before we
 820                     // suggest a fallible conversion, check if the value can never fit in the
 821                     // expected type.
 822                     let msg = format!("`{}` cannot fit into type `{}`", src, expected_ty);
 823                     err.note(&msg);
 824                     return;
 825                 } else if in_const_context {
 826                     // Do not recommend `into` or `try_into` in const contexts.
 827                     return;
 828                 } else if found_to_exp_is_fallible {
 829                     return suggest_fallible_into_or_lhs_from(err, exp_to_found_is_fallible);
 830                 } else {
 831                     &msg
 832                 };
 833                 let suggestion = if literal_is_ty_suffixed(expr) {
 834                     suffix_suggestion.clone()
 835                 } else {
 836                     into_suggestion.clone()
 837                 };
 838                 err.span_suggestion(expr.span, msg, suggestion, Applicability::MachineApplicable);
 839             };
 840
 841         match (&expected_ty.kind(), &checked_ty.kind()) {
 842             (&ty::Int(ref exp), &ty::Int(ref found)) => {
 843                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
 844                 {
 845                     (Some(exp), Some(found)) if exp < found => (true, false),
 846                     (Some(exp), Some(found)) if exp > found => (false, true),
 847                     (None, Some(8 | 16)) => (false, true),
 848                     (Some(8 | 16), None) => (true, false),
 849                     (None, _) | (_, None) => (true, true),
 850                     _ => (false, false),
 851                 };
 852                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
 853                 true
 854             }
 855             (&ty::Uint(ref exp), &ty::Uint(ref found)) => {
 856                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
 857                 {
 858                     (Some(exp), Some(found)) if exp < found => (true, false),
 859                     (Some(exp), Some(found)) if exp > found => (false, true),
 860                     (None, Some(8 | 16)) => (false, true),
 861                     (Some(8 | 16), None) => (true, false),
 862                     (None, _) | (_, None) => (true, true),
 863                     _ => (false, false),
 864                 };
 865                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
 866                 true
 867             }
 868             (&ty::Int(exp), &ty::Uint(found)) => {
 869                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
 870                 {
 871                     (Some(exp), Some(found)) if found < exp => (false, true),
 872                     (None, Some(8)) => (false, true),
 873                     _ => (true, true),
 874                 };
 875                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
 876                 true
 877             }
 878             (&ty::Uint(exp), &ty::Int(found)) => {
 879                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
 880                 {
 881                     (Some(exp), Some(found)) if found > exp => (true, false),
 882                     (Some(8), None) => (true, false),
 883                     _ => (true, true),
 884                 };
 885                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
 886                 true
 887             }
 888             (&ty::Float(ref exp), &ty::Float(ref found)) => {
 889                 if found.bit_width() < exp.bit_width() {
 890                     suggest_to_change_suffix_or_into(err, false, true);
 891                 } else if literal_is_ty_suffixed(expr) {
 892                     err.span_suggestion(
 893                         expr.span,
 894                         &lit_msg,
 895                         suffix_suggestion,
 896                         Applicability::MachineApplicable,
 897                     );
 898                 } else if can_cast {
 899                     // Missing try_into implementation for `f64` to `f32`
 900                     err.span_suggestion(
 901                         expr.span,
 902                         &format!("{}, producing the closest possible value", cast_msg),
 903                         cast_suggestion,
 904                         Applicability::MaybeIncorrect, // lossy conversion
 905                     );
 906                 }
 907                 true
 908             }
 909             (&ty::Uint(_) | &ty::Int(_), &ty::Float(_)) => {
 910                 if literal_is_ty_suffixed(expr) {
 911                     err.span_suggestion(
 912                         expr.span,
 913                         &lit_msg,
 914                         suffix_suggestion,
 915                         Applicability::MachineApplicable,
 916                     );
 917                 } else if can_cast {
 918                     // Missing try_into implementation for `{float}` to `{integer}`
 919                     err.span_suggestion(
 920                         expr.span,
 921                         &format!("{}, rounding the float towards zero", msg),
 922                         cast_suggestion,
 923                         Applicability::MaybeIncorrect, // lossy conversion
 924                     );
 925                 }
 926                 true
 927             }
 928             (&ty::Float(ref exp), &ty::Uint(ref found)) => {
 929                 // if `found` is `None` (meaning found is `usize`), don't suggest `.into()`
 930                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
 931                     err.span_suggestion(
 932                         expr.span,
 933                         &format!(
 934                             "{}, producing the floating point representation of the integer",
 935                             msg,
 936                         ),
 937                         into_suggestion,
 938                         Applicability::MachineApplicable,
 939                     );
 940                 } else if literal_is_ty_suffixed(expr) {
 941                     err.span_suggestion(
 942                         expr.span,
 943                         &lit_msg,
 944                         suffix_suggestion,
 945                         Applicability::MachineApplicable,
 946                     );
 947                 } else {
 948                     // Missing try_into implementation for `{integer}` to `{float}`
 949                     err.span_suggestion(
 950                         expr.span,
 951                         &format!(
 952                             "{}, producing the floating point representation of the integer,
 953                                  rounded if necessary",
 954                             cast_msg,
 955                         ),
 956                         cast_suggestion,
 957                         Applicability::MaybeIncorrect, // lossy conversion
 958                     );
 959                 }
 960                 true
 961             }
 962             (&ty::Float(ref exp), &ty::Int(ref found)) => {
 963                 // if `found` is `None` (meaning found is `isize`), don't suggest `.into()`
 964                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
 965                     err.span_suggestion(
 966                         expr.span,
 967                         &format!(
 968                             "{}, producing the floating point representation of the integer",
 969                             &msg,
 970                         ),
 971                         into_suggestion,
 972                         Applicability::MachineApplicable,
 973                     );
 974                 } else if literal_is_ty_suffixed(expr) {
 975                     err.span_suggestion(
 976                         expr.span,
 977                         &lit_msg,
 978                         suffix_suggestion,
 979                         Applicability::MachineApplicable,
 980                     );
 981                 } else {
 982                     // Missing try_into implementation for `{integer}` to `{float}`
 983                     err.span_suggestion(
 984                         expr.span,
 985                         &format!(
 986                             "{}, producing the floating point representation of the integer, \
 987                                 rounded if necessary",
 988                             &msg,
 989                         ),
 990                         cast_suggestion,
 991                         Applicability::MaybeIncorrect, // lossy conversion
 992                     );
 993                 }
 994                 true
 995             }
 996             _ => false,
 997         }
 998     }
 999 }