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_no_capture_closure(err, expected, expr_ty);
  36         self.suggest_boxing_when_appropriate(err, expr, expected, expr_ty);
  37         self.suggest_missing_parentheses(err, expr);
  38         self.note_need_for_fn_pointer(err, expected, expr_ty);
  39         self.note_internal_mutation_in_method(err, expr, expected, expr_ty);
  40     }
  41
  42     // Requires that the two types unify, and prints an error message if
  43     // they don't.
  44     pub fn demand_suptype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  45         if let Some(mut e) = self.demand_suptype_diag(sp, expected, actual) {
  46             e.emit();
  47         }
  48     }
  49
  50     pub fn demand_suptype_diag(
  51         &self,
  52         sp: Span,
  53         expected: Ty<'tcx>,
  54         actual: Ty<'tcx>,
  55     ) -> Option<DiagnosticBuilder<'tcx>> {
  56         self.demand_suptype_with_origin(&self.misc(sp), expected, actual)
  57     }
  58
  59     pub fn demand_suptype_with_origin(
  60         &self,
  61         cause: &ObligationCause<'tcx>,
  62         expected: Ty<'tcx>,
  63         actual: Ty<'tcx>,
  64     ) -> Option<DiagnosticBuilder<'tcx>> {
  65         match self.at(cause, self.param_env).sup(expected, actual) {
  66             Ok(InferOk { obligations, value: () }) => {
  67                 self.register_predicates(obligations);
  68                 None
  69             }
  70             Err(e) => Some(self.report_mismatched_types(&cause, expected, actual, e)),
  71         }
  72     }
  73
  74     pub fn demand_eqtype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  75         if let Some(mut err) = self.demand_eqtype_diag(sp, expected, actual) {
  76             err.emit();
  77         }
  78     }
  79
  80     pub fn demand_eqtype_diag(
  81         &self,
  82         sp: Span,
  83         expected: Ty<'tcx>,
  84         actual: Ty<'tcx>,
  85     ) -> Option<DiagnosticBuilder<'tcx>> {
  86         self.demand_eqtype_with_origin(&self.misc(sp), expected, actual)
  87     }
  88
  89     pub fn demand_eqtype_with_origin(
  90         &self,
  91         cause: &ObligationCause<'tcx>,
  92         expected: Ty<'tcx>,
  93         actual: Ty<'tcx>,
  94     ) -> Option<DiagnosticBuilder<'tcx>> {
  95         match self.at(cause, self.param_env).eq(expected, actual) {
  96             Ok(InferOk { obligations, value: () }) => {
  97                 self.register_predicates(obligations);
  98                 None
  99             }
 100             Err(e) => Some(self.report_mismatched_types(cause, expected, actual, e)),
 101         }
 102     }
 103
 104     pub fn demand_coerce(
 105         &self,
 106         expr: &hir::Expr<'_>,
 107         checked_ty: Ty<'tcx>,
 108         expected: Ty<'tcx>,
 109         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 110         allow_two_phase: AllowTwoPhase,
 111     ) -> Ty<'tcx> {
 112         let (ty, err) =
 113             self.demand_coerce_diag(expr, checked_ty, expected, expected_ty_expr, allow_two_phase);
 114         if let Some(mut err) = err {
 115             err.emit();
 116         }
 117         ty
 118     }
 119
 120     /// Checks that the type of `expr` can be coerced to `expected`.
 121     ///
 122     /// N.B., this code relies on `self.diverges` to be accurate. In particular, assignments to `!`
 123     /// will be permitted if the diverges flag is currently "always".
 124     pub fn demand_coerce_diag(
 125         &self,
 126         expr: &hir::Expr<'_>,
 127         checked_ty: Ty<'tcx>,
 128         expected: Ty<'tcx>,
 129         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 130         allow_two_phase: AllowTwoPhase,
 131     ) -> (Ty<'tcx>, Option<DiagnosticBuilder<'tcx>>) {
 132         let expected = self.resolve_vars_with_obligations(expected);
 133
 134         let e = match self.try_coerce(expr, checked_ty, expected, allow_two_phase) {
 135             Ok(ty) => return (ty, None),
 136             Err(e) => e,
 137         };
 138
 139         let expr = expr.peel_drop_temps();
 140         let cause = self.misc(expr.span);
 141         let expr_ty = self.resolve_vars_with_obligations(checked_ty);
 142         let mut err = self.report_mismatched_types(&cause, expected, expr_ty, e);
 143
 144         if self.is_assign_to_bool(expr, expected) {
 145             // Error reported in `check_assign` so avoid emitting error again.
 146             err.delay_as_bug();
 147             return (expected, None);
 148         }
 149
 150         self.emit_coerce_suggestions(&mut err, expr, expr_ty, expected, expected_ty_expr);
 151
 152         (expected, Some(err))
 153     }
 154
 155     fn annotate_expected_due_to_let_ty(
 156         &self,
 157         err: &mut DiagnosticBuilder<'_>,
 158         expr: &hir::Expr<'_>,
 159     ) {
 160         let parent = self.tcx.hir().get_parent_node(expr.hir_id);
 161         if let Some(hir::Node::Local(hir::Local { ty: Some(ty), init: Some(init), .. })) =
 162             self.tcx.hir().find(parent)
 163         {
 164             if init.hir_id == expr.hir_id {
 165                 // Point at `let` assignment type.
 166                 err.span_label(ty.span, "expected due to this");
 167             }
 168         }
 169     }
 170
 171     /// Returns whether the expected type is `bool` and the expression is `x = y`.
 172     pub fn is_assign_to_bool(&self, expr: &hir::Expr<'_>, expected: Ty<'tcx>) -> bool {
 173         if let hir::ExprKind::Assign(..) = expr.kind {
 174             return expected == self.tcx.types.bool;
 175         }
 176         false
 177     }
 178
 179     /// If the expected type is an enum (Issue #55250) with any variants whose
 180     /// sole field is of the found type, suggest such variants. (Issue #42764)
 181     fn suggest_compatible_variants(
 182         &self,
 183         err: &mut DiagnosticBuilder<'_>,
 184         expr: &hir::Expr<'_>,
 185         expected: Ty<'tcx>,
 186         expr_ty: Ty<'tcx>,
 187     ) {
 188         if let ty::Adt(expected_adt, substs) = expected.kind() {
 189             if !expected_adt.is_enum() {
 190                 return;
 191             }
 192
 193             let mut compatible_variants = expected_adt
 194                 .variants
 195                 .iter()
 196                 .filter(|variant| variant.fields.len() == 1)
 197                 .filter_map(|variant| {
 198                     let sole_field = &variant.fields[0];
 199                     let sole_field_ty = sole_field.ty(self.tcx, substs);
 200                     if self.can_coerce(expr_ty, sole_field_ty) {
 201                         let variant_path = self.tcx.def_path_str(variant.def_id);
 202                         // FIXME #56861: DRYer prelude filtering
 203                         Some(variant_path.trim_start_matches("std::prelude::v1::").to_string())
 204                     } else {
 205                         None
 206                     }
 207                 })
 208                 .peekable();
 209
 210             if compatible_variants.peek().is_some() {
 211                 if let Ok(expr_text) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
 212                     let suggestions = compatible_variants.map(|v| format!("{}({})", v, expr_text));
 213                     let msg = "try using a variant of the expected enum";
 214                     err.span_suggestions(
 215                         expr.span,
 216                         msg,
 217                         suggestions,
 218                         Applicability::MaybeIncorrect,
 219                     );
 220                 }
 221             }
 222         }
 223     }
 224
 225     pub fn get_conversion_methods(
 226         &self,
 227         span: Span,
 228         expected: Ty<'tcx>,
 229         checked_ty: Ty<'tcx>,
 230         hir_id: hir::HirId,
 231     ) -> Vec<AssocItem> {
 232         let mut methods =
 233             self.probe_for_return_type(span, probe::Mode::MethodCall, expected, checked_ty, hir_id);
 234         methods.retain(|m| {
 235             self.has_only_self_parameter(m)
 236                 && self
 237                     .tcx
 238                     .get_attrs(m.def_id)
 239                     .iter()
 240                     // This special internal attribute is used to permit
 241                     // "identity-like" conversion methods to be suggested here.
 242                     //
 243                     // FIXME (#46459 and #46460): ideally
 244                     // `std::convert::Into::into` and `std::borrow:ToOwned` would
 245                     // also be `#[rustc_conversion_suggestion]`, if not for
 246                     // method-probing false-positives and -negatives (respectively).
 247                     //
 248                     // FIXME? Other potential candidate methods: `as_ref` and
 249                     // `as_mut`?
 250                     .any(|a| self.sess().check_name(a, sym::rustc_conversion_suggestion))
 251         });
 252
 253         methods
 254     }
 255
 256     /// This function checks whether the method is not static and does not accept other parameters than `self`.
 257     fn has_only_self_parameter(&self, method: &AssocItem) -> bool {
 258         match method.kind {
 259             ty::AssocKind::Fn => {
 260                 method.fn_has_self_parameter
 261                     && self.tcx.fn_sig(method.def_id).inputs().skip_binder().len() == 1
 262             }
 263             _ => false,
 264         }
 265     }
 266
 267     /// Identify some cases where `as_ref()` would be appropriate and suggest it.
 268     ///
 269     /// Given the following code:
 270     /// ```
 271     /// struct Foo;
 272     /// fn takes_ref(_: &Foo) {}
 273     /// let ref opt = Some(Foo);
 274     ///
 275     /// opt.map(|param| takes_ref(param));
 276     /// ```
 277     /// Suggest using `opt.as_ref().map(|param| takes_ref(param));` instead.
 278     ///
 279     /// It only checks for `Option` and `Result` and won't work with
 280     /// ```
 281     /// opt.map(|param| { takes_ref(param) });
 282     /// ```
 283     fn can_use_as_ref(&self, expr: &hir::Expr<'_>) -> Option<(Span, &'static str, String)> {
 284         let path = match expr.kind {
 285             hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => path,
 286             _ => return None,
 287         };
 288
 289         let local_id = match path.res {
 290             hir::def::Res::Local(id) => id,
 291             _ => return None,
 292         };
 293
 294         let local_parent = self.tcx.hir().get_parent_node(local_id);
 295         let param_hir_id = match self.tcx.hir().find(local_parent) {
 296             Some(Node::Param(hir::Param { hir_id, .. })) => hir_id,
 297             _ => return None,
 298         };
 299
 300         let param_parent = self.tcx.hir().get_parent_node(*param_hir_id);
 301         let (expr_hir_id, closure_fn_decl) = match self.tcx.hir().find(param_parent) {
 302             Some(Node::Expr(hir::Expr {
 303                 hir_id,
 304                 kind: hir::ExprKind::Closure(_, decl, ..),
 305                 ..
 306             })) => (hir_id, decl),
 307             _ => return None,
 308         };
 309
 310         let expr_parent = self.tcx.hir().get_parent_node(*expr_hir_id);
 311         let hir = self.tcx.hir().find(expr_parent);
 312         let closure_params_len = closure_fn_decl.inputs.len();
 313         let (method_path, method_span, method_expr) = match (hir, closure_params_len) {
 314             (
 315                 Some(Node::Expr(hir::Expr {
 316                     kind: hir::ExprKind::MethodCall(path, span, expr, _),
 317                     ..
 318                 })),
 319                 1,
 320             ) => (path, span, expr),
 321             _ => return None,
 322         };
 323
 324         let self_ty = self.typeck_results.borrow().node_type(method_expr[0].hir_id);
 325         let self_ty = format!("{:?}", self_ty);
 326         let name = method_path.ident.name;
 327         let is_as_ref_able = (self_ty.starts_with("&std::option::Option")
 328             || self_ty.starts_with("&std::result::Result")
 329             || self_ty.starts_with("std::option::Option")
 330             || self_ty.starts_with("std::result::Result"))
 331             && (name == sym::map || name == sym::and_then);
 332         match (is_as_ref_able, self.sess().source_map().span_to_snippet(*method_span)) {
 333             (true, Ok(src)) => {
 334                 let suggestion = format!("as_ref().{}", src);
 335                 Some((*method_span, "consider using `as_ref` instead", suggestion))
 336             }
 337             _ => None,
 338         }
 339     }
 340
 341     crate fn is_hir_id_from_struct_pattern_shorthand_field(
 342         &self,
 343         hir_id: hir::HirId,
 344         sp: Span,
 345     ) -> bool {
 346         let sm = self.sess().source_map();
 347         let parent_id = self.tcx.hir().get_parent_node(hir_id);
 348         if let Some(parent) = self.tcx.hir().find(parent_id) {
 349             // Account for fields
 350             if let Node::Expr(hir::Expr { kind: hir::ExprKind::Struct(_, fields, ..), .. }) = parent
 351             {
 352                 if let Ok(src) = sm.span_to_snippet(sp) {
 353                     for field in *fields {
 354                         if field.ident.as_str() == src && field.is_shorthand {
 355                             return true;
 356                         }
 357                     }
 358                 }
 359             }
 360         }
 361         false
 362     }
 363
 364     /// This function is used to determine potential "simple" improvements or users' errors and
 365     /// provide them useful help. For example:
 366     ///
 367     /// ```
 368     /// fn some_fn(s: &str) {}
 369     ///
 370     /// let x = "hey!".to_owned();
 371     /// some_fn(x); // error
 372     /// ```
 373     ///
 374     /// No need to find every potential function which could make a coercion to transform a
 375     /// `String` into a `&str` since a `&` would do the trick!
 376     ///
 377     /// In addition of this check, it also checks between references mutability state. If the
 378     /// expected is mutable but the provided isn't, maybe we could just say "Hey, try with
 379     /// `&mut`!".
 380     pub fn check_ref(
 381         &self,
 382         expr: &hir::Expr<'_>,
 383         checked_ty: Ty<'tcx>,
 384         expected: Ty<'tcx>,
 385     ) -> Option<(Span, &'static str, String, Applicability)> {
 386         let sm = self.sess().source_map();
 387         let sp = expr.span;
 388         if sm.is_imported(sp) {
 389             // Ignore if span is from within a macro #41858, #58298. We previously used the macro
 390             // call span, but that breaks down when the type error comes from multiple calls down.
 391             return None;
 392         }
 393
 394         let replace_prefix = |s: &str, old: &str, new: &str| {
 395             s.strip_prefix(old).map(|stripped| new.to_string() + stripped)
 396         };
 397
 398         let is_struct_pat_shorthand_field =
 399             self.is_hir_id_from_struct_pattern_shorthand_field(expr.hir_id, sp);
 400
 401         // If the span is from a macro, then it's hard to extract the text
 402         // and make a good suggestion, so don't bother.
 403         let is_macro = sp.from_expansion() && sp.desugaring_kind().is_none();
 404
 405         // `ExprKind::DropTemps` is semantically irrelevant for these suggestions.
 406         let expr = expr.peel_drop_temps();
 407
 408         match (&expr.kind, expected.kind(), checked_ty.kind()) {
 409             (_, &ty::Ref(_, exp, _), &ty::Ref(_, check, _)) => match (exp.kind(), check.kind()) {
 410                 (&ty::Str, &ty::Array(arr, _) | &ty::Slice(arr)) if arr == self.tcx.types.u8 => {
 411                     if let hir::ExprKind::Lit(_) = expr.kind {
 412                         if let Ok(src) = sm.span_to_snippet(sp) {
 413                             if let Some(src) = replace_prefix(&src, "b\"", "\"") {
 414                                 return Some((
 415                                     sp,
 416                                     "consider removing the leading `b`",
 417                                     src,
 418                                     Applicability::MachineApplicable,
 419                                 ));
 420                             }
 421                         }
 422                     }
 423                 }
 424                 (&ty::Array(arr, _) | &ty::Slice(arr), &ty::Str) if arr == self.tcx.types.u8 => {
 425                     if let hir::ExprKind::Lit(_) = expr.kind {
 426                         if let Ok(src) = sm.span_to_snippet(sp) {
 427                             if let Some(src) = replace_prefix(&src, "\"", "b\"") {
 428                                 return Some((
 429                                     sp,
 430                                     "consider adding a leading `b`",
 431                                     src,
 432                                     Applicability::MachineApplicable,
 433                                 ));
 434                             }
 435                         }
 436                     }
 437                 }
 438                 _ => {}
 439             },
 440             (_, &ty::Ref(_, _, mutability), _) => {
 441                 // Check if it can work when put into a ref. For example:
 442                 //
 443                 // ```
 444                 // fn bar(x: &mut i32) {}
 445                 //
 446                 // let x = 0u32;
 447                 // bar(&x); // error, expected &mut
 448                 // ```
 449                 let ref_ty = match mutability {
 450                     hir::Mutability::Mut => {
 451                         self.tcx.mk_mut_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 452                     }
 453                     hir::Mutability::Not => {
 454                         self.tcx.mk_imm_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 455                     }
 456                 };
 457                 if self.can_coerce(ref_ty, expected) {
 458                     let mut sugg_sp = sp;
 459                     if let hir::ExprKind::MethodCall(ref segment, sp, ref args, _) = expr.kind {
 460                         let clone_trait = self.tcx.require_lang_item(LangItem::Clone, Some(sp));
 461                         if let ([arg], Some(true), sym::clone) = (
 462                             &args[..],
 463                             self.typeck_results.borrow().type_dependent_def_id(expr.hir_id).map(
 464                                 |did| {
 465                                     let ai = self.tcx.associated_item(did);
 466                                     ai.container == ty::TraitContainer(clone_trait)
 467                                 },
 468                             ),
 469                             segment.ident.name,
 470                         ) {
 471                             // If this expression had a clone call when suggesting borrowing
 472                             // we want to suggest removing it because it'd now be unnecessary.
 473                             sugg_sp = arg.span;
 474                         }
 475                     }
 476                     if let Ok(src) = sm.span_to_snippet(sugg_sp) {
 477                         let needs_parens = match expr.kind {
 478                             // parenthesize if needed (Issue #46756)
 479                             hir::ExprKind::Cast(_, _) | hir::ExprKind::Binary(_, _, _) => true,
 480                             // parenthesize borrows of range literals (Issue #54505)
 481                             _ if is_range_literal(expr) => true,
 482                             _ => false,
 483                         };
 484                         let sugg_expr = if needs_parens { format!("({})", src) } else { src };
 485
 486                         if let Some(sugg) = self.can_use_as_ref(expr) {
 487                             return Some((
 488                                 sugg.0,
 489                                 sugg.1,
 490                                 sugg.2,
 491                                 Applicability::MachineApplicable,
 492                             ));
 493                         }
 494                         let field_name = if is_struct_pat_shorthand_field {
 495                             format!("{}: ", sugg_expr)
 496                         } else {
 497                             String::new()
 498                         };
 499                         if let Some(hir::Node::Expr(hir::Expr {
 500                             kind: hir::ExprKind::Assign(left_expr, ..),
 501                             ..
 502                         })) = self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
 503                         {
 504                             if mutability == hir::Mutability::Mut {
 505                                 // Found the following case:
 506                                 // fn foo(opt: &mut Option<String>){ opt = None }
 507                                 //                                   ---   ^^^^
 508                                 //                                   |     |
 509                                 //    consider dereferencing here: `*opt`  |
 510                                 // expected mutable reference, found enum `Option`
 511                                 if let Ok(src) = sm.span_to_snippet(left_expr.span) {
 512                                     return Some((
 513                                         left_expr.span,
 514                                         "consider dereferencing here to assign to the mutable \
 515                                          borrowed piece of memory",
 516                                         format!("*{}", src),
 517                                         Applicability::MachineApplicable,
 518                                     ));
 519                                 }
 520                             }
 521                         }
 522
 523                         return Some(match mutability {
 524                             hir::Mutability::Mut => (
 525                                 sp,
 526                                 "consider mutably borrowing here",
 527                                 format!("{}&mut {}", field_name, sugg_expr),
 528                                 Applicability::MachineApplicable,
 529                             ),
 530                             hir::Mutability::Not => (
 531                                 sp,
 532                                 "consider borrowing here",
 533                                 format!("{}&{}", field_name, sugg_expr),
 534                                 Applicability::MachineApplicable,
 535                             ),
 536                         });
 537                     }
 538                 }
 539             }
 540             (
 541                 hir::ExprKind::AddrOf(hir::BorrowKind::Ref, _, ref expr),
 542                 _,
 543                 &ty::Ref(_, checked, _),
 544             ) if {
 545                 self.infcx.can_sub(self.param_env, checked, &expected).is_ok() && !is_macro
 546             } =>
 547             {
 548                 // We have `&T`, check if what was expected was `T`. If so,
 549                 // we may want to suggest removing a `&`.
 550                 if sm.is_imported(expr.span) {
 551                     if let Ok(src) = sm.span_to_snippet(sp) {
 552                         if let Some(src) = src.strip_prefix('&') {
 553                             return Some((
 554                                 sp,
 555                                 "consider removing the borrow",
 556                                 src.to_string(),
 557                                 Applicability::MachineApplicable,
 558                             ));
 559                         }
 560                     }
 561                     return None;
 562                 }
 563                 if let Ok(code) = sm.span_to_snippet(expr.span) {
 564                     return Some((
 565                         sp,
 566                         "consider removing the borrow",
 567                         code,
 568                         Applicability::MachineApplicable,
 569                     ));
 570                 }
 571             }
 572             (
 573                 _,
 574                 &ty::RawPtr(TypeAndMut { ty: ty_b, mutbl: mutbl_b }),
 575                 &ty::Ref(_, ty_a, mutbl_a),
 576             ) => {
 577                 if let Some(steps) = self.deref_steps(ty_a, ty_b) {
 578                     // Only suggest valid if dereferencing needed.
 579                     if steps > 0 {
 580                         // The pointer type implements `Copy` trait so the suggestion is always valid.
 581                         if let Ok(src) = sm.span_to_snippet(sp) {
 582                             let derefs = &"*".repeat(steps);
 583                             if let Some((src, applicability)) = match mutbl_b {
 584                                 hir::Mutability::Mut => {
 585                                     let new_prefix = "&mut ".to_owned() + derefs;
 586                                     match mutbl_a {
 587                                         hir::Mutability::Mut => {
 588                                             replace_prefix(&src, "&mut ", &new_prefix)
 589                                                 .map(|s| (s, Applicability::MachineApplicable))
 590                                         }
 591                                         hir::Mutability::Not => {
 592                                             replace_prefix(&src, "&", &new_prefix)
 593                                                 .map(|s| (s, Applicability::Unspecified))
 594                                         }
 595                                     }
 596                                 }
 597                                 hir::Mutability::Not => {
 598                                     let new_prefix = "&".to_owned() + derefs;
 599                                     match mutbl_a {
 600                                         hir::Mutability::Mut => {
 601                                             replace_prefix(&src, "&mut ", &new_prefix)
 602                                                 .map(|s| (s, Applicability::MachineApplicable))
 603                                         }
 604                                         hir::Mutability::Not => {
 605                                             replace_prefix(&src, "&", &new_prefix)
 606                                                 .map(|s| (s, Applicability::MachineApplicable))
 607                                         }
 608                                     }
 609                                 }
 610                             } {
 611                                 return Some((sp, "consider dereferencing", src, applicability));
 612                             }
 613                         }
 614                     }
 615                 }
 616             }
 617             _ if sp == expr.span && !is_macro => {
 618                 if let Some(steps) = self.deref_steps(checked_ty, expected) {
 619                     if steps == 1 {
 620                         // For a suggestion to make sense, the type would need to be `Copy`.
 621                         if self.infcx.type_is_copy_modulo_regions(self.param_env, expected, sp) {
 622                             if let Ok(code) = sm.span_to_snippet(sp) {
 623                                 let message = if checked_ty.is_region_ptr() {
 624                                     "consider dereferencing the borrow"
 625                                 } else {
 626                                     "consider dereferencing the type"
 627                                 };
 628                                 let suggestion = if is_struct_pat_shorthand_field {
 629                                     format!("{}: *{}", code, code)
 630                                 } else {
 631                                     format!("*{}", code)
 632                                 };
 633                                 return Some((
 634                                     sp,
 635                                     message,
 636                                     suggestion,
 637                                     Applicability::MachineApplicable,
 638                                 ));
 639                             }
 640                         }
 641                     }
 642                 }
 643             }
 644             _ => {}
 645         }
 646         None
 647     }
 648
 649     pub fn check_for_cast(
 650         &self,
 651         err: &mut DiagnosticBuilder<'_>,
 652         expr: &hir::Expr<'_>,
 653         checked_ty: Ty<'tcx>,
 654         expected_ty: Ty<'tcx>,
 655         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 656     ) -> bool {
 657         if self.tcx.sess.source_map().is_imported(expr.span) {
 658             // Ignore if span is from within a macro.
 659             return false;
 660         }
 661
 662         let src = if let Ok(src) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
 663             src
 664         } else {
 665             return false;
 666         };
 667
 668         // If casting this expression to a given numeric type would be appropriate in case of a type
 669         // mismatch.
 670         //
 671         // We want to minimize the amount of casting operations that are suggested, as it can be a
 672         // lossy operation with potentially bad side effects, so we only suggest when encountering
 673         // an expression that indicates that the original type couldn't be directly changed.
 674         //
 675         // For now, don't suggest casting with `as`.
 676         let can_cast = false;
 677
 678         let prefix = if let Some(hir::Node::Expr(hir::Expr {
 679             kind: hir::ExprKind::Struct(_, fields, _),
 680             ..
 681         })) = self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
 682         {
 683             // `expr` is a literal field for a struct, only suggest if appropriate
 684             match (*fields)
 685                 .iter()
 686                 .find(|field| field.expr.hir_id == expr.hir_id && field.is_shorthand)
 687             {
 688                 // This is a field literal
 689                 Some(field) => format!("{}: ", field.ident),
 690                 // Likely a field was meant, but this field wasn't found. Do not suggest anything.
 691                 None => return false,
 692             }
 693         } else {
 694             String::new()
 695         };
 696
 697         if let hir::ExprKind::Call(path, args) = &expr.kind {
 698             if let (hir::ExprKind::Path(hir::QPath::TypeRelative(base_ty, path_segment)), 1) =
 699                 (&path.kind, args.len())
 700             {
 701                 // `expr` is a conversion like `u32::from(val)`, do not suggest anything (#63697).
 702                 if let (hir::TyKind::Path(hir::QPath::Resolved(None, base_ty_path)), sym::from) =
 703                     (&base_ty.kind, path_segment.ident.name)
 704                 {
 705                     if let Some(ident) = &base_ty_path.segments.iter().map(|s| s.ident).next() {
 706                         match ident.name {
 707                             sym::i128
 708                             | sym::i64
 709                             | sym::i32
 710                             | sym::i16
 711                             | sym::i8
 712                             | sym::u128
 713                             | sym::u64
 714                             | sym::u32
 715                             | sym::u16
 716                             | sym::u8
 717                             | sym::isize
 718                             | sym::usize
 719                                 if base_ty_path.segments.len() == 1 =>
 720                             {
 721                                 return false;
 722                             }
 723                             _ => {}
 724                         }
 725                     }
 726                 }
 727             }
 728         }
 729
 730         let msg = format!(
 731             "you can convert {} `{}` to {} `{}`",
 732             checked_ty.kind().article(),
 733             checked_ty,
 734             expected_ty.kind().article(),
 735             expected_ty,
 736         );
 737         let cast_msg = format!(
 738             "you can cast {} `{}` to {} `{}`",
 739             checked_ty.kind().article(),
 740             checked_ty,
 741             expected_ty.kind().article(),
 742             expected_ty,
 743         );
 744         let lit_msg = format!(
 745             "change the type of the numeric literal from `{}` to `{}`",
 746             checked_ty, expected_ty,
 747         );
 748
 749         let with_opt_paren: fn(&dyn fmt::Display) -> String =
 750             if expr.precedence().order() < PREC_POSTFIX {
 751                 |s| format!("({})", s)
 752             } else {
 753                 |s| s.to_string()
 754             };
 755
 756         let cast_suggestion = format!("{}{} as {}", prefix, with_opt_paren(&src), expected_ty);
 757         let into_suggestion = format!("{}{}.into()", prefix, with_opt_paren(&src));
 758         let suffix_suggestion = with_opt_paren(&format_args!(
 759             "{}{}",
 760             if matches!(
 761                 (&expected_ty.kind(), &checked_ty.kind()),
 762                 (ty::Int(_) | ty::Uint(_), ty::Float(_))
 763             ) {
 764                 // Remove fractional part from literal, for example `42.0f32` into `42`
 765                 let src = src.trim_end_matches(&checked_ty.to_string());
 766                 src.split('.').next().unwrap()
 767             } else {
 768                 src.trim_end_matches(&checked_ty.to_string())
 769             },
 770             expected_ty,
 771         ));
 772         let literal_is_ty_suffixed = |expr: &hir::Expr<'_>| {
 773             if let hir::ExprKind::Lit(lit) = &expr.kind { lit.node.is_suffixed() } else { false }
 774         };
 775         let is_negative_int =
 776             |expr: &hir::Expr<'_>| matches!(expr.kind, hir::ExprKind::Unary(hir::UnOp::Neg, ..));
 777         let is_uint = |ty: Ty<'_>| matches!(ty.kind(), ty::Uint(..));
 778
 779         let in_const_context = self.tcx.hir().is_inside_const_context(expr.hir_id);
 780
 781         let suggest_fallible_into_or_lhs_from =
 782             |err: &mut DiagnosticBuilder<'_>, exp_to_found_is_fallible: bool| {
 783                 // If we know the expression the expected type is derived from, we might be able
 784                 // to suggest a widening conversion rather than a narrowing one (which may
 785                 // panic). For example, given x: u8 and y: u32, if we know the span of "x",
 786                 //   x > y
 787                 // can be given the suggestion "u32::from(x) > y" rather than
 788                 // "x > y.try_into().unwrap()".
 789                 let lhs_expr_and_src = expected_ty_expr.and_then(|expr| {
 790                     self.tcx
 791                         .sess
 792                         .source_map()
 793                         .span_to_snippet(expr.span)
 794                         .ok()
 795                         .map(|src| (expr, src))
 796                 });
 797                 let (span, msg, suggestion) = if let (Some((lhs_expr, lhs_src)), false) =
 798                     (lhs_expr_and_src, exp_to_found_is_fallible)
 799                 {
 800                     let msg = format!(
 801                         "you can convert `{}` from `{}` to `{}`, matching the type of `{}`",
 802                         lhs_src, expected_ty, checked_ty, src
 803                     );
 804                     let suggestion = format!("{}::from({})", checked_ty, lhs_src);
 805                     (lhs_expr.span, msg, suggestion)
 806                 } else {
 807                     let msg = format!("{} and panic if the converted value doesn't fit", msg);
 808                     let suggestion =
 809                         format!("{}{}.try_into().unwrap()", prefix, with_opt_paren(&src));
 810                     (expr.span, msg, suggestion)
 811                 };
 812                 err.span_suggestion(span, &msg, suggestion, Applicability::MachineApplicable);
 813             };
 814
 815         let suggest_to_change_suffix_or_into =
 816             |err: &mut DiagnosticBuilder<'_>,
 817              found_to_exp_is_fallible: bool,
 818              exp_to_found_is_fallible: bool| {
 819                 let exp_is_lhs =
 820                     expected_ty_expr.map(|e| self.tcx.hir().is_lhs(e.hir_id)).unwrap_or(false);
 821
 822                 if exp_is_lhs {
 823                     return;
 824                 }
 825
 826                 let always_fallible = found_to_exp_is_fallible
 827                     && (exp_to_found_is_fallible || expected_ty_expr.is_none());
 828                 let msg = if literal_is_ty_suffixed(expr) {
 829                     &lit_msg
 830                 } else if always_fallible && (is_negative_int(expr) && is_uint(expected_ty)) {
 831                     // We now know that converting either the lhs or rhs is fallible. Before we
 832                     // suggest a fallible conversion, check if the value can never fit in the
 833                     // expected type.
 834                     let msg = format!("`{}` cannot fit into type `{}`", src, expected_ty);
 835                     err.note(&msg);
 836                     return;
 837                 } else if in_const_context {
 838                     // Do not recommend `into` or `try_into` in const contexts.
 839                     return;
 840                 } else if found_to_exp_is_fallible {
 841                     return suggest_fallible_into_or_lhs_from(err, exp_to_found_is_fallible);
 842                 } else {
 843                     &msg
 844                 };
 845                 let suggestion = if literal_is_ty_suffixed(expr) {
 846                     suffix_suggestion.clone()
 847                 } else {
 848                     into_suggestion.clone()
 849                 };
 850                 err.span_suggestion(expr.span, msg, suggestion, Applicability::MachineApplicable);
 851             };
 852
 853         match (&expected_ty.kind(), &checked_ty.kind()) {
 854             (&ty::Int(ref exp), &ty::Int(ref found)) => {
 855                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
 856                 {
 857                     (Some(exp), Some(found)) if exp < found => (true, false),
 858                     (Some(exp), Some(found)) if exp > found => (false, true),
 859                     (None, Some(8 | 16)) => (false, true),
 860                     (Some(8 | 16), None) => (true, false),
 861                     (None, _) | (_, None) => (true, true),
 862                     _ => (false, false),
 863                 };
 864                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
 865                 true
 866             }
 867             (&ty::Uint(ref exp), &ty::Uint(ref found)) => {
 868                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
 869                 {
 870                     (Some(exp), Some(found)) if exp < found => (true, false),
 871                     (Some(exp), Some(found)) if exp > found => (false, true),
 872                     (None, Some(8 | 16)) => (false, true),
 873                     (Some(8 | 16), None) => (true, false),
 874                     (None, _) | (_, None) => (true, true),
 875                     _ => (false, false),
 876                 };
 877                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
 878                 true
 879             }
 880             (&ty::Int(exp), &ty::Uint(found)) => {
 881                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
 882                 {
 883                     (Some(exp), Some(found)) if found < exp => (false, true),
 884                     (None, Some(8)) => (false, true),
 885                     _ => (true, true),
 886                 };
 887                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
 888                 true
 889             }
 890             (&ty::Uint(exp), &ty::Int(found)) => {
 891                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
 892                 {
 893                     (Some(exp), Some(found)) if found > exp => (true, false),
 894                     (Some(8), None) => (true, false),
 895                     _ => (true, true),
 896                 };
 897                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
 898                 true
 899             }
 900             (&ty::Float(ref exp), &ty::Float(ref found)) => {
 901                 if found.bit_width() < exp.bit_width() {
 902                     suggest_to_change_suffix_or_into(err, false, true);
 903                 } else if literal_is_ty_suffixed(expr) {
 904                     err.span_suggestion(
 905                         expr.span,
 906                         &lit_msg,
 907                         suffix_suggestion,
 908                         Applicability::MachineApplicable,
 909                     );
 910                 } else if can_cast {
 911                     // Missing try_into implementation for `f64` to `f32`
 912                     err.span_suggestion(
 913                         expr.span,
 914                         &format!("{}, producing the closest possible value", cast_msg),
 915                         cast_suggestion,
 916                         Applicability::MaybeIncorrect, // lossy conversion
 917                     );
 918                 }
 919                 true
 920             }
 921             (&ty::Uint(_) | &ty::Int(_), &ty::Float(_)) => {
 922                 if literal_is_ty_suffixed(expr) {
 923                     err.span_suggestion(
 924                         expr.span,
 925                         &lit_msg,
 926                         suffix_suggestion,
 927                         Applicability::MachineApplicable,
 928                     );
 929                 } else if can_cast {
 930                     // Missing try_into implementation for `{float}` to `{integer}`
 931                     err.span_suggestion(
 932                         expr.span,
 933                         &format!("{}, rounding the float towards zero", msg),
 934                         cast_suggestion,
 935                         Applicability::MaybeIncorrect, // lossy conversion
 936                     );
 937                 }
 938                 true
 939             }
 940             (&ty::Float(ref exp), &ty::Uint(ref found)) => {
 941                 // if `found` is `None` (meaning found is `usize`), don't suggest `.into()`
 942                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
 943                     err.span_suggestion(
 944                         expr.span,
 945                         &format!(
 946                             "{}, producing the floating point representation of the integer",
 947                             msg,
 948                         ),
 949                         into_suggestion,
 950                         Applicability::MachineApplicable,
 951                     );
 952                 } else if literal_is_ty_suffixed(expr) {
 953                     err.span_suggestion(
 954                         expr.span,
 955                         &lit_msg,
 956                         suffix_suggestion,
 957                         Applicability::MachineApplicable,
 958                     );
 959                 } else {
 960                     // Missing try_into implementation for `{integer}` to `{float}`
 961                     err.span_suggestion(
 962                         expr.span,
 963                         &format!(
 964                             "{}, producing the floating point representation of the integer,
 965                                  rounded if necessary",
 966                             cast_msg,
 967                         ),
 968                         cast_suggestion,
 969                         Applicability::MaybeIncorrect, // lossy conversion
 970                     );
 971                 }
 972                 true
 973             }
 974             (&ty::Float(ref exp), &ty::Int(ref found)) => {
 975                 // if `found` is `None` (meaning found is `isize`), don't suggest `.into()`
 976                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
 977                     err.span_suggestion(
 978                         expr.span,
 979                         &format!(
 980                             "{}, producing the floating point representation of the integer",
 981                             &msg,
 982                         ),
 983                         into_suggestion,
 984                         Applicability::MachineApplicable,
 985                     );
 986                 } else if literal_is_ty_suffixed(expr) {
 987                     err.span_suggestion(
 988                         expr.span,
 989                         &lit_msg,
 990                         suffix_suggestion,
 991                         Applicability::MachineApplicable,
 992                     );
 993                 } else {
 994                     // Missing try_into implementation for `{integer}` to `{float}`
 995                     err.span_suggestion(
 996                         expr.span,
 997                         &format!(
 998                             "{}, producing the floating point representation of the integer, \
 999                                 rounded if necessary",
1000                             &msg,
1001                         ),
1002                         cast_suggestion,
1003                         Applicability::MaybeIncorrect, // lossy conversion
1004                     );
1005                 }
1006                 true
1007             }
1008             _ => false,
1009         }
1010     }
1011 }