src/librustc_typeck/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_await(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
 123     // particular, assignments to `!` will be permitted if the
 124     // diverges flag is currently "always".
 125     pub fn demand_coerce_diag(
 126         &self,
 127         expr: &hir::Expr<'_>,
 128         checked_ty: Ty<'tcx>,
 129         expected: Ty<'tcx>,
 130         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 131         allow_two_phase: AllowTwoPhase,
 132     ) -> (Ty<'tcx>, Option<DiagnosticBuilder<'tcx>>) {
 133         let expected = self.resolve_vars_with_obligations(expected);
 134
 135         let e = match self.try_coerce(expr, checked_ty, expected, allow_two_phase) {
 136             Ok(ty) => return (ty, None),
 137             Err(e) => e,
 138         };
 139
 140         let expr = expr.peel_drop_temps();
 141         let cause = self.misc(expr.span);
 142         let expr_ty = self.resolve_vars_with_obligations(checked_ty);
 143         let mut err = self.report_mismatched_types(&cause, expected, expr_ty, e);
 144
 145         if self.is_assign_to_bool(expr, expected) {
 146             // Error reported in `check_assign` so avoid emitting error again.
 147             err.delay_as_bug();
 148             return (expected, None);
 149         }
 150
 151         self.emit_coerce_suggestions(&mut err, expr, expr_ty, expected, expected_ty_expr);
 152
 153         (expected, Some(err))
 154     }
 155
 156     fn annotate_expected_due_to_let_ty(
 157         &self,
 158         err: &mut DiagnosticBuilder<'_>,
 159         expr: &hir::Expr<'_>,
 160     ) {
 161         let parent = self.tcx.hir().get_parent_node(expr.hir_id);
 162         if let Some(hir::Node::Local(hir::Local { ty: Some(ty), init: Some(init), .. })) =
 163             self.tcx.hir().find(parent)
 164         {
 165             if init.hir_id == expr.hir_id {
 166                 // Point at `let` assignment type.
 167                 err.span_label(ty.span, "expected due to this");
 168             }
 169         }
 170     }
 171
 172     /// Returns whether the expected type is `bool` and the expression is `x = y`.
 173     pub fn is_assign_to_bool(&self, expr: &hir::Expr<'_>, expected: Ty<'tcx>) -> bool {
 174         if let hir::ExprKind::Assign(..) = expr.kind {
 175             return expected == self.tcx.types.bool;
 176         }
 177         false
 178     }
 179
 180     /// If the expected type is an enum (Issue #55250) with any variants whose
 181     /// sole field is of the found type, suggest such variants. (Issue #42764)
 182     fn suggest_compatible_variants(
 183         &self,
 184         err: &mut DiagnosticBuilder<'_>,
 185         expr: &hir::Expr<'_>,
 186         expected: Ty<'tcx>,
 187         expr_ty: Ty<'tcx>,
 188     ) {
 189         if let ty::Adt(expected_adt, substs) = expected.kind {
 190             if !expected_adt.is_enum() {
 191                 return;
 192             }
 193
 194             let mut compatible_variants = expected_adt
 195                 .variants
 196                 .iter()
 197                 .filter(|variant| variant.fields.len() == 1)
 198                 .filter_map(|variant| {
 199                     let sole_field = &variant.fields[0];
 200                     let sole_field_ty = sole_field.ty(self.tcx, substs);
 201                     if self.can_coerce(expr_ty, sole_field_ty) {
 202                         let variant_path = self.tcx.def_path_str(variant.def_id);
 203                         // FIXME #56861: DRYer prelude filtering
 204                         Some(variant_path.trim_start_matches("std::prelude::v1::").to_string())
 205                     } else {
 206                         None
 207                     }
 208                 })
 209                 .peekable();
 210
 211             if compatible_variants.peek().is_some() {
 212                 if let Ok(expr_text) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
 213                     let suggestions = compatible_variants.map(|v| format!("{}({})", v, expr_text));
 214                     let msg = "try using a variant of the expected enum";
 215                     err.span_suggestions(
 216                         expr.span,
 217                         msg,
 218                         suggestions,
 219                         Applicability::MaybeIncorrect,
 220                     );
 221                 }
 222             }
 223         }
 224     }
 225
 226     pub fn get_conversion_methods(
 227         &self,
 228         span: Span,
 229         expected: Ty<'tcx>,
 230         checked_ty: Ty<'tcx>,
 231         hir_id: hir::HirId,
 232     ) -> Vec<AssocItem> {
 233         let mut methods =
 234             self.probe_for_return_type(span, probe::Mode::MethodCall, expected, checked_ty, hir_id);
 235         methods.retain(|m| {
 236             self.has_only_self_parameter(m)
 237                 && self
 238                     .tcx
 239                     .get_attrs(m.def_id)
 240                     .iter()
 241                     // This special internal attribute is used to permit
 242                     // "identity-like" conversion methods to be suggested here.
 243                     //
 244                     // FIXME (#46459 and #46460): ideally
 245                     // `std::convert::Into::into` and `std::borrow:ToOwned` would
 246                     // also be `#[rustc_conversion_suggestion]`, if not for
 247                     // method-probing false-positives and -negatives (respectively).
 248                     //
 249                     // FIXME? Other potential candidate methods: `as_ref` and
 250                     // `as_mut`?
 251                     .any(|a| self.sess().check_name(a, sym::rustc_conversion_suggestion))
 252         });
 253
 254         methods
 255     }
 256
 257     /// This function checks whether the method is not static and does not accept other parameters than `self`.
 258     fn has_only_self_parameter(&self, method: &AssocItem) -> bool {
 259         match method.kind {
 260             ty::AssocKind::Fn => {
 261                 method.fn_has_self_parameter
 262                     && self.tcx.fn_sig(method.def_id).inputs().skip_binder().len() == 1
 263             }
 264             _ => false,
 265         }
 266     }
 267
 268     /// Identify some cases where `as_ref()` would be appropriate and suggest it.
 269     ///
 270     /// Given the following code:
 271     /// ```
 272     /// struct Foo;
 273     /// fn takes_ref(_: &Foo) {}
 274     /// let ref opt = Some(Foo);
 275     ///
 276     /// opt.map(|param| takes_ref(param));
 277     /// ```
 278     /// Suggest using `opt.as_ref().map(|param| takes_ref(param));` instead.
 279     ///
 280     /// It only checks for `Option` and `Result` and won't work with
 281     /// ```
 282     /// opt.map(|param| { takes_ref(param) });
 283     /// ```
 284     fn can_use_as_ref(&self, expr: &hir::Expr<'_>) -> Option<(Span, &'static str, String)> {
 285         let path = match expr.kind {
 286             hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => path,
 287             _ => return None,
 288         };
 289
 290         let local_id = match path.res {
 291             hir::def::Res::Local(id) => id,
 292             _ => return None,
 293         };
 294
 295         let local_parent = self.tcx.hir().get_parent_node(local_id);
 296         let param_hir_id = match self.tcx.hir().find(local_parent) {
 297             Some(Node::Param(hir::Param { hir_id, .. })) => hir_id,
 298             _ => return None,
 299         };
 300
 301         let param_parent = self.tcx.hir().get_parent_node(*param_hir_id);
 302         let (expr_hir_id, closure_fn_decl) = match self.tcx.hir().find(param_parent) {
 303             Some(Node::Expr(hir::Expr {
 304                 hir_id,
 305                 kind: hir::ExprKind::Closure(_, decl, ..),
 306                 ..
 307             })) => (hir_id, decl),
 308             _ => return None,
 309         };
 310
 311         let expr_parent = self.tcx.hir().get_parent_node(*expr_hir_id);
 312         let hir = self.tcx.hir().find(expr_parent);
 313         let closure_params_len = closure_fn_decl.inputs.len();
 314         let (method_path, method_span, method_expr) = match (hir, closure_params_len) {
 315             (
 316                 Some(Node::Expr(hir::Expr {
 317                     kind: hir::ExprKind::MethodCall(path, span, expr, _),
 318                     ..
 319                 })),
 320                 1,
 321             ) => (path, span, expr),
 322             _ => return None,
 323         };
 324
 325         let self_ty = self.typeck_results.borrow().node_type(method_expr[0].hir_id);
 326         let self_ty = format!("{:?}", self_ty);
 327         let name = method_path.ident.name;
 328         let is_as_ref_able = (self_ty.starts_with("&std::option::Option")
 329             || self_ty.starts_with("&std::result::Result")
 330             || self_ty.starts_with("std::option::Option")
 331             || self_ty.starts_with("std::result::Result"))
 332             && (name == sym::map || name == sym::and_then);
 333         match (is_as_ref_able, self.sess().source_map().span_to_snippet(*method_span)) {
 334             (true, Ok(src)) => {
 335                 let suggestion = format!("as_ref().{}", src);
 336                 Some((*method_span, "consider using `as_ref` instead", suggestion))
 337             }
 338             _ => None,
 339         }
 340     }
 341
 342     crate fn is_hir_id_from_struct_pattern_shorthand_field(
 343         &self,
 344         hir_id: hir::HirId,
 345         sp: Span,
 346     ) -> bool {
 347         let sm = self.sess().source_map();
 348         let parent_id = self.tcx.hir().get_parent_node(hir_id);
 349         if let Some(parent) = self.tcx.hir().find(parent_id) {
 350             // Account for fields
 351             if let Node::Expr(hir::Expr { kind: hir::ExprKind::Struct(_, fields, ..), .. }) = parent
 352             {
 353                 if let Ok(src) = sm.span_to_snippet(sp) {
 354                     for field in *fields {
 355                         if field.ident.as_str() == src && field.is_shorthand {
 356                             return true;
 357                         }
 358                     }
 359                 }
 360             }
 361         }
 362         false
 363     }
 364
 365     fn replace_prefix<A, B, C>(&self, s: A, old: B, new: C) -> Option<String>
 366     where
 367         A: AsRef<str>,
 368         B: AsRef<str>,
 369         C: AsRef<str>,
 370     {
 371         let s = s.as_ref();
 372         let old = old.as_ref();
 373         if s.starts_with(old) { Some(new.as_ref().to_owned() + &s[old.len()..]) } else { None }
 374     }
 375
 376     /// This function is used to determine potential "simple" improvements or users' errors and
 377     /// provide them useful help. For example:
 378     ///
 379     /// ```
 380     /// fn some_fn(s: &str) {}
 381     ///
 382     /// let x = "hey!".to_owned();
 383     /// some_fn(x); // error
 384     /// ```
 385     ///
 386     /// No need to find every potential function which could make a coercion to transform a
 387     /// `String` into a `&str` since a `&` would do the trick!
 388     ///
 389     /// In addition of this check, it also checks between references mutability state. If the
 390     /// expected is mutable but the provided isn't, maybe we could just say "Hey, try with
 391     /// `&mut`!".
 392     pub fn check_ref(
 393         &self,
 394         expr: &hir::Expr<'_>,
 395         checked_ty: Ty<'tcx>,
 396         expected: Ty<'tcx>,
 397     ) -> Option<(Span, &'static str, String, Applicability)> {
 398         let sm = self.sess().source_map();
 399         let sp = expr.span;
 400         if sm.is_imported(sp) {
 401             // Ignore if span is from within a macro #41858, #58298. We previously used the macro
 402             // call span, but that breaks down when the type error comes from multiple calls down.
 403             return None;
 404         }
 405
 406         let is_struct_pat_shorthand_field =
 407             self.is_hir_id_from_struct_pattern_shorthand_field(expr.hir_id, sp);
 408
 409         // If the span is from a macro, then it's hard to extract the text
 410         // and make a good suggestion, so don't bother.
 411         let is_macro = sp.from_expansion() && sp.desugaring_kind().is_none();
 412
 413         // `ExprKind::DropTemps` is semantically irrelevant for these suggestions.
 414         let expr = expr.peel_drop_temps();
 415
 416         match (&expr.kind, &expected.kind, &checked_ty.kind) {
 417             (_, &ty::Ref(_, exp, _), &ty::Ref(_, check, _)) => match (&exp.kind, &check.kind) {
 418                 (&ty::Str, &ty::Array(arr, _) | &ty::Slice(arr)) if arr == self.tcx.types.u8 => {
 419                     if let hir::ExprKind::Lit(_) = expr.kind {
 420                         if let Ok(src) = sm.span_to_snippet(sp) {
 421                             if let Some(src) = self.replace_prefix(src, "b\"", "\"") {
 422                                 return Some((
 423                                     sp,
 424                                     "consider removing the leading `b`",
 425                                     src,
 426                                     Applicability::MachineApplicable,
 427                                 ));
 428                             }
 429                         }
 430                     }
 431                 }
 432                 (&ty::Array(arr, _) | &ty::Slice(arr), &ty::Str) if arr == self.tcx.types.u8 => {
 433                     if let hir::ExprKind::Lit(_) = expr.kind {
 434                         if let Ok(src) = sm.span_to_snippet(sp) {
 435                             if let Some(src) = self.replace_prefix(src, "\"", "b\"") {
 436                                 return Some((
 437                                     sp,
 438                                     "consider adding a leading `b`",
 439                                     src,
 440                                     Applicability::MachineApplicable,
 441                                 ));
 442                             }
 443                         }
 444                     }
 445                 }
 446                 _ => {}
 447             },
 448             (_, &ty::Ref(_, _, mutability), _) => {
 449                 // Check if it can work when put into a ref. For example:
 450                 //
 451                 // ```
 452                 // fn bar(x: &mut i32) {}
 453                 //
 454                 // let x = 0u32;
 455                 // bar(&x); // error, expected &mut
 456                 // ```
 457                 let ref_ty = match mutability {
 458                     hir::Mutability::Mut => {
 459                         self.tcx.mk_mut_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 460                     }
 461                     hir::Mutability::Not => {
 462                         self.tcx.mk_imm_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 463                     }
 464                 };
 465                 if self.can_coerce(ref_ty, expected) {
 466                     let mut sugg_sp = sp;
 467                     if let hir::ExprKind::MethodCall(ref segment, sp, ref args, _) = expr.kind {
 468                         let clone_trait = self.tcx.require_lang_item(LangItem::Clone, Some(sp));
 469                         if let ([arg], Some(true), sym::clone) = (
 470                             &args[..],
 471                             self.typeck_results.borrow().type_dependent_def_id(expr.hir_id).map(
 472                                 |did| {
 473                                     let ai = self.tcx.associated_item(did);
 474                                     ai.container == ty::TraitContainer(clone_trait)
 475                                 },
 476                             ),
 477                             segment.ident.name,
 478                         ) {
 479                             // If this expression had a clone call when suggesting borrowing
 480                             // we want to suggest removing it because it'd now be unnecessary.
 481                             sugg_sp = arg.span;
 482                         }
 483                     }
 484                     if let Ok(src) = sm.span_to_snippet(sugg_sp) {
 485                         let needs_parens = match expr.kind {
 486                             // parenthesize if needed (Issue #46756)
 487                             hir::ExprKind::Cast(_, _) | hir::ExprKind::Binary(_, _, _) => true,
 488                             // parenthesize borrows of range literals (Issue #54505)
 489                             _ if is_range_literal(expr) => true,
 490                             _ => false,
 491                         };
 492                         let sugg_expr = if needs_parens { format!("({})", src) } else { src };
 493
 494                         if let Some(sugg) = self.can_use_as_ref(expr) {
 495                             return Some((
 496                                 sugg.0,
 497                                 sugg.1,
 498                                 sugg.2,
 499                                 Applicability::MachineApplicable,
 500                             ));
 501                         }
 502                         let field_name = if is_struct_pat_shorthand_field {
 503                             format!("{}: ", sugg_expr)
 504                         } else {
 505                             String::new()
 506                         };
 507                         if let Some(hir::Node::Expr(hir::Expr {
 508                             kind: hir::ExprKind::Assign(left_expr, ..),
 509                             ..
 510                         })) = self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
 511                         {
 512                             if mutability == hir::Mutability::Mut {
 513                                 // Found the following case:
 514                                 // fn foo(opt: &mut Option<String>){ opt = None }
 515                                 //                                   ---   ^^^^
 516                                 //                                   |     |
 517                                 //    consider dereferencing here: `*opt`  |
 518                                 // expected mutable reference, found enum `Option`
 519                                 if let Ok(src) = sm.span_to_snippet(left_expr.span) {
 520                                     return Some((
 521                                         left_expr.span,
 522                                         "consider dereferencing here to assign to the mutable \
 523                                          borrowed piece of memory",
 524                                         format!("*{}", src),
 525                                         Applicability::MachineApplicable,
 526                                     ));
 527                                 }
 528                             }
 529                         }
 530
 531                         return Some(match mutability {
 532                             hir::Mutability::Mut => (
 533                                 sp,
 534                                 "consider mutably borrowing here",
 535                                 format!("{}&mut {}", field_name, sugg_expr),
 536                                 Applicability::MachineApplicable,
 537                             ),
 538                             hir::Mutability::Not => (
 539                                 sp,
 540                                 "consider borrowing here",
 541                                 format!("{}&{}", field_name, sugg_expr),
 542                                 Applicability::MachineApplicable,
 543                             ),
 544                         });
 545                     }
 546                 }
 547             }
 548             (
 549                 hir::ExprKind::AddrOf(hir::BorrowKind::Ref, _, ref expr),
 550                 _,
 551                 &ty::Ref(_, checked, _),
 552             ) if {
 553                 self.infcx.can_sub(self.param_env, checked, &expected).is_ok() && !is_macro
 554             } =>
 555             {
 556                 // We have `&T`, check if what was expected was `T`. If so,
 557                 // we may want to suggest removing a `&`.
 558                 if sm.is_imported(expr.span) {
 559                     if let Ok(src) = sm.span_to_snippet(sp) {
 560                         if let Some(src) = self.replace_prefix(src, "&", "") {
 561                             return Some((
 562                                 sp,
 563                                 "consider removing the borrow",
 564                                 src,
 565                                 Applicability::MachineApplicable,
 566                             ));
 567                         }
 568                     }
 569                     return None;
 570                 }
 571                 if let Ok(code) = sm.span_to_snippet(expr.span) {
 572                     return Some((
 573                         sp,
 574                         "consider removing the borrow",
 575                         code,
 576                         Applicability::MachineApplicable,
 577                     ));
 578                 }
 579             }
 580             (
 581                 _,
 582                 &ty::RawPtr(TypeAndMut { ty: ty_b, mutbl: mutbl_b }),
 583                 &ty::Ref(_, ty_a, mutbl_a),
 584             ) => {
 585                 if let Some(steps) = self.deref_steps(ty_a, ty_b) {
 586                     // Only suggest valid if dereferencing needed.
 587                     if steps > 0 {
 588                         // The pointer type implements `Copy` trait so the suggestion is always valid.
 589                         if let Ok(src) = sm.span_to_snippet(sp) {
 590                             let derefs = &"*".repeat(steps);
 591                             if let Some((src, applicability)) = match mutbl_b {
 592                                 hir::Mutability::Mut => {
 593                                     let new_prefix = "&mut ".to_owned() + derefs;
 594                                     match mutbl_a {
 595                                         hir::Mutability::Mut => {
 596                                             if let Some(s) =
 597                                                 self.replace_prefix(src, "&mut ", new_prefix)
 598                                             {
 599                                                 Some((s, Applicability::MachineApplicable))
 600                                             } else {
 601                                                 None
 602                                             }
 603                                         }
 604                                         hir::Mutability::Not => {
 605                                             if let Some(s) =
 606                                                 self.replace_prefix(src, "&", new_prefix)
 607                                             {
 608                                                 Some((s, Applicability::Unspecified))
 609                                             } else {
 610                                                 None
 611                                             }
 612                                         }
 613                                     }
 614                                 }
 615                                 hir::Mutability::Not => {
 616                                     let new_prefix = "&".to_owned() + derefs;
 617                                     match mutbl_a {
 618                                         hir::Mutability::Mut => {
 619                                             if let Some(s) =
 620                                                 self.replace_prefix(src, "&mut ", new_prefix)
 621                                             {
 622                                                 Some((s, Applicability::MachineApplicable))
 623                                             } else {
 624                                                 None
 625                                             }
 626                                         }
 627                                         hir::Mutability::Not => {
 628                                             if let Some(s) =
 629                                                 self.replace_prefix(src, "&", new_prefix)
 630                                             {
 631                                                 Some((s, Applicability::MachineApplicable))
 632                                             } else {
 633                                                 None
 634                                             }
 635                                         }
 636                                     }
 637                                 }
 638                             } {
 639                                 return Some((sp, "consider dereferencing", src, applicability));
 640                             }
 641                         }
 642                     }
 643                 }
 644             }
 645             _ if sp == expr.span && !is_macro => {
 646                 if let Some(steps) = self.deref_steps(checked_ty, expected) {
 647                     if steps == 1 {
 648                         // For a suggestion to make sense, the type would need to be `Copy`.
 649                         if self.infcx.type_is_copy_modulo_regions(self.param_env, expected, sp) {
 650                             if let Ok(code) = sm.span_to_snippet(sp) {
 651                                 let message = if checked_ty.is_region_ptr() {
 652                                     "consider dereferencing the borrow"
 653                                 } else {
 654                                     "consider dereferencing the type"
 655                                 };
 656                                 let suggestion = if is_struct_pat_shorthand_field {
 657                                     format!("{}: *{}", code, code)
 658                                 } else {
 659                                     format!("*{}", code)
 660                                 };
 661                                 return Some((
 662                                     sp,
 663                                     message,
 664                                     suggestion,
 665                                     Applicability::MachineApplicable,
 666                                 ));
 667                             }
 668                         }
 669                     }
 670                 }
 671             }
 672             _ => {}
 673         }
 674         None
 675     }
 676
 677     pub fn check_for_cast(
 678         &self,
 679         err: &mut DiagnosticBuilder<'_>,
 680         expr: &hir::Expr<'_>,
 681         checked_ty: Ty<'tcx>,
 682         expected_ty: Ty<'tcx>,
 683         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 684     ) -> bool {
 685         if self.tcx.sess.source_map().is_imported(expr.span) {
 686             // Ignore if span is from within a macro.
 687             return false;
 688         }
 689
 690         let src = if let Ok(src) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
 691             src
 692         } else {
 693             return false;
 694         };
 695
 696         // If casting this expression to a given numeric type would be appropriate in case of a type
 697         // mismatch.
 698         //
 699         // We want to minimize the amount of casting operations that are suggested, as it can be a
 700         // lossy operation with potentially bad side effects, so we only suggest when encountering
 701         // an expression that indicates that the original type couldn't be directly changed.
 702         //
 703         // For now, don't suggest casting with `as`.
 704         let can_cast = false;
 705
 706         let prefix = if let Some(hir::Node::Expr(hir::Expr {
 707             kind: hir::ExprKind::Struct(_, fields, _),
 708             ..
 709         })) = self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
 710         {
 711             // `expr` is a literal field for a struct, only suggest if appropriate
 712             match (*fields)
 713                 .iter()
 714                 .find(|field| field.expr.hir_id == expr.hir_id && field.is_shorthand)
 715             {
 716                 // This is a field literal
 717                 Some(field) => format!("{}: ", field.ident),
 718                 // Likely a field was meant, but this field wasn't found. Do not suggest anything.
 719                 None => return false,
 720             }
 721         } else {
 722             String::new()
 723         };
 724
 725         if let hir::ExprKind::Call(path, args) = &expr.kind {
 726             if let (hir::ExprKind::Path(hir::QPath::TypeRelative(base_ty, path_segment)), 1) =
 727                 (&path.kind, args.len())
 728             {
 729                 // `expr` is a conversion like `u32::from(val)`, do not suggest anything (#63697).
 730                 if let (hir::TyKind::Path(hir::QPath::Resolved(None, base_ty_path)), sym::from) =
 731                     (&base_ty.kind, path_segment.ident.name)
 732                 {
 733                     if let Some(ident) = &base_ty_path.segments.iter().map(|s| s.ident).next() {
 734                         match ident.name {
 735                             sym::i128
 736                             | sym::i64
 737                             | sym::i32
 738                             | sym::i16
 739                             | sym::i8
 740                             | sym::u128
 741                             | sym::u64
 742                             | sym::u32
 743                             | sym::u16
 744                             | sym::u8
 745                             | sym::isize
 746                             | sym::usize
 747                                 if base_ty_path.segments.len() == 1 =>
 748                             {
 749                                 return false;
 750                             }
 751                             _ => {}
 752                         }
 753                     }
 754                 }
 755             }
 756         }
 757
 758         let msg = format!("you can convert an `{}` to `{}`", checked_ty, expected_ty);
 759         let cast_msg = format!("you can cast an `{} to `{}`", checked_ty, expected_ty);
 760         let lit_msg = format!(
 761             "change the type of the numeric literal from `{}` to `{}`",
 762             checked_ty, expected_ty,
 763         );
 764
 765         let with_opt_paren: fn(&dyn fmt::Display) -> String =
 766             if expr.precedence().order() < PREC_POSTFIX {
 767                 |s| format!("({})", s)
 768             } else {
 769                 |s| s.to_string()
 770             };
 771
 772         let cast_suggestion = format!("{}{} as {}", prefix, with_opt_paren(&src), expected_ty);
 773         let into_suggestion = format!("{}{}.into()", prefix, with_opt_paren(&src));
 774         let suffix_suggestion = with_opt_paren(&format_args!(
 775             "{}{}",
 776             if matches!(
 777                 (&expected_ty.kind, &checked_ty.kind),
 778                 (ty::Int(_) | ty::Uint(_), ty::Float(_))
 779             ) {
 780                 // Remove fractional part from literal, for example `42.0f32` into `42`
 781                 let src = src.trim_end_matches(&checked_ty.to_string());
 782                 src.split('.').next().unwrap()
 783             } else {
 784                 src.trim_end_matches(&checked_ty.to_string())
 785             },
 786             expected_ty,
 787         ));
 788         let literal_is_ty_suffixed = |expr: &hir::Expr<'_>| {
 789             if let hir::ExprKind::Lit(lit) = &expr.kind { lit.node.is_suffixed() } else { false }
 790         };
 791         let is_negative_int =
 792             |expr: &hir::Expr<'_>| matches!(expr.kind, hir::ExprKind::Unary(hir::UnOp::UnNeg, ..));
 793         let is_uint = |ty: Ty<'_>| matches!(ty.kind, ty::Uint(..));
 794
 795         let in_const_context = self.tcx.hir().is_inside_const_context(expr.hir_id);
 796
 797         let suggest_fallible_into_or_lhs_from =
 798             |err: &mut DiagnosticBuilder<'_>, exp_to_found_is_fallible: bool| {
 799                 // If we know the expression the expected type is derived from, we might be able
 800                 // to suggest a widening conversion rather than a narrowing one (which may
 801                 // panic). For example, given x: u8 and y: u32, if we know the span of "x",
 802                 //   x > y
 803                 // can be given the suggestion "u32::from(x) > y" rather than
 804                 // "x > y.try_into().unwrap()".
 805                 let lhs_expr_and_src = expected_ty_expr.and_then(|expr| {
 806                     match self.tcx.sess.source_map().span_to_snippet(expr.span).ok() {
 807                         Some(src) => Some((expr, src)),
 808                         None => None,
 809                     }
 810                 });
 811                 let (span, msg, suggestion) = if let (Some((lhs_expr, lhs_src)), false) =
 812                     (lhs_expr_and_src, exp_to_found_is_fallible)
 813                 {
 814                     let msg = format!(
 815                         "you can convert `{}` from `{}` to `{}`, matching the type of `{}`",
 816                         lhs_src, expected_ty, checked_ty, src
 817                     );
 818                     let suggestion = format!("{}::from({})", checked_ty, lhs_src);
 819                     (lhs_expr.span, msg, suggestion)
 820                 } else {
 821                     let msg = format!("{} and panic if the converted value wouldn't fit", msg);
 822                     let suggestion =
 823                         format!("{}{}.try_into().unwrap()", prefix, with_opt_paren(&src));
 824                     (expr.span, msg, suggestion)
 825                 };
 826                 err.span_suggestion(span, &msg, suggestion, Applicability::MachineApplicable);
 827             };
 828
 829         let suggest_to_change_suffix_or_into =
 830             |err: &mut DiagnosticBuilder<'_>,
 831              found_to_exp_is_fallible: bool,
 832              exp_to_found_is_fallible: bool| {
 833                 let always_fallible = found_to_exp_is_fallible
 834                     && (exp_to_found_is_fallible || expected_ty_expr.is_none());
 835                 let msg = if literal_is_ty_suffixed(expr) {
 836                     &lit_msg
 837                 } else if always_fallible && (is_negative_int(expr) && is_uint(expected_ty)) {
 838                     // We now know that converting either the lhs or rhs is fallible. Before we
 839                     // suggest a fallible conversion, check if the value can never fit in the
 840                     // expected type.
 841                     let msg = format!("`{}` cannot fit into type `{}`", src, expected_ty);
 842                     err.note(&msg);
 843                     return;
 844                 } else if in_const_context {
 845                     // Do not recommend `into` or `try_into` in const contexts.
 846                     return;
 847                 } else if found_to_exp_is_fallible {
 848                     return suggest_fallible_into_or_lhs_from(err, exp_to_found_is_fallible);
 849                 } else {
 850                     &msg
 851                 };
 852                 let suggestion = if literal_is_ty_suffixed(expr) {
 853                     suffix_suggestion.clone()
 854                 } else {
 855                     into_suggestion.clone()
 856                 };
 857                 err.span_suggestion(expr.span, msg, suggestion, Applicability::MachineApplicable);
 858             };
 859
 860         match (&expected_ty.kind, &checked_ty.kind) {
 861             (&ty::Int(ref exp), &ty::Int(ref found)) => {
 862                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
 863                 {
 864                     (Some(exp), Some(found)) if exp < found => (true, false),
 865                     (Some(exp), Some(found)) if exp > found => (false, true),
 866                     (None, Some(8 | 16)) => (false, true),
 867                     (Some(8 | 16), None) => (true, false),
 868                     (None, _) | (_, None) => (true, true),
 869                     _ => (false, false),
 870                 };
 871                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
 872                 true
 873             }
 874             (&ty::Uint(ref exp), &ty::Uint(ref found)) => {
 875                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
 876                 {
 877                     (Some(exp), Some(found)) if exp < found => (true, false),
 878                     (Some(exp), Some(found)) if exp > found => (false, true),
 879                     (None, Some(8 | 16)) => (false, true),
 880                     (Some(8 | 16), None) => (true, false),
 881                     (None, _) | (_, None) => (true, true),
 882                     _ => (false, false),
 883                 };
 884                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
 885                 true
 886             }
 887             (&ty::Int(exp), &ty::Uint(found)) => {
 888                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
 889                 {
 890                     (Some(exp), Some(found)) if found < exp => (false, true),
 891                     (None, Some(8)) => (false, true),
 892                     _ => (true, true),
 893                 };
 894                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
 895                 true
 896             }
 897             (&ty::Uint(exp), &ty::Int(found)) => {
 898                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
 899                 {
 900                     (Some(exp), Some(found)) if found > exp => (true, false),
 901                     (Some(8), None) => (true, false),
 902                     _ => (true, true),
 903                 };
 904                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
 905                 true
 906             }
 907             (&ty::Float(ref exp), &ty::Float(ref found)) => {
 908                 if found.bit_width() < exp.bit_width() {
 909                     suggest_to_change_suffix_or_into(err, false, true);
 910                 } else 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 `f64` to `f32`
 919                     err.span_suggestion(
 920                         expr.span,
 921                         &format!("{}, producing the closest possible value", cast_msg),
 922                         cast_suggestion,
 923                         Applicability::MaybeIncorrect, // lossy conversion
 924                     );
 925                 }
 926                 true
 927             }
 928             (&ty::Uint(_) | &ty::Int(_), &ty::Float(_)) => {
 929                 if literal_is_ty_suffixed(expr) {
 930                     err.span_suggestion(
 931                         expr.span,
 932                         &lit_msg,
 933                         suffix_suggestion,
 934                         Applicability::MachineApplicable,
 935                     );
 936                 } else if can_cast {
 937                     // Missing try_into implementation for `{float}` to `{integer}`
 938                     err.span_suggestion(
 939                         expr.span,
 940                         &format!("{}, rounding the float towards zero", msg),
 941                         cast_suggestion,
 942                         Applicability::MaybeIncorrect, // lossy conversion
 943                     );
 944                 }
 945                 true
 946             }
 947             (&ty::Float(ref exp), &ty::Uint(ref found)) => {
 948                 // if `found` is `None` (meaning found is `usize`), don't suggest `.into()`
 949                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
 950                     err.span_suggestion(
 951                         expr.span,
 952                         &format!(
 953                             "{}, producing the floating point representation of the integer",
 954                             msg,
 955                         ),
 956                         into_suggestion,
 957                         Applicability::MachineApplicable,
 958                     );
 959                 } else if literal_is_ty_suffixed(expr) {
 960                     err.span_suggestion(
 961                         expr.span,
 962                         &lit_msg,
 963                         suffix_suggestion,
 964                         Applicability::MachineApplicable,
 965                     );
 966                 } else {
 967                     // Missing try_into implementation for `{integer}` to `{float}`
 968                     err.span_suggestion(
 969                         expr.span,
 970                         &format!(
 971                             "{}, producing the floating point representation of the integer,
 972                                  rounded if necessary",
 973                             cast_msg,
 974                         ),
 975                         cast_suggestion,
 976                         Applicability::MaybeIncorrect, // lossy conversion
 977                     );
 978                 }
 979                 true
 980             }
 981             (&ty::Float(ref exp), &ty::Int(ref found)) => {
 982                 // if `found` is `None` (meaning found is `isize`), don't suggest `.into()`
 983                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
 984                     err.span_suggestion(
 985                         expr.span,
 986                         &format!(
 987                             "{}, producing the floating point representation of the integer",
 988                             &msg,
 989                         ),
 990                         into_suggestion,
 991                         Applicability::MachineApplicable,
 992                     );
 993                 } else if literal_is_ty_suffixed(expr) {
 994                     err.span_suggestion(
 995                         expr.span,
 996                         &lit_msg,
 997                         suffix_suggestion,
 998                         Applicability::MachineApplicable,
 999                     );
1000                 } else {
1001                     // Missing try_into implementation for `{integer}` to `{float}`
1002                     err.span_suggestion(
1003                         expr.span,
1004                         &format!(
1005                             "{}, producing the floating point representation of the integer, \
1006                                 rounded if necessary",
1007                             &msg,
1008                         ),
1009                         cast_suggestion,
1010                         Applicability::MaybeIncorrect, // lossy conversion
1011                     );
1012                 }
1013                 true
1014             }
1015             _ => false,
1016         }
1017     }
1018 }