src/librustc_typeck/check/demand.rs

   1 use crate::check::FnCtxt;
   2 use rustc::infer::InferOk;
   3 use rustc::traits::{self, ObligationCause, ObligationCauseCode};
   4
   5 use syntax::symbol::sym;
   6 use syntax::util::parser::PREC_POSTFIX;
   7 use syntax_pos::Span;
   8 use rustc::hir;
   9 use rustc::hir::Node;
  10 use rustc::hir::{print, lowering::is_range_literal};
  11 use rustc::ty::{self, Ty, AssocItem};
  12 use rustc::ty::adjustment::AllowTwoPhase;
  13 use errors::{Applicability, DiagnosticBuilder};
  14
  15 use super::method::probe;
  16
  17 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  18     // Requires that the two types unify, and prints an error message if
  19     // they don't.
  20     pub fn demand_suptype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  21         self.demand_suptype_diag(sp, expected, actual).map(|mut e| e.emit());
  22     }
  23
  24     pub fn demand_suptype_diag(&self,
  25                                sp: Span,
  26                                expected: Ty<'tcx>,
  27                                actual: Ty<'tcx>) -> Option<DiagnosticBuilder<'tcx>> {
  28         let cause = &self.misc(sp);
  29         match self.at(cause, self.param_env).sup(expected, actual) {
  30             Ok(InferOk { obligations, value: () }) => {
  31                 self.register_predicates(obligations);
  32                 None
  33             },
  34             Err(e) => {
  35                 Some(self.report_mismatched_types(&cause, expected, actual, e))
  36             }
  37         }
  38     }
  39
  40     pub fn demand_eqtype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  41         if let Some(mut err) = self.demand_eqtype_diag(sp, expected, actual) {
  42             err.emit();
  43         }
  44     }
  45
  46     pub fn demand_eqtype_diag(&self,
  47                              sp: Span,
  48                              expected: Ty<'tcx>,
  49                              actual: Ty<'tcx>) -> Option<DiagnosticBuilder<'tcx>> {
  50         self.demand_eqtype_with_origin(&self.misc(sp), expected, actual)
  51     }
  52
  53     pub fn demand_eqtype_with_origin(&self,
  54                                      cause: &ObligationCause<'tcx>,
  55                                      expected: Ty<'tcx>,
  56                                      actual: Ty<'tcx>) -> Option<DiagnosticBuilder<'tcx>> {
  57         match self.at(cause, self.param_env).eq(expected, actual) {
  58             Ok(InferOk { obligations, value: () }) => {
  59                 self.register_predicates(obligations);
  60                 None
  61             }
  62             Err(e) => {
  63                 Some(self.report_mismatched_types(cause, expected, actual, e))
  64             }
  65         }
  66     }
  67
  68     pub fn demand_eqtype_pat(
  69         &self,
  70         cause_span: Span,
  71         expected: Ty<'tcx>,
  72         actual: Ty<'tcx>,
  73         match_expr_span: Option<Span>,
  74     ) {
  75         let cause = if let Some(span) = match_expr_span {
  76             self.cause(
  77                 cause_span,
  78                 ObligationCauseCode::MatchExpressionArmPattern { span, ty: expected },
  79             )
  80         } else {
  81             self.misc(cause_span)
  82         };
  83         self.demand_eqtype_with_origin(&cause, expected, actual).map(|mut err| err.emit());
  84     }
  85
  86
  87     pub fn demand_coerce(&self,
  88                          expr: &hir::Expr,
  89                          checked_ty: Ty<'tcx>,
  90                          expected: Ty<'tcx>,
  91                          allow_two_phase: AllowTwoPhase)
  92                          -> Ty<'tcx> {
  93         let (ty, err) = self.demand_coerce_diag(expr, checked_ty, expected, allow_two_phase);
  94         if let Some(mut err) = err {
  95             err.emit();
  96         }
  97         ty
  98     }
  99
 100     // Checks that the type of `expr` can be coerced to `expected`.
 101     //
 102     // N.B., this code relies on `self.diverges` to be accurate. In
 103     // particular, assignments to `!` will be permitted if the
 104     // diverges flag is currently "always".
 105     pub fn demand_coerce_diag(&self,
 106                               expr: &hir::Expr,
 107                               checked_ty: Ty<'tcx>,
 108                               expected: Ty<'tcx>,
 109                               allow_two_phase: AllowTwoPhase)
 110                               -> (Ty<'tcx>, Option<DiagnosticBuilder<'tcx>>) {
 111         let expected = self.resolve_vars_with_obligations(expected);
 112
 113         let e = match self.try_coerce(expr, checked_ty, expected, allow_two_phase) {
 114             Ok(ty) => return (ty, None),
 115             Err(e) => e
 116         };
 117
 118         let expr = expr.peel_drop_temps();
 119         let cause = self.misc(expr.span);
 120         let expr_ty = self.resolve_vars_with_obligations(checked_ty);
 121         let mut err = self.report_mismatched_types(&cause, expected, expr_ty, e);
 122
 123         if self.is_assign_to_bool(expr, expected) {
 124             // Error reported in `check_assign` so avoid emitting error again.
 125             err.delay_as_bug();
 126             return (expected, None)
 127         }
 128
 129         self.suggest_compatible_variants(&mut err, expr, expected, expr_ty);
 130         self.suggest_ref_or_into(&mut err, expr, expected, expr_ty);
 131         self.suggest_boxing_when_appropriate(&mut err, expr, expected, expr_ty);
 132         self.suggest_missing_await(&mut err, expr, expected, expr_ty);
 133
 134         (expected, Some(err))
 135     }
 136
 137     /// Returns whether the expected type is `bool` and the expression is `x = y`.
 138     pub fn is_assign_to_bool(&self, expr: &hir::Expr, expected: Ty<'tcx>) -> bool {
 139         if let hir::ExprKind::Assign(..) = expr.kind {
 140             return expected == self.tcx.types.bool;
 141         }
 142         false
 143     }
 144
 145     /// If the expected type is an enum (Issue #55250) with any variants whose
 146     /// sole field is of the found type, suggest such variants. (Issue #42764)
 147     fn suggest_compatible_variants(
 148         &self,
 149         err: &mut DiagnosticBuilder<'_>,
 150         expr: &hir::Expr,
 151         expected: Ty<'tcx>,
 152         expr_ty: Ty<'tcx>,
 153     ) {
 154         if let ty::Adt(expected_adt, substs) = expected.kind {
 155             if !expected_adt.is_enum() {
 156                 return;
 157             }
 158
 159             let mut compatible_variants = expected_adt.variants
 160                 .iter()
 161                 .filter(|variant| variant.fields.len() == 1)
 162                 .filter_map(|variant| {
 163                     let sole_field = &variant.fields[0];
 164                     let sole_field_ty = sole_field.ty(self.tcx, substs);
 165                     if self.can_coerce(expr_ty, sole_field_ty) {
 166                         let variant_path = self.tcx.def_path_str(variant.def_id);
 167                         // FIXME #56861: DRYer prelude filtering
 168                         Some(variant_path.trim_start_matches("std::prelude::v1::").to_string())
 169                     } else {
 170                         None
 171                     }
 172                 }).peekable();
 173
 174             if compatible_variants.peek().is_some() {
 175                 let expr_text = self.tcx.sess
 176                     .source_map()
 177                     .span_to_snippet(expr.span)
 178                     .unwrap_or_else(|_| {
 179                         print::to_string(print::NO_ANN, |s| s.print_expr(expr))
 180                     });
 181                 let suggestions = compatible_variants
 182                     .map(|v| format!("{}({})", v, expr_text));
 183                 let msg = "try using a variant of the expected enum";
 184                 err.span_suggestions(expr.span, msg, suggestions, Applicability::MaybeIncorrect);
 185             }
 186         }
 187     }
 188
 189     pub fn get_conversion_methods(&self, span: Span, expected: Ty<'tcx>, checked_ty: Ty<'tcx>)
 190                               -> Vec<AssocItem> {
 191         let mut methods = self.probe_for_return_type(span,
 192                                                      probe::Mode::MethodCall,
 193                                                      expected,
 194                                                      checked_ty,
 195                                                      hir::DUMMY_HIR_ID);
 196         methods.retain(|m| {
 197             self.has_no_input_arg(m) &&
 198                 self.tcx.get_attrs(m.def_id).iter()
 199                 // This special internal attribute is used to whitelist
 200                 // "identity-like" conversion methods to be suggested here.
 201                 //
 202                 // FIXME (#46459 and #46460): ideally
 203                 // `std::convert::Into::into` and `std::borrow:ToOwned` would
 204                 // also be `#[rustc_conversion_suggestion]`, if not for
 205                 // method-probing false-positives and -negatives (respectively).
 206                 //
 207                 // FIXME? Other potential candidate methods: `as_ref` and
 208                 // `as_mut`?
 209                 .find(|a| a.check_name(sym::rustc_conversion_suggestion)).is_some()
 210         });
 211
 212         methods
 213     }
 214
 215     // This function checks if the method isn't static and takes other arguments than `self`.
 216     fn has_no_input_arg(&self, method: &AssocItem) -> bool {
 217         match method.kind {
 218             ty::AssocKind::Method => {
 219                 self.tcx.fn_sig(method.def_id).inputs().skip_binder().len() == 1
 220             }
 221             _ => false,
 222         }
 223     }
 224
 225     /// Identify some cases where `as_ref()` would be appropriate and suggest it.
 226     ///
 227     /// Given the following code:
 228     /// ```
 229     /// struct Foo;
 230     /// fn takes_ref(_: &Foo) {}
 231     /// let ref opt = Some(Foo);
 232     ///
 233     /// opt.map(|param| takes_ref(param));
 234     /// ```
 235     /// Suggest using `opt.as_ref().map(|param| takes_ref(param));` instead.
 236     ///
 237     /// It only checks for `Option` and `Result` and won't work with
 238     /// ```
 239     /// opt.map(|param| { takes_ref(param) });
 240     /// ```
 241     fn can_use_as_ref(
 242         &self,
 243         expr: &hir::Expr,
 244     ) -> Option<(Span, &'static str, String)> {
 245         let path = match expr.kind {
 246             hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => path,
 247             _ => return None
 248         };
 249
 250         let local_id = match path.res {
 251             hir::def::Res::Local(id) => id,
 252             _ => return None
 253         };
 254
 255         let local_parent = self.tcx.hir().get_parent_node(local_id);
 256         let param_hir_id = match self.tcx.hir().find(local_parent) {
 257             Some(Node::Param(hir::Param { hir_id, .. })) => hir_id,
 258             _ => return None
 259         };
 260
 261         let param_parent = self.tcx.hir().get_parent_node(*param_hir_id);
 262         let (expr_hir_id, closure_fn_decl) = match self.tcx.hir().find(param_parent) {
 263             Some(Node::Expr(
 264                 hir::Expr { hir_id, kind: hir::ExprKind::Closure(_, decl, ..), .. }
 265             )) => (hir_id, decl),
 266             _ => return None
 267         };
 268
 269         let expr_parent = self.tcx.hir().get_parent_node(*expr_hir_id);
 270         let hir = self.tcx.hir().find(expr_parent);
 271         let closure_params_len = closure_fn_decl.inputs.len();
 272         let (method_path, method_span, method_expr) = match (hir, closure_params_len) {
 273             (Some(Node::Expr(
 274                 hir::Expr { kind: hir::ExprKind::MethodCall(path, span, expr), .. }
 275             )), 1) => (path, span, expr),
 276             _ => return None
 277         };
 278
 279         let self_ty = self.tables.borrow().node_type(method_expr[0].hir_id);
 280         let self_ty = format!("{:?}", self_ty);
 281         let name = method_path.ident.as_str();
 282         let is_as_ref_able = (
 283             self_ty.starts_with("&std::option::Option") ||
 284             self_ty.starts_with("&std::result::Result") ||
 285             self_ty.starts_with("std::option::Option") ||
 286             self_ty.starts_with("std::result::Result")
 287         ) && (name == "map" || name == "and_then");
 288         match (is_as_ref_able, self.sess().source_map().span_to_snippet(*method_span)) {
 289             (true, Ok(src)) => {
 290                 let suggestion = format!("as_ref().{}", src);
 291                 Some((*method_span, "consider using `as_ref` instead", suggestion))
 292             },
 293             _ => None
 294         }
 295     }
 296
 297     crate fn is_hir_id_from_struct_pattern_shorthand_field(
 298         &self,
 299         hir_id: hir::HirId,
 300         sp: Span,
 301     ) -> bool {
 302         let cm = self.sess().source_map();
 303         let parent_id = self.tcx.hir().get_parent_node(hir_id);
 304         if let Some(parent) = self.tcx.hir().find(parent_id) {
 305             // Account for fields
 306             if let Node::Expr(hir::Expr {
 307                 kind: hir::ExprKind::Struct(_, fields, ..), ..
 308             }) = parent {
 309                 if let Ok(src) = cm.span_to_snippet(sp) {
 310                     for field in fields {
 311                         if field.ident.as_str() == src.as_str() && field.is_shorthand {
 312                             return true;
 313                         }
 314                     }
 315                 }
 316             }
 317         }
 318         false
 319     }
 320
 321     /// This function is used to determine potential "simple" improvements or users' errors and
 322     /// provide them useful help. For example:
 323     ///
 324     /// ```
 325     /// fn some_fn(s: &str) {}
 326     ///
 327     /// let x = "hey!".to_owned();
 328     /// some_fn(x); // error
 329     /// ```
 330     ///
 331     /// No need to find every potential function which could make a coercion to transform a
 332     /// `String` into a `&str` since a `&` would do the trick!
 333     ///
 334     /// In addition of this check, it also checks between references mutability state. If the
 335     /// expected is mutable but the provided isn't, maybe we could just say "Hey, try with
 336     /// `&mut`!".
 337     pub fn check_ref(
 338         &self,
 339         expr: &hir::Expr,
 340         checked_ty: Ty<'tcx>,
 341         expected: Ty<'tcx>,
 342     ) -> Option<(Span, &'static str, String)> {
 343         let cm = self.sess().source_map();
 344         let sp = expr.span;
 345         if !cm.span_to_filename(sp).is_real() {
 346             // Ignore if span is from within a macro #41858, #58298. We previously used the macro
 347             // call span, but that breaks down when the type error comes from multiple calls down.
 348             return None;
 349         }
 350
 351         let is_struct_pat_shorthand_field = self.is_hir_id_from_struct_pattern_shorthand_field(
 352             expr.hir_id,
 353             sp,
 354         );
 355
 356         // If the span is from a macro, then it's hard to extract the text
 357         // and make a good suggestion, so don't bother.
 358         let is_macro = sp.from_expansion() && sp.desugaring_kind().is_none();
 359
 360         // `ExprKind::DropTemps` is semantically irrelevant for these suggestions.
 361         let expr = expr.peel_drop_temps();
 362
 363         match (&expr.kind, &expected.kind, &checked_ty.kind) {
 364             (_, &ty::Ref(_, exp, _), &ty::Ref(_, check, _)) => match (&exp.kind, &check.kind) {
 365                 (&ty::Str, &ty::Array(arr, _)) |
 366                 (&ty::Str, &ty::Slice(arr)) if arr == self.tcx.types.u8 => {
 367                     if let hir::ExprKind::Lit(_) = expr.kind {
 368                         if let Ok(src) = cm.span_to_snippet(sp) {
 369                             if src.starts_with("b\"") {
 370                                 return Some((sp,
 371                                              "consider removing the leading `b`",
 372                                              src[1..].to_string()));
 373                             }
 374                         }
 375                     }
 376                 },
 377                 (&ty::Array(arr, _), &ty::Str) |
 378                 (&ty::Slice(arr), &ty::Str) if arr == self.tcx.types.u8 => {
 379                     if let hir::ExprKind::Lit(_) = expr.kind {
 380                         if let Ok(src) = cm.span_to_snippet(sp) {
 381                             if src.starts_with("\"") {
 382                                 return Some((sp,
 383                                              "consider adding a leading `b`",
 384                                              format!("b{}", src)));
 385                             }
 386                         }
 387                     }
 388                 }
 389                 _ => {}
 390             },
 391             (_, &ty::Ref(_, _, mutability), _) => {
 392                 // Check if it can work when put into a ref. For example:
 393                 //
 394                 // ```
 395                 // fn bar(x: &mut i32) {}
 396                 //
 397                 // let x = 0u32;
 398                 // bar(&x); // error, expected &mut
 399                 // ```
 400                 let ref_ty = match mutability {
 401                     hir::Mutability::MutMutable => {
 402                         self.tcx.mk_mut_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 403                     }
 404                     hir::Mutability::MutImmutable => {
 405                         self.tcx.mk_imm_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 406                     }
 407                 };
 408                 if self.can_coerce(ref_ty, expected) {
 409                     let mut sugg_sp = sp;
 410                     if let hir::ExprKind::MethodCall(segment, _sp, args) = &expr.kind {
 411                         let clone_trait = self.tcx.lang_items().clone_trait().unwrap();
 412                         if let ([arg], Some(true), "clone") = (
 413                             &args[..],
 414                             self.tables.borrow().type_dependent_def_id(expr.hir_id).map(|did| {
 415                                 let ai = self.tcx.associated_item(did);
 416                                 ai.container == ty::TraitContainer(clone_trait)
 417                             }),
 418                             &segment.ident.as_str()[..],
 419                         ) {
 420                             // If this expression had a clone call when suggesting borrowing
 421                             // we want to suggest removing it because it'd now be unecessary.
 422                             sugg_sp = arg.span;
 423                         }
 424                     }
 425                     if let Ok(src) = cm.span_to_snippet(sugg_sp) {
 426                         let needs_parens = match expr.kind {
 427                             // parenthesize if needed (Issue #46756)
 428                             hir::ExprKind::Cast(_, _) |
 429                             hir::ExprKind::Binary(_, _, _) => true,
 430                             // parenthesize borrows of range literals (Issue #54505)
 431                             _ if is_range_literal(self.tcx.sess, expr) => true,
 432                             _ => false,
 433                         };
 434                         let sugg_expr = if needs_parens {
 435                             format!("({})", src)
 436                         } else {
 437                             src
 438                         };
 439
 440                         if let Some(sugg) = self.can_use_as_ref(expr) {
 441                             return Some(sugg);
 442                         }
 443                         let field_name = if is_struct_pat_shorthand_field {
 444                             format!("{}: ", sugg_expr)
 445                         } else {
 446                             String::new()
 447                         };
 448                         if let Some(hir::Node::Expr(hir::Expr {
 449                             kind: hir::ExprKind::Assign(left_expr, _),
 450                             ..
 451                         })) = self.tcx.hir().find(
 452                             self.tcx.hir().get_parent_node(expr.hir_id),
 453                         ) {
 454                             if mutability == hir::Mutability::MutMutable {
 455                                 // Found the following case:
 456                                 // fn foo(opt: &mut Option<String>){ opt = None }
 457                                 //                                   ---   ^^^^
 458                                 //                                   |     |
 459                                 //    consider dereferencing here: `*opt`  |
 460                                 // expected mutable reference, found enum `Option`
 461                                 if let Ok(src) = cm.span_to_snippet(left_expr.span) {
 462                                     return Some((
 463                                         left_expr.span,
 464                                         "consider dereferencing here to assign to the mutable \
 465                                          borrowed piece of memory",
 466                                         format!("*{}", src),
 467                                     ));
 468                                 }
 469                             }
 470                         }
 471
 472                         return Some(match mutability {
 473                             hir::Mutability::MutMutable => (
 474                                 sp,
 475                                 "consider mutably borrowing here",
 476                                 format!("{}&mut {}", field_name, sugg_expr),
 477                             ),
 478                             hir::Mutability::MutImmutable => (
 479                                 sp,
 480                                 "consider borrowing here",
 481                                 format!("{}&{}", field_name, sugg_expr),
 482                             ),
 483                         });
 484                     }
 485                 }
 486             },
 487             (hir::ExprKind::AddrOf(_, ref expr), _, &ty::Ref(_, checked, _)) if {
 488                 self.infcx.can_sub(self.param_env, checked, &expected).is_ok() && !is_macro
 489             } => {
 490                 // We have `&T`, check if what was expected was `T`. If so,
 491                 // we may want to suggest removing a `&`.
 492                 if !cm.span_to_filename(expr.span).is_real() {
 493                     if let Ok(code) = cm.span_to_snippet(sp) {
 494                         if code.chars().next() == Some('&') {
 495                             return Some((
 496                                 sp,
 497                                 "consider removing the borrow",
 498                                 code[1..].to_string(),
 499                             ));
 500                         }
 501                     }
 502                     return None;
 503                 }
 504                 if let Ok(code) = cm.span_to_snippet(expr.span) {
 505                     return Some((sp, "consider removing the borrow", code));
 506                 }
 507             },
 508             _ if sp == expr.span && !is_macro => {
 509                 // Check for `Deref` implementations by constructing a predicate to
 510                 // prove: `<T as Deref>::Output == U`
 511                 let deref_trait = self.tcx.lang_items().deref_trait().unwrap();
 512                 let item_def_id = self.tcx.associated_items(deref_trait).next().unwrap().def_id;
 513                 let predicate = ty::Predicate::Projection(ty::Binder::bind(ty::ProjectionPredicate {
 514                     // `<T as Deref>::Output`
 515                     projection_ty: ty::ProjectionTy {
 516                         // `T`
 517                         substs: self.tcx.mk_substs_trait(
 518                             checked_ty,
 519                             self.fresh_substs_for_item(sp, item_def_id),
 520                         ),
 521                         // `Deref::Output`
 522                         item_def_id,
 523                     },
 524                     // `U`
 525                     ty: expected,
 526                 }));
 527                 let obligation = traits::Obligation::new(self.misc(sp), self.param_env, predicate);
 528                 let impls_deref = self.infcx.predicate_may_hold(&obligation);
 529
 530                 // For a suggestion to make sense, the type would need to be `Copy`.
 531                 let is_copy = self.infcx.type_is_copy_modulo_regions(self.param_env, expected, sp);
 532
 533                 if is_copy && impls_deref {
 534                     if let Ok(code) = cm.span_to_snippet(sp) {
 535                         let message = if checked_ty.is_region_ptr() {
 536                             "consider dereferencing the borrow"
 537                         } else {
 538                             "consider dereferencing the type"
 539                         };
 540                         let suggestion = if is_struct_pat_shorthand_field {
 541                             format!("{}: *{}", code, code)
 542                         } else {
 543                             format!("*{}", code)
 544                         };
 545                         return Some((sp, message, suggestion));
 546                     }
 547                 }
 548             }
 549             _ => {}
 550         }
 551         None
 552     }
 553
 554     pub fn check_for_cast(
 555         &self,
 556         err: &mut DiagnosticBuilder<'tcx>,
 557         expr: &hir::Expr,
 558         checked_ty: Ty<'tcx>,
 559         expected_ty: Ty<'tcx>,
 560     ) -> bool {
 561         if self.tcx.hir().is_const_context(expr.hir_id) {
 562             // Shouldn't suggest `.into()` on `const`s.
 563             // FIXME(estebank): modify once we decide to suggest `as` casts
 564             return false;
 565         }
 566         if !self.tcx.sess.source_map().span_to_filename(expr.span).is_real() {
 567             // Ignore if span is from within a macro.
 568             return false;
 569         }
 570
 571         // If casting this expression to a given numeric type would be appropriate in case of a type
 572         // mismatch.
 573         //
 574         // We want to minimize the amount of casting operations that are suggested, as it can be a
 575         // lossy operation with potentially bad side effects, so we only suggest when encountering
 576         // an expression that indicates that the original type couldn't be directly changed.
 577         //
 578         // For now, don't suggest casting with `as`.
 579         let can_cast = false;
 580
 581         let mut prefix = String::new();
 582         if let Some(hir::Node::Expr(hir::Expr {
 583             kind: hir::ExprKind::Struct(_, fields, _),
 584             ..
 585         })) = self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id)) {
 586             // `expr` is a literal field for a struct, only suggest if appropriate
 587             for field in fields {
 588                 if field.expr.hir_id == expr.hir_id && field.is_shorthand {
 589                     // This is a field literal
 590                     prefix = format!("{}: ", field.ident);
 591                     break;
 592                 }
 593             }
 594             if &prefix == "" {
 595                 // Likely a field was meant, but this field wasn't found. Do not suggest anything.
 596                 return false;
 597             }
 598         }
 599         if let hir::ExprKind::Call(path, args) = &expr.kind {
 600             if let (
 601                 hir::ExprKind::Path(hir::QPath::TypeRelative(base_ty, path_segment)),
 602                 1,
 603             ) = (&path.kind, args.len()) {
 604                 // `expr` is a conversion like `u32::from(val)`, do not suggest anything (#63697).
 605                 if let (
 606                     hir::TyKind::Path(hir::QPath::Resolved(None, base_ty_path)),
 607                     sym::from,
 608                 ) = (&base_ty.kind, path_segment.ident.name) {
 609                     if let Some(ident) = &base_ty_path.segments.iter().map(|s| s.ident).next() {
 610                         match ident.name {
 611                             sym::i128 | sym::i64 | sym::i32 | sym::i16 | sym::i8 |
 612                             sym::u128 | sym::u64 | sym::u32 | sym::u16 | sym::u8 |
 613                             sym::isize | sym::usize
 614                             if base_ty_path.segments.len() == 1 => {
 615                                 return false;
 616                             }
 617                             _ => {}
 618                         }
 619                     }
 620                 }
 621             }
 622         }
 623
 624         let msg = format!("you can convert an `{}` to `{}`", checked_ty, expected_ty);
 625         let cast_msg = format!("you can cast an `{} to `{}`", checked_ty, expected_ty);
 626         let try_msg = format!("{} and panic if the converted value wouldn't fit", msg);
 627         let lit_msg = format!(
 628             "change the type of the numeric literal from `{}` to `{}`",
 629             checked_ty,
 630             expected_ty,
 631         );
 632
 633         let needs_paren = expr.precedence().order() < (PREC_POSTFIX as i8);
 634
 635         if let Ok(src) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
 636             let cast_suggestion = format!(
 637                 "{}{}{}{} as {}",
 638                 prefix,
 639                 if needs_paren { "(" } else { "" },
 640                 src,
 641                 if needs_paren { ")" } else { "" },
 642                 expected_ty,
 643             );
 644             let try_into_suggestion = format!(
 645                 "{}{}{}{}.try_into().unwrap()",
 646                 prefix,
 647                 if needs_paren { "(" } else { "" },
 648                 src,
 649                 if needs_paren { ")" } else { "" },
 650             );
 651             let into_suggestion = format!(
 652                 "{}{}{}{}.into()",
 653                 prefix,
 654                 if needs_paren { "(" } else { "" },
 655                 src,
 656                 if needs_paren { ")" } else { "" },
 657             );
 658             let suffix_suggestion = format!(
 659                 "{}{}{}{}",
 660                 if needs_paren { "(" } else { "" },
 661                 if let (ty::Int(_), ty::Float(_)) | (ty::Uint(_), ty::Float(_)) = (
 662                     &expected_ty.kind,
 663                     &checked_ty.kind,
 664                 ) {
 665                     // Remove fractional part from literal, for example `42.0f32` into `42`
 666                     let src = src.trim_end_matches(&checked_ty.to_string());
 667                     src.split(".").next().unwrap()
 668                 } else {
 669                     src.trim_end_matches(&checked_ty.to_string())
 670                 },
 671                 expected_ty,
 672                 if needs_paren { ")" } else { "" },
 673             );
 674             let literal_is_ty_suffixed = |expr: &hir::Expr| {
 675                 if let hir::ExprKind::Lit(lit) = &expr.kind {
 676                     lit.node.is_suffixed()
 677                 } else {
 678                     false
 679                 }
 680             };
 681
 682             let suggest_to_change_suffix_or_into = |
 683                 err: &mut DiagnosticBuilder<'_>,
 684                 is_fallible: bool,
 685             | {
 686                 let into_sugg = into_suggestion.clone();
 687                 err.span_suggestion(
 688                     expr.span,
 689                     if literal_is_ty_suffixed(expr) {
 690                         &lit_msg
 691                     } else if is_fallible {
 692                         &try_msg
 693                     } else {
 694                         &msg
 695                     },
 696                     if literal_is_ty_suffixed(expr) {
 697                         suffix_suggestion.clone()
 698                     } else if is_fallible {
 699                         try_into_suggestion
 700                     } else {
 701                         into_sugg
 702                     },
 703                     Applicability::MachineApplicable,
 704                 );
 705             };
 706
 707             match (&expected_ty.kind, &checked_ty.kind) {
 708                 (&ty::Int(ref exp), &ty::Int(ref found)) => {
 709                     let is_fallible = match (found.bit_width(), exp.bit_width()) {
 710                         (Some(found), Some(exp)) if found > exp => true,
 711                         (None, _) | (_, None) => true,
 712                         _ => false,
 713                     };
 714                     suggest_to_change_suffix_or_into(err, is_fallible);
 715                     true
 716                 }
 717                 (&ty::Uint(ref exp), &ty::Uint(ref found)) => {
 718                     let is_fallible = match (found.bit_width(), exp.bit_width()) {
 719                         (Some(found), Some(exp)) if found > exp => true,
 720                         (None, _) | (_, None) => true,
 721                         _ => false,
 722                     };
 723                     suggest_to_change_suffix_or_into(err, is_fallible);
 724                     true
 725                 }
 726                 (&ty::Int(_), &ty::Uint(_)) | (&ty::Uint(_), &ty::Int(_)) => {
 727                     suggest_to_change_suffix_or_into(err, true);
 728                     true
 729                 }
 730                 (&ty::Float(ref exp), &ty::Float(ref found)) => {
 731                     if found.bit_width() < exp.bit_width() {
 732                         suggest_to_change_suffix_or_into(err, false);
 733                     } else if literal_is_ty_suffixed(expr) {
 734                         err.span_suggestion(
 735                             expr.span,
 736                             &lit_msg,
 737                             suffix_suggestion,
 738                             Applicability::MachineApplicable,
 739                         );
 740                     } else if can_cast { // Missing try_into implementation for `f64` to `f32`
 741                         err.span_suggestion(
 742                             expr.span,
 743                             &format!("{}, producing the closest possible value", cast_msg),
 744                             cast_suggestion,
 745                             Applicability::MaybeIncorrect,  // lossy conversion
 746                         );
 747                     }
 748                     true
 749                 }
 750                 (&ty::Uint(_), &ty::Float(_)) | (&ty::Int(_), &ty::Float(_)) => {
 751                     if literal_is_ty_suffixed(expr) {
 752                         err.span_suggestion(
 753                             expr.span,
 754                             &lit_msg,
 755                             suffix_suggestion,
 756                             Applicability::MachineApplicable,
 757                         );
 758                     } else if can_cast {
 759                         // Missing try_into implementation for `{float}` to `{integer}`
 760                         err.span_suggestion(
 761                             expr.span,
 762                             &format!("{}, rounding the float towards zero", msg),
 763                             cast_suggestion,
 764                             Applicability::MaybeIncorrect  // lossy conversion
 765                         );
 766                         err.warn("if the rounded value cannot be represented by the target \
 767                                   integer type, including `Inf` and `NaN`, casting will cause \
 768                                   undefined behavior \
 769                                   (https://github.com/rust-lang/rust/issues/10184)");
 770                     }
 771                     true
 772                 }
 773                 (&ty::Float(ref exp), &ty::Uint(ref found)) => {
 774                     // if `found` is `None` (meaning found is `usize`), don't suggest `.into()`
 775                     if exp.bit_width() > found.bit_width().unwrap_or(256) {
 776                         err.span_suggestion(
 777                             expr.span,
 778                             &format!(
 779                                 "{}, producing the floating point representation of the integer",
 780                                 msg,
 781                             ),
 782                             into_suggestion,
 783                             Applicability::MachineApplicable
 784                         );
 785                     } else if literal_is_ty_suffixed(expr) {
 786                         err.span_suggestion(
 787                             expr.span,
 788                             &lit_msg,
 789                             suffix_suggestion,
 790                             Applicability::MachineApplicable,
 791                         );
 792                     } else {
 793                         // Missing try_into implementation for `{integer}` to `{float}`
 794                         err.span_suggestion(
 795                             expr.span,
 796                             &format!(
 797                                 "{}, producing the floating point representation of the integer,
 798                                  rounded if necessary",
 799                                 cast_msg,
 800                             ),
 801                             cast_suggestion,
 802                             Applicability::MaybeIncorrect  // lossy conversion
 803                         );
 804                     }
 805                     true
 806                 }
 807                 (&ty::Float(ref exp), &ty::Int(ref found)) => {
 808                     // if `found` is `None` (meaning found is `isize`), don't suggest `.into()`
 809                     if exp.bit_width() > found.bit_width().unwrap_or(256) {
 810                         err.span_suggestion(
 811                             expr.span,
 812                             &format!(
 813                                 "{}, producing the floating point representation of the integer",
 814                                 &msg,
 815                             ),
 816                             into_suggestion,
 817                             Applicability::MachineApplicable
 818                         );
 819                     } else if literal_is_ty_suffixed(expr) {
 820                         err.span_suggestion(
 821                             expr.span,
 822                             &lit_msg,
 823                             suffix_suggestion,
 824                             Applicability::MachineApplicable,
 825                         );
 826                     } else {
 827                         // Missing try_into implementation for `{integer}` to `{float}`
 828                         err.span_suggestion(
 829                             expr.span,
 830                             &format!(
 831                                 "{}, producing the floating point representation of the integer, \
 832                                  rounded if necessary",
 833                                 &msg,
 834                             ),
 835                             cast_suggestion,
 836                             Applicability::MaybeIncorrect  // lossy conversion
 837                         );
 838                     }
 839                     true
 840                 }
 841                 _ => false,
 842             }
 843         } else {
 844             false
 845         }
 846     }
 847 }