compiler/rustc_typeck/src/check/fn_ctxt/suggestions.rs

   1 use super::FnCtxt;
   2 use crate::astconv::AstConv;
   3
   4 use rustc_ast::util::parser::ExprPrecedence;
   5 use rustc_span::{self, MultiSpan, Span};
   6
   7 use rustc_errors::{Applicability, Diagnostic};
   8 use rustc_hir as hir;
   9 use rustc_hir::def::{CtorOf, DefKind};
  10 use rustc_hir::lang_items::LangItem;
  11 use rustc_hir::{
  12     Expr, ExprKind, GenericBound, ItemKind, Node, Path, QPath, Stmt, StmtKind, TyKind,
  13     WherePredicate,
  14 };
  15 use rustc_infer::infer::{self, TyCtxtInferExt};
  16
  17 use rustc_middle::lint::in_external_macro;
  18 use rustc_middle::ty::{self, Binder, Ty};
  19 use rustc_span::symbol::{kw, sym};
  20
  21 use rustc_middle::ty::subst::GenericArgKind;
  22 use std::iter;
  23
  24 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  25     pub(in super::super) fn suggest_semicolon_at_end(&self, span: Span, err: &mut Diagnostic) {
  26         err.span_suggestion_short(
  27             span.shrink_to_hi(),
  28             "consider using a semicolon here",
  29             ";".to_string(),
  30             Applicability::MachineApplicable,
  31         );
  32     }
  33
  34     /// On implicit return expressions with mismatched types, provides the following suggestions:
  35     ///
  36     /// - Points out the method's return type as the reason for the expected type.
  37     /// - Possible missing semicolon.
  38     /// - Possible missing return type if the return type is the default, and not `fn main()`.
  39     pub fn suggest_mismatched_types_on_tail(
  40         &self,
  41         err: &mut Diagnostic,
  42         expr: &'tcx hir::Expr<'tcx>,
  43         expected: Ty<'tcx>,
  44         found: Ty<'tcx>,
  45         blk_id: hir::HirId,
  46     ) -> bool {
  47         let expr = expr.peel_drop_temps();
  48         // If the expression is from an external macro, then do not suggest
  49         // adding a semicolon, because there's nowhere to put it.
  50         // See issue #81943.
  51         if expr.can_have_side_effects() && !in_external_macro(self.tcx.sess, expr.span) {
  52             self.suggest_missing_semicolon(err, expr, expected);
  53         }
  54         let mut pointing_at_return_type = false;
  55         if let Some((fn_decl, can_suggest)) = self.get_fn_decl(blk_id) {
  56             let fn_id = self.tcx.hir().get_return_block(blk_id).unwrap();
  57             pointing_at_return_type = self.suggest_missing_return_type(
  58                 err,
  59                 &fn_decl,
  60                 expected,
  61                 found,
  62                 can_suggest,
  63                 fn_id,
  64             );
  65             self.suggest_missing_break_or_return_expr(
  66                 err, expr, &fn_decl, expected, found, blk_id, fn_id,
  67             );
  68         }
  69         pointing_at_return_type
  70     }
  71
  72     /// When encountering an fn-like ctor that needs to unify with a value, check whether calling
  73     /// the ctor would successfully solve the type mismatch and if so, suggest it:
  74     /// ```
  75     /// fn foo(x: usize) -> usize { x }
  76     /// let x: usize = foo;  // suggest calling the `foo` function: `foo(42)`
  77     /// ```
  78     fn suggest_fn_call(
  79         &self,
  80         err: &mut Diagnostic,
  81         expr: &hir::Expr<'_>,
  82         expected: Ty<'tcx>,
  83         found: Ty<'tcx>,
  84     ) -> bool {
  85         let hir = self.tcx.hir();
  86         let (def_id, sig) = match *found.kind() {
  87             ty::FnDef(def_id, _) => (def_id, found.fn_sig(self.tcx)),
  88             ty::Closure(def_id, substs) => (def_id, substs.as_closure().sig()),
  89             _ => return false,
  90         };
  91
  92         let sig = self.replace_bound_vars_with_fresh_vars(expr.span, infer::FnCall, sig).0;
  93         let sig = self.normalize_associated_types_in(expr.span, sig);
  94         if self.can_coerce(sig.output(), expected) {
  95             let (mut sugg_call, applicability) = if sig.inputs().is_empty() {
  96                 (String::new(), Applicability::MachineApplicable)
  97             } else {
  98                 ("...".to_string(), Applicability::HasPlaceholders)
  99             };
 100             let mut msg = "call this function";
 101             match hir.get_if_local(def_id) {
 102                 Some(
 103                     Node::Item(hir::Item { kind: ItemKind::Fn(.., body_id), .. })
 104                     | Node::ImplItem(hir::ImplItem {
 105                         kind: hir::ImplItemKind::Fn(_, body_id), ..
 106                     })
 107                     | Node::TraitItem(hir::TraitItem {
 108                         kind: hir::TraitItemKind::Fn(.., hir::TraitFn::Provided(body_id)),
 109                         ..
 110                     }),
 111                 ) => {
 112                     let body = hir.body(*body_id);
 113                     sugg_call = body
 114                         .params
 115                         .iter()
 116                         .map(|param| match &param.pat.kind {
 117                             hir::PatKind::Binding(_, _, ident, None)
 118                                 if ident.name != kw::SelfLower =>
 119                             {
 120                                 ident.to_string()
 121                             }
 122                             _ => "_".to_string(),
 123                         })
 124                         .collect::<Vec<_>>()
 125                         .join(", ");
 126                 }
 127                 Some(Node::Expr(hir::Expr {
 128                     kind: ExprKind::Closure(_, _, body_id, _, _),
 129                     span: full_closure_span,
 130                     ..
 131                 })) => {
 132                     if *full_closure_span == expr.span {
 133                         return false;
 134                     }
 135                     msg = "call this closure";
 136                     let body = hir.body(*body_id);
 137                     sugg_call = body
 138                         .params
 139                         .iter()
 140                         .map(|param| match &param.pat.kind {
 141                             hir::PatKind::Binding(_, _, ident, None)
 142                                 if ident.name != kw::SelfLower =>
 143                             {
 144                                 ident.to_string()
 145                             }
 146                             _ => "_".to_string(),
 147                         })
 148                         .collect::<Vec<_>>()
 149                         .join(", ");
 150                 }
 151                 Some(Node::Ctor(hir::VariantData::Tuple(fields, _))) => {
 152                     sugg_call = fields.iter().map(|_| "_").collect::<Vec<_>>().join(", ");
 153                     match def_id.as_local().map(|def_id| hir.def_kind(def_id)) {
 154                         Some(DefKind::Ctor(hir::def::CtorOf::Variant, _)) => {
 155                             msg = "instantiate this tuple variant";
 156                         }
 157                         Some(DefKind::Ctor(CtorOf::Struct, _)) => {
 158                             msg = "instantiate this tuple struct";
 159                         }
 160                         _ => {}
 161                     }
 162                 }
 163                 Some(Node::ForeignItem(hir::ForeignItem {
 164                     kind: hir::ForeignItemKind::Fn(_, idents, _),
 165                     ..
 166                 })) => {
 167                     sugg_call = idents
 168                         .iter()
 169                         .map(|ident| {
 170                             if ident.name != kw::SelfLower {
 171                                 ident.to_string()
 172                             } else {
 173                                 "_".to_string()
 174                             }
 175                         })
 176                         .collect::<Vec<_>>()
 177                         .join(", ")
 178                 }
 179                 Some(Node::TraitItem(hir::TraitItem {
 180                     kind: hir::TraitItemKind::Fn(.., hir::TraitFn::Required(idents)),
 181                     ..
 182                 })) => {
 183                     sugg_call = idents
 184                         .iter()
 185                         .map(|ident| {
 186                             if ident.name != kw::SelfLower {
 187                                 ident.to_string()
 188                             } else {
 189                                 "_".to_string()
 190                             }
 191                         })
 192                         .collect::<Vec<_>>()
 193                         .join(", ")
 194                 }
 195                 _ => {}
 196             }
 197             err.span_suggestion_verbose(
 198                 expr.span.shrink_to_hi(),
 199                 &format!("use parentheses to {}", msg),
 200                 format!("({})", sugg_call),
 201                 applicability,
 202             );
 203             return true;
 204         }
 205         false
 206     }
 207
 208     pub fn suggest_deref_ref_or_into(
 209         &self,
 210         err: &mut Diagnostic,
 211         expr: &hir::Expr<'tcx>,
 212         expected: Ty<'tcx>,
 213         found: Ty<'tcx>,
 214         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 215     ) {
 216         let expr = expr.peel_blocks();
 217         if let Some((sp, msg, suggestion, applicability, verbose)) =
 218             self.check_ref(expr, found, expected)
 219         {
 220             if verbose {
 221                 err.span_suggestion_verbose(sp, &msg, suggestion, applicability);
 222             } else {
 223                 err.span_suggestion(sp, &msg, suggestion, applicability);
 224             }
 225         } else if let (ty::FnDef(def_id, ..), true) =
 226             (&found.kind(), self.suggest_fn_call(err, expr, expected, found))
 227         {
 228             if let Some(sp) = self.tcx.hir().span_if_local(*def_id) {
 229                 let sp = self.sess().source_map().guess_head_span(sp);
 230                 err.span_label(sp, &format!("{} defined here", found));
 231             }
 232         } else if !self.check_for_cast(err, expr, found, expected, expected_ty_expr) {
 233             let is_struct_pat_shorthand_field =
 234                 self.maybe_get_struct_pattern_shorthand_field(expr).is_some();
 235             let methods = self.get_conversion_methods(expr.span, expected, found, expr.hir_id);
 236             if !methods.is_empty() {
 237                 if let Ok(expr_text) = self.sess().source_map().span_to_snippet(expr.span) {
 238                     let mut suggestions = iter::zip(iter::repeat(&expr_text), &methods)
 239                         .filter_map(|(receiver, method)| {
 240                             let method_call = format!(".{}()", method.name);
 241                             if receiver.ends_with(&method_call) {
 242                                 None // do not suggest code that is already there (#53348)
 243                             } else {
 244                                 let method_call_list = [".to_vec()", ".to_string()"];
 245                                 let mut sugg = if receiver.ends_with(".clone()")
 246                                     && method_call_list.contains(&method_call.as_str())
 247                                 {
 248                                     let max_len = receiver.rfind('.').unwrap();
 249                                     vec![(
 250                                         expr.span,
 251                                         format!("{}{}", &receiver[..max_len], method_call),
 252                                     )]
 253                                 } else {
 254                                     if expr.precedence().order()
 255                                         < ExprPrecedence::MethodCall.order()
 256                                     {
 257                                         vec![
 258                                             (expr.span.shrink_to_lo(), "(".to_string()),
 259                                             (expr.span.shrink_to_hi(), format!("){}", method_call)),
 260                                         ]
 261                                     } else {
 262                                         vec![(expr.span.shrink_to_hi(), method_call)]
 263                                     }
 264                                 };
 265                                 if is_struct_pat_shorthand_field {
 266                                     sugg.insert(
 267                                         0,
 268                                         (expr.span.shrink_to_lo(), format!("{}: ", receiver)),
 269                                     );
 270                                 }
 271                                 Some(sugg)
 272                             }
 273                         })
 274                         .peekable();
 275                     if suggestions.peek().is_some() {
 276                         err.multipart_suggestions(
 277                             "try using a conversion method",
 278                             suggestions,
 279                             Applicability::MaybeIncorrect,
 280                         );
 281                     }
 282                 }
 283             } else if found.to_string().starts_with("Option<")
 284                 && expected.to_string() == "Option<&str>"
 285             {
 286                 if let ty::Adt(_def, subst) = found.kind() {
 287                     if subst.len() != 0 {
 288                         if let GenericArgKind::Type(ty) = subst[0].unpack() {
 289                             let peeled = ty.peel_refs().to_string();
 290                             if peeled == "String" {
 291                                 let ref_cnt = ty.to_string().len() - peeled.len();
 292                                 let result = format!(".map(|x| &*{}x)", "*".repeat(ref_cnt));
 293                                 err.span_suggestion_verbose(
 294                                     expr.span.shrink_to_hi(),
 295                                     "try converting the passed type into a `&str`",
 296                                     result,
 297                                     Applicability::MaybeIncorrect,
 298                                 );
 299                             }
 300                         }
 301                     }
 302                 }
 303             }
 304         }
 305     }
 306
 307     /// When encountering the expected boxed value allocated in the stack, suggest allocating it
 308     /// in the heap by calling `Box::new()`.
 309     pub(in super::super) fn suggest_boxing_when_appropriate(
 310         &self,
 311         err: &mut Diagnostic,
 312         expr: &hir::Expr<'_>,
 313         expected: Ty<'tcx>,
 314         found: Ty<'tcx>,
 315     ) {
 316         if self.tcx.hir().is_inside_const_context(expr.hir_id) {
 317             // Do not suggest `Box::new` in const context.
 318             return;
 319         }
 320         if !expected.is_box() || found.is_box() {
 321             return;
 322         }
 323         let boxed_found = self.tcx.mk_box(found);
 324         if self.can_coerce(boxed_found, expected) {
 325             err.multipart_suggestion(
 326                 "store this in the heap by calling `Box::new`",
 327                 vec![
 328                     (expr.span.shrink_to_lo(), "Box::new(".to_string()),
 329                     (expr.span.shrink_to_hi(), ")".to_string()),
 330                 ],
 331                 Applicability::MachineApplicable,
 332             );
 333             err.note(
 334                 "for more on the distinction between the stack and the heap, read \
 335                  https://doc.rust-lang.org/book/ch15-01-box.html, \
 336                  https://doc.rust-lang.org/rust-by-example/std/box.html, and \
 337                  https://doc.rust-lang.org/std/boxed/index.html",
 338             );
 339         }
 340     }
 341
 342     /// When encountering a closure that captures variables, where a FnPtr is expected,
 343     /// suggest a non-capturing closure
 344     pub(in super::super) fn suggest_no_capture_closure(
 345         &self,
 346         err: &mut Diagnostic,
 347         expected: Ty<'tcx>,
 348         found: Ty<'tcx>,
 349     ) {
 350         if let (ty::FnPtr(_), ty::Closure(def_id, _)) = (expected.kind(), found.kind()) {
 351             if let Some(upvars) = self.tcx.upvars_mentioned(*def_id) {
 352                 // Report upto four upvars being captured to reduce the amount error messages
 353                 // reported back to the user.
 354                 let spans_and_labels = upvars
 355                     .iter()
 356                     .take(4)
 357                     .map(|(var_hir_id, upvar)| {
 358                         let var_name = self.tcx.hir().name(*var_hir_id).to_string();
 359                         let msg = format!("`{}` captured here", var_name);
 360                         (upvar.span, msg)
 361                     })
 362                     .collect::<Vec<_>>();
 363
 364                 let mut multi_span: MultiSpan =
 365                     spans_and_labels.iter().map(|(sp, _)| *sp).collect::<Vec<_>>().into();
 366                 for (sp, label) in spans_and_labels {
 367                     multi_span.push_span_label(sp, label);
 368                 }
 369                 err.span_note(
 370                     multi_span,
 371                     "closures can only be coerced to `fn` types if they do not capture any variables"
 372                 );
 373             }
 374         }
 375     }
 376
 377     /// When encountering an `impl Future` where `BoxFuture` is expected, suggest `Box::pin`.
 378     #[instrument(skip(self, err))]
 379     pub(in super::super) fn suggest_calling_boxed_future_when_appropriate(
 380         &self,
 381         err: &mut Diagnostic,
 382         expr: &hir::Expr<'_>,
 383         expected: Ty<'tcx>,
 384         found: Ty<'tcx>,
 385     ) -> bool {
 386         // Handle #68197.
 387
 388         if self.tcx.hir().is_inside_const_context(expr.hir_id) {
 389             // Do not suggest `Box::new` in const context.
 390             return false;
 391         }
 392         let pin_did = self.tcx.lang_items().pin_type();
 393         // This guards the `unwrap` and `mk_box` below.
 394         if pin_did.is_none() || self.tcx.lang_items().owned_box().is_none() {
 395             return false;
 396         }
 397         let box_found = self.tcx.mk_box(found);
 398         let pin_box_found = self.tcx.mk_lang_item(box_found, LangItem::Pin).unwrap();
 399         let pin_found = self.tcx.mk_lang_item(found, LangItem::Pin).unwrap();
 400         match expected.kind() {
 401             ty::Adt(def, _) if Some(def.did()) == pin_did => {
 402                 if self.can_coerce(pin_box_found, expected) {
 403                     debug!("can coerce {:?} to {:?}, suggesting Box::pin", pin_box_found, expected);
 404                     match found.kind() {
 405                         ty::Adt(def, _) if def.is_box() => {
 406                             err.help("use `Box::pin`");
 407                         }
 408                         _ => {
 409                             err.multipart_suggestion(
 410                                 "you need to pin and box this expression",
 411                                 vec![
 412                                     (expr.span.shrink_to_lo(), "Box::pin(".to_string()),
 413                                     (expr.span.shrink_to_hi(), ")".to_string()),
 414                                 ],
 415                                 Applicability::MaybeIncorrect,
 416                             );
 417                         }
 418                     }
 419                     true
 420                 } else if self.can_coerce(pin_found, expected) {
 421                     match found.kind() {
 422                         ty::Adt(def, _) if def.is_box() => {
 423                             err.help("use `Box::pin`");
 424                             true
 425                         }
 426                         _ => false,
 427                     }
 428                 } else {
 429                     false
 430                 }
 431             }
 432             ty::Adt(def, _) if def.is_box() && self.can_coerce(box_found, expected) => {
 433                 // Check if the parent expression is a call to Pin::new.  If it
 434                 // is and we were expecting a Box, ergo Pin<Box<expected>>, we
 435                 // can suggest Box::pin.
 436                 let parent = self.tcx.hir().get_parent_node(expr.hir_id);
 437                 let Some(Node::Expr(Expr { kind: ExprKind::Call(fn_name, _), .. })) = self.tcx.hir().find(parent) else {
 438                     return false;
 439                 };
 440                 match fn_name.kind {
 441                     ExprKind::Path(QPath::TypeRelative(
 442                         hir::Ty {
 443                             kind: TyKind::Path(QPath::Resolved(_, Path { res: recv_ty, .. })),
 444                             ..
 445                         },
 446                         method,
 447                     )) if recv_ty.opt_def_id() == pin_did && method.ident.name == sym::new => {
 448                         err.span_suggestion(
 449                             fn_name.span,
 450                             "use `Box::pin` to pin and box this expression",
 451                             "Box::pin".to_string(),
 452                             Applicability::MachineApplicable,
 453                         );
 454                         true
 455                     }
 456                     _ => false,
 457                 }
 458             }
 459             _ => false,
 460         }
 461     }
 462
 463     /// A common error is to forget to add a semicolon at the end of a block, e.g.,
 464     ///
 465     /// ```
 466     /// fn foo() {
 467     ///     bar_that_returns_u32()
 468     /// }
 469     /// ```
 470     ///
 471     /// This routine checks if the return expression in a block would make sense on its own as a
 472     /// statement and the return type has been left as default or has been specified as `()`. If so,
 473     /// it suggests adding a semicolon.
 474     fn suggest_missing_semicolon(
 475         &self,
 476         err: &mut Diagnostic,
 477         expression: &'tcx hir::Expr<'tcx>,
 478         expected: Ty<'tcx>,
 479     ) {
 480         if expected.is_unit() {
 481             // `BlockTailExpression` only relevant if the tail expr would be
 482             // useful on its own.
 483             match expression.kind {
 484                 ExprKind::Call(..)
 485                 | ExprKind::MethodCall(..)
 486                 | ExprKind::Loop(..)
 487                 | ExprKind::If(..)
 488                 | ExprKind::Match(..)
 489                 | ExprKind::Block(..)
 490                     if expression.can_have_side_effects() =>
 491                 {
 492                     err.span_suggestion(
 493                         expression.span.shrink_to_hi(),
 494                         "consider using a semicolon here",
 495                         ";".to_string(),
 496                         Applicability::MachineApplicable,
 497                     );
 498                 }
 499                 _ => (),
 500             }
 501         }
 502     }
 503
 504     /// A possible error is to forget to add a return type that is needed:
 505     ///
 506     /// ```
 507     /// fn foo() {
 508     ///     bar_that_returns_u32()
 509     /// }
 510     /// ```
 511     ///
 512     /// This routine checks if the return type is left as default, the method is not part of an
 513     /// `impl` block and that it isn't the `main` method. If so, it suggests setting the return
 514     /// type.
 515     pub(in super::super) fn suggest_missing_return_type(
 516         &self,
 517         err: &mut Diagnostic,
 518         fn_decl: &hir::FnDecl<'_>,
 519         expected: Ty<'tcx>,
 520         found: Ty<'tcx>,
 521         can_suggest: bool,
 522         fn_id: hir::HirId,
 523     ) -> bool {
 524         let found =
 525             self.resolve_numeric_literals_with_default(self.resolve_vars_if_possible(found));
 526         // Only suggest changing the return type for methods that
 527         // haven't set a return type at all (and aren't `fn main()` or an impl).
 528         match (&fn_decl.output, found.is_suggestable(), can_suggest, expected.is_unit()) {
 529             (&hir::FnRetTy::DefaultReturn(span), true, true, true) => {
 530                 err.span_suggestion(
 531                     span,
 532                     "try adding a return type",
 533                     format!("-> {} ", found),
 534                     Applicability::MachineApplicable,
 535                 );
 536                 true
 537             }
 538             (&hir::FnRetTy::DefaultReturn(span), false, true, true) => {
 539                 // FIXME: if `found` could be `impl Iterator` or `impl Fn*`, we should suggest
 540                 // that.
 541                 err.span_suggestion(
 542                     span,
 543                     "a return type might be missing here",
 544                     "-> _ ".to_string(),
 545                     Applicability::HasPlaceholders,
 546                 );
 547                 true
 548             }
 549             (&hir::FnRetTy::DefaultReturn(span), _, false, true) => {
 550                 // `fn main()` must return `()`, do not suggest changing return type
 551                 err.span_label(span, "expected `()` because of default return type");
 552                 true
 553             }
 554             // expectation was caused by something else, not the default return
 555             (&hir::FnRetTy::DefaultReturn(_), _, _, false) => false,
 556             (&hir::FnRetTy::Return(ref ty), _, _, _) => {
 557                 // Only point to return type if the expected type is the return type, as if they
 558                 // are not, the expectation must have been caused by something else.
 559                 debug!("suggest_missing_return_type: return type {:?} node {:?}", ty, ty.kind);
 560                 let sp = ty.span;
 561                 let ty = <dyn AstConv<'_>>::ast_ty_to_ty(self, ty);
 562                 debug!("suggest_missing_return_type: return type {:?}", ty);
 563                 debug!("suggest_missing_return_type: expected type {:?}", ty);
 564                 let bound_vars = self.tcx.late_bound_vars(fn_id);
 565                 let ty = Binder::bind_with_vars(ty, bound_vars);
 566                 let ty = self.normalize_associated_types_in(sp, ty);
 567                 let ty = self.tcx.erase_late_bound_regions(ty);
 568                 if self.can_coerce(expected, ty) {
 569                     err.span_label(sp, format!("expected `{}` because of return type", expected));
 570                     self.try_suggest_return_impl_trait(err, expected, ty, fn_id);
 571                     return true;
 572                 }
 573                 false
 574             }
 575         }
 576     }
 577
 578     /// check whether the return type is a generic type with a trait bound
 579     /// only suggest this if the generic param is not present in the arguments
 580     /// if this is true, hint them towards changing the return type to `impl Trait`
 581     /// ```
 582     /// fn cant_name_it<T: Fn() -> u32>() -> T {
 583     ///     || 3
 584     /// }
 585     /// ```
 586     fn try_suggest_return_impl_trait(
 587         &self,
 588         err: &mut Diagnostic,
 589         expected: Ty<'tcx>,
 590         found: Ty<'tcx>,
 591         fn_id: hir::HirId,
 592     ) {
 593         // Only apply the suggestion if:
 594         //  - the return type is a generic parameter
 595         //  - the generic param is not used as a fn param
 596         //  - the generic param has at least one bound
 597         //  - the generic param doesn't appear in any other bounds where it's not the Self type
 598         // Suggest:
 599         //  - Changing the return type to be `impl <all bounds>`
 600
 601         debug!("try_suggest_return_impl_trait, expected = {:?}, found = {:?}", expected, found);
 602
 603         let ty::Param(expected_ty_as_param) = expected.kind() else { return };
 604
 605         let fn_node = self.tcx.hir().find(fn_id);
 606
 607         let Some(hir::Node::Item(hir::Item {
 608             kind:
 609                 hir::ItemKind::Fn(
 610                     hir::FnSig { decl: hir::FnDecl { inputs: fn_parameters, output: fn_return, .. }, .. },
 611                     hir::Generics { params, where_clause, .. },
 612                     _body_id,
 613                 ),
 614             ..
 615         })) = fn_node else { return };
 616
 617         let Some(expected_generic_param) = params.get(expected_ty_as_param.index as usize) else { return };
 618
 619         // get all where BoundPredicates here, because they are used in to cases below
 620         let where_predicates = where_clause
 621             .predicates
 622             .iter()
 623             .filter_map(|p| match p {
 624                 WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
 625                     bounds,
 626                     bounded_ty,
 627                     ..
 628                 }) => {
 629                     // FIXME: Maybe these calls to `ast_ty_to_ty` can be removed (and the ones below)
 630                     let ty = <dyn AstConv<'_>>::ast_ty_to_ty(self, bounded_ty);
 631                     Some((ty, bounds))
 632                 }
 633                 _ => None,
 634             })
 635             .map(|(ty, bounds)| match ty.kind() {
 636                 ty::Param(param_ty) if param_ty == expected_ty_as_param => Ok(Some(bounds)),
 637                 // check whether there is any predicate that contains our `T`, like `Option<T>: Send`
 638                 _ => match ty.contains(expected) {
 639                     true => Err(()),
 640                     false => Ok(None),
 641                 },
 642             })
 643             .collect::<Result<Vec<_>, _>>();
 644
 645         let Ok(where_predicates) = where_predicates else { return };
 646
 647         // now get all predicates in the same types as the where bounds, so we can chain them
 648         let predicates_from_where =
 649             where_predicates.iter().flatten().map(|bounds| bounds.iter()).flatten();
 650
 651         // extract all bounds from the source code using their spans
 652         let all_matching_bounds_strs = expected_generic_param
 653             .bounds
 654             .iter()
 655             .chain(predicates_from_where)
 656             .filter_map(|bound| match bound {
 657                 GenericBound::Trait(_, _) => {
 658                     self.tcx.sess.source_map().span_to_snippet(bound.span()).ok()
 659                 }
 660                 _ => None,
 661             })
 662             .collect::<Vec<String>>();
 663
 664         if all_matching_bounds_strs.len() == 0 {
 665             return;
 666         }
 667
 668         let all_bounds_str = all_matching_bounds_strs.join(" + ");
 669
 670         let ty_param_used_in_fn_params = fn_parameters.iter().any(|param| {
 671                 let ty = <dyn AstConv<'_>>::ast_ty_to_ty(self, param);
 672                 matches!(ty.kind(), ty::Param(fn_param_ty_param) if expected_ty_as_param == fn_param_ty_param)
 673             });
 674
 675         if ty_param_used_in_fn_params {
 676             return;
 677         }
 678
 679         err.span_suggestion(
 680             fn_return.span(),
 681             "consider using an impl return type",
 682             format!("impl {}", all_bounds_str),
 683             Applicability::MaybeIncorrect,
 684         );
 685     }
 686
 687     pub(in super::super) fn suggest_missing_break_or_return_expr(
 688         &self,
 689         err: &mut Diagnostic,
 690         expr: &'tcx hir::Expr<'tcx>,
 691         fn_decl: &hir::FnDecl<'_>,
 692         expected: Ty<'tcx>,
 693         found: Ty<'tcx>,
 694         id: hir::HirId,
 695         fn_id: hir::HirId,
 696     ) {
 697         if !expected.is_unit() {
 698             return;
 699         }
 700         let found = self.resolve_vars_with_obligations(found);
 701
 702         let in_loop = self.is_loop(id)
 703             || self.tcx.hir().parent_iter(id).any(|(parent_id, _)| self.is_loop(parent_id));
 704
 705         let in_local_statement = self.is_local_statement(id)
 706             || self
 707                 .tcx
 708                 .hir()
 709                 .parent_iter(id)
 710                 .any(|(parent_id, _)| self.is_local_statement(parent_id));
 711
 712         if in_loop && in_local_statement {
 713             err.multipart_suggestion(
 714                 "you might have meant to break the loop with this value",
 715                 vec![
 716                     (expr.span.shrink_to_lo(), "break ".to_string()),
 717                     (expr.span.shrink_to_hi(), ";".to_string()),
 718                 ],
 719                 Applicability::MaybeIncorrect,
 720             );
 721             return;
 722         }
 723
 724         if let hir::FnRetTy::Return(ty) = fn_decl.output {
 725             let ty = <dyn AstConv<'_>>::ast_ty_to_ty(self, ty);
 726             let bound_vars = self.tcx.late_bound_vars(fn_id);
 727             let ty = self.tcx.erase_late_bound_regions(Binder::bind_with_vars(ty, bound_vars));
 728             let ty = self.normalize_associated_types_in(expr.span, ty);
 729             let ty = match self.tcx.asyncness(fn_id.owner) {
 730                 hir::IsAsync::Async => self
 731                     .tcx
 732                     .infer_ctxt()
 733                     .enter(|infcx| {
 734                         infcx.get_impl_future_output_ty(ty).unwrap_or_else(|| {
 735                             span_bug!(
 736                                 fn_decl.output.span(),
 737                                 "failed to get output type of async function"
 738                             )
 739                         })
 740                     })
 741                     .skip_binder(),
 742                 hir::IsAsync::NotAsync => ty,
 743             };
 744             if self.can_coerce(found, ty) {
 745                 err.multipart_suggestion(
 746                     "you might have meant to return this value",
 747                     vec![
 748                         (expr.span.shrink_to_lo(), "return ".to_string()),
 749                         (expr.span.shrink_to_hi(), ";".to_string()),
 750                     ],
 751                     Applicability::MaybeIncorrect,
 752                 );
 753             }
 754         }
 755     }
 756
 757     pub(in super::super) fn suggest_missing_parentheses(
 758         &self,
 759         err: &mut Diagnostic,
 760         expr: &hir::Expr<'_>,
 761     ) {
 762         let sp = self.tcx.sess.source_map().start_point(expr.span);
 763         if let Some(sp) = self.tcx.sess.parse_sess.ambiguous_block_expr_parse.borrow().get(&sp) {
 764             // `{ 42 } &&x` (#61475) or `{ 42 } && if x { 1 } else { 0 }`
 765             self.tcx.sess.parse_sess.expr_parentheses_needed(err, *sp);
 766         }
 767     }
 768
 769     /// Given an expression type mismatch, peel any `&` expressions until we get to
 770     /// a block expression, and then suggest replacing the braces with square braces
 771     /// if it was possibly mistaken array syntax.
 772     pub(crate) fn suggest_block_to_brackets_peeling_refs(
 773         &self,
 774         diag: &mut Diagnostic,
 775         mut expr: &hir::Expr<'_>,
 776         mut expr_ty: Ty<'tcx>,
 777         mut expected_ty: Ty<'tcx>,
 778     ) {
 779         loop {
 780             match (&expr.kind, expr_ty.kind(), expected_ty.kind()) {
 781                 (
 782                     hir::ExprKind::AddrOf(_, _, inner_expr),
 783                     ty::Ref(_, inner_expr_ty, _),
 784                     ty::Ref(_, inner_expected_ty, _),
 785                 ) => {
 786                     expr = *inner_expr;
 787                     expr_ty = *inner_expr_ty;
 788                     expected_ty = *inner_expected_ty;
 789                 }
 790                 (hir::ExprKind::Block(blk, _), _, _) => {
 791                     self.suggest_block_to_brackets(diag, *blk, expr_ty, expected_ty);
 792                     break;
 793                 }
 794                 _ => break,
 795             }
 796         }
 797     }
 798
 799     /// Suggest wrapping the block in square brackets instead of curly braces
 800     /// in case the block was mistaken array syntax, e.g. `{ 1 }` -> `[ 1 ]`.
 801     pub(crate) fn suggest_block_to_brackets(
 802         &self,
 803         diag: &mut Diagnostic,
 804         blk: &hir::Block<'_>,
 805         blk_ty: Ty<'tcx>,
 806         expected_ty: Ty<'tcx>,
 807     ) {
 808         if let ty::Slice(elem_ty) | ty::Array(elem_ty, _) = expected_ty.kind() {
 809             if self.can_coerce(blk_ty, *elem_ty)
 810                 && blk.stmts.is_empty()
 811                 && blk.rules == hir::BlockCheckMode::DefaultBlock
 812             {
 813                 let source_map = self.tcx.sess.source_map();
 814                 if let Ok(snippet) = source_map.span_to_snippet(blk.span) {
 815                     if snippet.starts_with('{') && snippet.ends_with('}') {
 816                         diag.multipart_suggestion_verbose(
 817                             "to create an array, use square brackets instead of curly braces",
 818                             vec![
 819                                 (
 820                                     blk.span
 821                                         .shrink_to_lo()
 822                                         .with_hi(rustc_span::BytePos(blk.span.lo().0 + 1)),
 823                                     "[".to_string(),
 824                                 ),
 825                                 (
 826                                     blk.span
 827                                         .shrink_to_hi()
 828                                         .with_lo(rustc_span::BytePos(blk.span.hi().0 - 1)),
 829                                     "]".to_string(),
 830                                 ),
 831                             ],
 832                             Applicability::MachineApplicable,
 833                         );
 834                     }
 835                 }
 836             }
 837         }
 838     }
 839
 840     fn is_loop(&self, id: hir::HirId) -> bool {
 841         let node = self.tcx.hir().get(id);
 842         matches!(node, Node::Expr(Expr { kind: ExprKind::Loop(..), .. }))
 843     }
 844
 845     fn is_local_statement(&self, id: hir::HirId) -> bool {
 846         let node = self.tcx.hir().get(id);
 847         matches!(node, Node::Stmt(Stmt { kind: StmtKind::Local(..), .. }))
 848     }
 849 }