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