Recover suggestions and useful information lost in previous PR

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -333,6 +333,9 @@ pub fn same_type_modulo_infer<'tcx>(a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
         )
         | (&ty::Infer(ty::InferTy::TyVar(_)), _)
         | (_, &ty::Infer(ty::InferTy::TyVar(_))) => true,
+        (&ty::Ref(reg_a, ty_a, mut_a), &ty::Ref(reg_b, ty_b, mut_b)) => {
+            reg_a == reg_b && mut_a == mut_b && same_type_modulo_infer(*ty_a, *ty_b)
+        }
         _ => a == b,
     }
 }
@@ -602,7 +605,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         match *cause.code() {
             ObligationCauseCode::Pattern { origin_expr: true, span: Some(span), root_ty } => {
                 let ty = self.resolve_vars_if_possible(root_ty);
-                if ty.is_suggestable() {
+                if !matches!(ty.kind(), ty::Infer(ty::InferTy::TyVar(_) | ty::InferTy::FreshTy(_)))
+                {
                     // don't show type `_`
                     err.span_label(span, format!("this expression has type `{}`", ty));
                 }

compiler/rustc_infer/src/infer/mod.rs

@@ -1434,6 +1434,17 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         value.fold_with(&mut r)
     }
 
+    pub fn resolve_numeric_literals_with_default<T>(&self, value: T) -> T
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        if !value.needs_infer() {
+            return value; // Avoid duplicated subst-folding.
+        }
+        let mut r = InferenceLiteralEraser { tcx: self.tcx };
+        value.fold_with(&mut r)
+    }
+
     /// Returns the first unresolved variable contained in `T`. In the
     /// process of visiting `T`, this will resolve (where possible)
     /// type variables in `T`, but it never constructs the final,
@@ -1785,6 +1796,26 @@ impl<'tcx> TyOrConstInferVar<'tcx> {
     }
 }
 
+/// Replace `{integer}` with `i32` and `{float}` with `f64`.
+/// Used only for diagnostics.
+struct InferenceLiteralEraser<'tcx> {
+    tcx: TyCtxt<'tcx>,
+}
+
+impl<'tcx> TypeFolder<'tcx> for InferenceLiteralEraser<'tcx> {
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        match ty.kind() {
+            ty::Infer(ty::IntVar(_) | ty::FreshIntTy(_)) => self.tcx.types.i32,
+            ty::Infer(ty::FloatVar(_) | ty::FreshFloatTy(_)) => self.tcx.types.f64,
+            _ => ty.super_fold_with(self),
+        }
+    }
+}
+
 struct ShallowResolver<'a, 'tcx> {
     infcx: &'a InferCtxt<'a, 'tcx>,
 }

compiler/rustc_typeck/src/astconv/generics.rs

@@ -11,6 +11,7 @@ use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::DefId;
 use rustc_hir::GenericArg;
+use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::ty::{
     self, subst, subst::SubstsRef, GenericParamDef, GenericParamDefKind, Ty, TyCtxt,
 };
@@ -83,7 +84,9 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     if let Some(param_local_id) = param.def_id.as_local() {
                         let param_hir_id = tcx.hir().local_def_id_to_hir_id(param_local_id);
                         let param_name = tcx.hir().ty_param_name(param_hir_id);
-                        let param_type = tcx.type_of(param.def_id);
+                        let param_type = tcx.infer_ctxt().enter(|infcx| {
+                            infcx.resolve_numeric_literals_with_default(tcx.type_of(param.def_id))
+                        });
                         if param_type.is_suggestable() {
                             err.span_suggestion(
                                 tcx.def_span(src_def_id),

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

@@ -521,20 +521,29 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         can_suggest: bool,
         fn_id: hir::HirId,
     ) -> bool {
+        let found =
+            self.resolve_numeric_literals_with_default(self.resolve_vars_if_possible(found));
         // Only suggest changing the return type for methods that
         // haven't set a return type at all (and aren't `fn main()` or an impl).
         match (&fn_decl.output, found.is_suggestable(), can_suggest, expected.is_unit()) {
             (&hir::FnRetTy::DefaultReturn(span), true, true, true) => {
                 err.span_suggestion(
                     span,
                     "try adding a return type",
-                    format!("-> {} ", self.resolve_vars_with_obligations(found)),
+                    format!("-> {} ", found),
                     Applicability::MachineApplicable,
                 );
                 true
             }
             (&hir::FnRetTy::DefaultReturn(span), false, true, true) => {
-                err.span_label(span, "possibly return type missing here?");
+                // FIXME: if `found` could be `impl Iterator` or `impl Fn*`, we should suggest
+                // that.
+                err.span_suggestion(
+                    span,
+                    "a return type might be missing here",
+                    "-> _ ".to_string(),
+                    Applicability::HasPlaceholders,
+                );
                 true
             }
             (&hir::FnRetTy::DefaultReturn(span), _, false, true) => {

src/test/ui/async-await/issue-61076.rs

@@ -87,6 +87,7 @@ async fn baz() -> Result<(), ()> {
 
 async fn match_() {
     match tuple() { //~ HELP consider `await`ing on the `Future`
+        //~^ NOTE this expression has type `impl Future<Output = Tuple>`
         Tuple(_) => {} //~ ERROR mismatched types
         //~^ NOTE expected opaque type, found struct `Tuple`
         //~| NOTE expected opaque type `impl Future<Output = Tuple>`

src/test/ui/async-await/issue-61076.stderr

@@ -56,8 +56,11 @@ LL |     struct_().await.method();
    |               ++++++
 
 error[E0308]: mismatched types
-  --> $DIR/issue-61076.rs:90:9
+  --> $DIR/issue-61076.rs:91:9
    |
+LL |     match tuple() {
+   |           ------- this expression has type `impl Future<Output = Tuple>`
+LL |
 LL |         Tuple(_) => {}
    |         ^^^^^^^^ expected opaque type, found struct `Tuple`
    |

src/test/ui/async-await/suggest-missing-await.stderr

@@ -91,6 +91,8 @@ LL ~         1 => dummy().await,
 error[E0308]: mismatched types
   --> $DIR/suggest-missing-await.rs:53:9
    |
+LL |     let _x = match dummy() {
+   |                    ------- this expression has type `impl Future<Output = ()>`
 LL |         () => {}
    |         ^^ expected opaque type, found `()`
    |
@@ -109,6 +111,9 @@ LL |     let _x = match dummy().await {
 error[E0308]: mismatched types
   --> $DIR/suggest-missing-await.rs:67:9
    |
+LL |     match dummy_result() {
+   |           -------------- this expression has type `impl Future<Output = Result<(), ()>>`
+...
 LL |         Ok(_) => {}
    |         ^^^^^ expected opaque type, found enum `Result`
    |
@@ -127,6 +132,9 @@ LL |     match dummy_result().await {
 error[E0308]: mismatched types
   --> $DIR/suggest-missing-await.rs:69:9
    |
+LL |     match dummy_result() {
+   |           -------------- this expression has type `impl Future<Output = Result<(), ()>>`
+...
 LL |         Err(_) => {}
    |         ^^^^^^ expected opaque type, found enum `Result`
    |

src/test/ui/blind/blind-item-block-middle.stderr

@@ -5,7 +5,7 @@ LL | mod foo { pub struct bar; }
    |           --------------- unit struct defined here
 ...
 LL |     let bar = 5;
-   |         ^^^
+   |         ^^^   - this expression has type `{integer}`
    |         |
    |         expected integer, found struct `bar`
    |         `bar` is interpreted as a unit struct, not a new binding

src/test/ui/block-result/issue-20862.stderr

@@ -2,7 +2,7 @@ error[E0308]: mismatched types
   --> $DIR/issue-20862.rs:2:5
    |
 LL | fn foo(x: i32) {
-   |                - possibly return type missing here?
+   |                - help: a return type might be missing here: `-> _`
 LL |     |y| x + y
    |     ^^^^^^^^^ expected `()`, found closure
    |

src/test/ui/destructuring-assignment/default-match-bindings-forbidden.stderr

@@ -2,7 +2,9 @@ error[E0308]: mismatched types
   --> $DIR/default-match-bindings-forbidden.rs:4:5
    |
 LL |     (x, y) = &(1, 2);
-   |     ^^^^^^ expected reference, found tuple
+   |     ^^^^^^   ------- this expression has type `&({integer}, {integer})`
+   |     |
+   |     expected reference, found tuple
    |
    = note: expected type `&({integer}, {integer})`
              found tuple `(_, _)`

src/test/ui/destructuring-assignment/tuple_destructure_fail.stderr

@@ -10,7 +10,9 @@ error[E0308]: mismatched types
   --> $DIR/tuple_destructure_fail.rs:6:5
    |
 LL |     (a, a, b) = (1, 2);
-   |     ^^^^^^^^^ expected a tuple with 2 elements, found one with 3 elements
+   |     ^^^^^^^^^   ------ this expression has type `({integer}, {integer})`
+   |     |
+   |     expected a tuple with 2 elements, found one with 3 elements
    |
    = note: expected type `({integer}, {integer})`
              found tuple `(_, _, _)`
@@ -27,7 +29,9 @@ error[E0308]: mismatched types
   --> $DIR/tuple_destructure_fail.rs:8:5
    |
 LL |     (_,) = (1, 2);
-   |     ^^^^ expected a tuple with 2 elements, found one with 1 element
+   |     ^^^^   ------ this expression has type `({integer}, {integer})`
+   |     |
+   |     expected a tuple with 2 elements, found one with 1 element
    |
    = note: expected type `({integer}, {integer})`
              found tuple `(_,)`

src/test/ui/half-open-range-patterns/exclusive_range_pattern_syntax_collision.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/exclusive_range_pattern_syntax_collision.rs:6:13
    |
+LL |     match [5..4, 99..105, 43..44] {
+   |           ----------------------- this expression has type `[std::ops::Range<{integer}>; 3]`
 LL |         [_, 99.., _] => {},
    |             ^^ expected struct `std::ops::Range`, found integer
    |

src/test/ui/half-open-range-patterns/exclusive_range_pattern_syntax_collision2.stderr

@@ -7,6 +7,8 @@ LL |         [_, 99..] => {},
 error[E0308]: mismatched types
   --> $DIR/exclusive_range_pattern_syntax_collision2.rs:6:13
    |
+LL |     match [5..4, 99..105, 43..44] {
+   |           ----------------------- this expression has type `[std::ops::Range<{integer}>; 3]`
 LL |         [_, 99..] => {},
    |             ^^ expected struct `std::ops::Range`, found integer
    |

src/test/ui/half-open-range-patterns/exclusive_range_pattern_syntax_collision3.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/exclusive_range_pattern_syntax_collision3.rs:6:12
    |
+LL |     match [5..4, 99..105, 43..44] {
+   |           ----------------------- this expression has type `[std::ops::Range<{integer}>; 3]`
 LL |         [..9, 99..100, _] => {},
    |            ^ expected struct `std::ops::Range`, found integer
    |
@@ -10,6 +12,8 @@ LL |         [..9, 99..100, _] => {},
 error[E0308]: mismatched types
   --> $DIR/exclusive_range_pattern_syntax_collision3.rs:6:15
    |
+LL |     match [5..4, 99..105, 43..44] {
+   |           ----------------------- this expression has type `[std::ops::Range<{integer}>; 3]`
 LL |         [..9, 99..100, _] => {},
    |               ^^  --- this is of type `{integer}`
    |               |
@@ -21,6 +25,8 @@ LL |         [..9, 99..100, _] => {},
 error[E0308]: mismatched types
   --> $DIR/exclusive_range_pattern_syntax_collision3.rs:6:19
    |
+LL |     match [5..4, 99..105, 43..44] {
+   |           ----------------------- this expression has type `[std::ops::Range<{integer}>; 3]`
 LL |         [..9, 99..100, _] => {},
    |               --  ^^^ expected struct `std::ops::Range`, found integer
    |               |

src/test/ui/half-open-range-patterns/pat-tuple-5.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/pat-tuple-5.rs:8:10
    |
+LL |     match (0, 1) {
+   |           ------ this expression has type `({integer}, {integer})`
 LL |         (PAT ..) => {}
    |          ^^^ expected tuple, found `u8`
    |

src/test/ui/issues/issue-11844.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/issue-11844.rs:6:9
    |
+LL |     match a {
+   |           - this expression has type `Option<Box<{integer}>>`
 LL |         Ok(a) =>
    |         ^^^^^ expected enum `Option`, found enum `Result`
    |

src/test/ui/issues/issue-12552.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/issue-12552.rs:6:5
    |
+LL |   match t {
+   |         - this expression has type `Result<_, {integer}>`
 LL |     Some(k) => match k {
    |     ^^^^^^^ expected enum `Result`, found enum `Option`
    |
@@ -10,6 +12,9 @@ LL |     Some(k) => match k {
 error[E0308]: mismatched types
   --> $DIR/issue-12552.rs:9:5
    |
+LL |   match t {
+   |         - this expression has type `Result<_, {integer}>`
+...
 LL |     None => ()
    |     ^^^^ expected enum `Result`, found enum `Option`
    |

src/test/ui/issues/issue-13466.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/issue-13466.rs:8:9
    |
+LL |     let _x: usize = match Some(1) {
+   |                           ------- this expression has type `Option<{integer}>`
 LL |         Ok(u) => u,
    |         ^^^^^ expected enum `Option`, found enum `Result`
    |
@@ -10,6 +12,9 @@ LL |         Ok(u) => u,
 error[E0308]: mismatched types
   --> $DIR/issue-13466.rs:14:9
    |
+LL |     let _x: usize = match Some(1) {
+   |                           ------- this expression has type `Option<{integer}>`
+...
 LL |         Err(e) => panic!(e)
    |         ^^^^^^ expected enum `Option`, found enum `Result`
    |

src/test/ui/issues/issue-33504.stderr

@@ -5,7 +5,7 @@ LL | struct Test;
    | ------------ unit struct defined here
 ...
 LL |         let Test = 1;
-   |             ^^^^
+   |             ^^^^   - this expression has type `{integer}`
    |             |
    |             expected integer, found struct `Test`
    |             `Test` is interpreted as a unit struct, not a new binding

src/test/ui/issues/issue-3680.stderr

@@ -1,6 +1,8 @@
 error[E0308]: mismatched types
   --> $DIR/issue-3680.rs:3:9
    |
+LL |     match None {
+   |           ---- this expression has type `Option<_>`
 LL |         Err(_) => ()
    |         ^^^^^^ expected enum `Option`, found enum `Result`
    |

src/test/ui/issues/issue-4968.stderr

@@ -5,11 +5,12 @@ LL | const A: (isize,isize) = (4,2);
    | ------------------------------- constant defined here
 LL | fn main() {
 LL |     match 42 { A => () }
-   |                ^
-   |                |
-   |                expected integer, found tuple
-   |                `A` is interpreted as a constant, not a new binding
-   |                help: introduce a new binding instead: `other_a`
+   |           --   ^
+   |           |    |
+   |           |    expected integer, found tuple
+   |           |    `A` is interpreted as a constant, not a new binding
+   |           |    help: introduce a new binding instead: `other_a`
+   |           this expression has type `{integer}`
    |
    = note: expected type `{integer}`
              found tuple `(isize, isize)`

src/test/ui/issues/issue-66706.stderr

@@ -36,23 +36,23 @@ error[E0308]: mismatched types
   --> $DIR/issue-66706.rs:2:5
    |
 LL | fn a() {
-   |        - possibly return type missing here?
+   |        - help: try adding a return type: `-> [i32; _]`
 LL |     [0; [|_: _ &_| ()].len()]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found array `[{integer}; _]`
 
 error[E0308]: mismatched types
   --> $DIR/issue-66706.rs:14:5
    |
 LL | fn c() {
-   |        - possibly return type missing here?
+   |        - help: try adding a return type: `-> [i32; _]`
 LL |     [0; [|&_: _ &_| {}; 0 ].len()]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found array `[{integer}; _]`
 
 error[E0308]: mismatched types
   --> $DIR/issue-66706.rs:20:5
    |
 LL | fn d() {
-   |        - possibly return type missing here?
+   |        - help: try adding a return type: `-> [i32; _]`
 LL |     [0; match [|f @ &ref _| () ] {} ]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found array `[{integer}; _]`
 

src/test/ui/issues/issue-72574-1.stderr

@@ -21,6 +21,8 @@ LL |         (_a, _x @ ..) => {}
 error[E0308]: mismatched types
   --> $DIR/issue-72574-1.rs:4:9
    |
+LL |     match x {
+   |           - this expression has type `({integer}, {integer}, {integer})`
 LL |         (_a, _x @ ..) => {}
    |         ^^^^^^^^^^^^^ expected a tuple with 3 elements, found one with 2 elements
    |