Rollup merge of rust-lang#96516 - oli-obk:impl_trait_inference_accidental_permitted, r=jackh726

Revert diagnostic duplication and accidental stabilization

fixes rust-lang#96460

this is an accidental stabilization that we should put into the beta. I believe it is low-risk, because it was literally what we had before rust-lang#94081

The effect on tests is massive, but mostly deduplication of diagnostics and some minor span changes.

Author: Dylan-DPC

compiler/rustc_typeck/src/check/check.rs

@@ -102,6 +102,12 @@ pub(super) fn check_fn<'a, 'tcx>(
             DUMMY_SP,
             param_env,
         ));
+    // HACK(oli-obk): we rewrite the declared return type, too, so that we don't end up inferring all
+    // unconstrained RPIT to have `()` as their hidden type. This would happen because further down we
+    // compare the ret_coercion with declared_ret_ty, and anything uninferred would be inferred to the
+    // opaque type itself. That again would cause writeback to assume we have a recursive call site
+    // and do the sadly stabilized fallback to `()`.
+    let declared_ret_ty = ret_ty;
     fcx.ret_coercion = Some(RefCell::new(CoerceMany::new(ret_ty)));
     fcx.ret_type_span = Some(decl.output.span());
 

src/test/ui/associated-types/impl-trait-return-missing-constraint.rs

@@ -24,7 +24,6 @@ fn bar() -> impl Bar {
 
 fn baz() -> impl Bar<Item = i32> {
     //~^ ERROR type mismatch resolving `<impl Bar as Foo>::Item == i32`
-    //~| ERROR type mismatch resolving `<impl Bar as Foo>::Item == i32`
     bar()
 }
 

src/test/ui/associated-types/impl-trait-return-missing-constraint.stderr

@@ -16,29 +16,6 @@ help: consider constraining the associated type `<impl Bar as Foo>::Item` to `i3
 LL | fn bar() -> impl Bar<Item = i32> {
    |                     ++++++++++++
 
-error[E0271]: type mismatch resolving `<impl Bar as Foo>::Item == i32`
-  --> $DIR/impl-trait-return-missing-constraint.rs:25:34
-   |
-LL |   fn bar() -> impl Bar {
-   |               -------- the expected opaque type
-...
-LL |   fn baz() -> impl Bar<Item = i32> {
-   |  __________________________________^
-LL | |
-LL | |
-LL | |     bar()
-LL | | }
-   | |_^ expected associated type, found `i32`
-   |
-   = note: expected associated type `<impl Bar as Foo>::Item`
-                         found type `i32`
-   = help: consider constraining the associated type `<impl Bar as Foo>::Item` to `i32` or calling a method that returns `<impl Bar as Foo>::Item`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
-help: consider constraining the associated type `<impl Bar as Foo>::Item` to `i32`
-   |
-LL | fn bar() -> impl Bar<Item = i32> {
-   |                     ++++++++++++
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0271`.

src/test/ui/conservative_impl_trait.rs

@@ -2,7 +2,6 @@
 
 fn will_ice(something: &u32) -> impl Iterator<Item = &u32> {
     //~^ ERROR `()` is not an iterator
-    //~| ERROR `()` is not an iterator
 }
 
 fn main() {}

src/test/ui/conservative_impl_trait.stderr

@@ -6,18 +6,6 @@ LL | fn will_ice(something: &u32) -> impl Iterator<Item = &u32> {
    |
    = help: the trait `Iterator` is not implemented for `()`
 
-error[E0277]: `()` is not an iterator
-  --> $DIR/conservative_impl_trait.rs:3:60
-   |
-LL |   fn will_ice(something: &u32) -> impl Iterator<Item = &u32> {
-   |  ____________________________________________________________^
-LL | |
-LL | |
-LL | | }
-   | |_^ `()` is not an iterator
-   |
-   = help: the trait `Iterator` is not implemented for `()`
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0277`.

src/test/ui/const-generics/defaults/rp_impl_trait_fail.rs

@@ -5,7 +5,6 @@ impl<const N: u32> Trait for Uwu<N> {}
 
 fn rawr() -> impl Trait {
     //~^ error: the trait bound `Uwu<10_u32, 12_u32>: Trait` is not satisfied
-    //~| error: the trait bound `Uwu<10_u32, 12_u32>: Trait` is not satisfied
     Uwu::<10, 12>
 }
 
@@ -17,13 +16,11 @@ impl Traitor<1, 2> for u64 {}
 
 fn uwu<const N: u8>() -> impl Traitor<N> {
     //~^ error: the trait bound `u32: Traitor<N, N>` is not satisfied
-    //~| error: the trait bound `u32: Traitor<N, N>` is not satisfied
     1_u32
 }
 
 fn owo() -> impl Traitor {
     //~^ error: the trait bound `u64: Traitor<1_u8, 1_u8>` is not satisfied
-    //~| error: the trait bound `u64: Traitor<1_u8, 1_u8>` is not satisfied
     1_u64
 }
 

src/test/ui/const-generics/defaults/rp_impl_trait_fail.stderr

@@ -7,22 +7,8 @@ LL | fn rawr() -> impl Trait {
    = help: the following implementations were found:
              <Uwu<N> as Trait>
 
-error[E0277]: the trait bound `Uwu<10_u32, 12_u32>: Trait` is not satisfied
-  --> $DIR/rp_impl_trait_fail.rs:6:25
-   |
-LL |   fn rawr() -> impl Trait {
-   |  _________________________^
-LL | |
-LL | |
-LL | |     Uwu::<10, 12>
-LL | | }
-   | |_^ the trait `Trait` is not implemented for `Uwu<10_u32, 12_u32>`
-   |
-   = help: the following implementations were found:
-             <Uwu<N> as Trait>
-
 error[E0277]: the trait bound `u32: Traitor<N, N>` is not satisfied
-  --> $DIR/rp_impl_trait_fail.rs:18:26
+  --> $DIR/rp_impl_trait_fail.rs:17:26
    |
 LL | fn uwu<const N: u8>() -> impl Traitor<N> {
    |                          ^^^^^^^^^^^^^^^ the trait `Traitor<N, N>` is not implemented for `u32`
@@ -31,23 +17,8 @@ LL | fn uwu<const N: u8>() -> impl Traitor<N> {
              <u32 as Traitor<N, 2_u8>>
              <u64 as Traitor<1_u8, 2_u8>>
 
-error[E0277]: the trait bound `u32: Traitor<N, N>` is not satisfied
-  --> $DIR/rp_impl_trait_fail.rs:18:42
-   |
-LL |   fn uwu<const N: u8>() -> impl Traitor<N> {
-   |  __________________________________________^
-LL | |
-LL | |
-LL | |     1_u32
-LL | | }
-   | |_^ the trait `Traitor<N, N>` is not implemented for `u32`
-   |
-   = help: the following implementations were found:
-             <u32 as Traitor<N, 2_u8>>
-             <u64 as Traitor<1_u8, 2_u8>>
-
 error[E0277]: the trait bound `u64: Traitor<1_u8, 1_u8>` is not satisfied
-  --> $DIR/rp_impl_trait_fail.rs:24:13
+  --> $DIR/rp_impl_trait_fail.rs:22:13
    |
 LL | fn owo() -> impl Traitor {
    |             ^^^^^^^^^^^^ the trait `Traitor<1_u8, 1_u8>` is not implemented for `u64`
@@ -56,21 +27,6 @@ LL | fn owo() -> impl Traitor {
              <u64 as Traitor<1_u8, 2_u8>>
              <u32 as Traitor<N, 2_u8>>
 
-error[E0277]: the trait bound `u64: Traitor<1_u8, 1_u8>` is not satisfied
-  --> $DIR/rp_impl_trait_fail.rs:24:26
-   |
-LL |   fn owo() -> impl Traitor {
-   |  __________________________^
-LL | |
-LL | |
-LL | |     1_u64
-LL | | }
-   | |_^ the trait `Traitor<1_u8, 1_u8>` is not implemented for `u64`
-   |
-   = help: the following implementations were found:
-             <u64 as Traitor<1_u8, 2_u8>>
-             <u32 as Traitor<N, 2_u8>>
-
-error: aborting due to 6 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

src/test/ui/generator/issue-88653.rs

@@ -7,13 +7,10 @@ use std::ops::Generator;
 
 fn foo(bar: bool) -> impl Generator<(bool,)> {
     //~^ ERROR: type mismatch in generator arguments [E0631]
-    //~| ERROR: type mismatch in generator arguments [E0631]
-    //~| NOTE: expected signature of `fn((bool,)) -> _`
     //~| NOTE: expected signature of `fn((bool,)) -> _`
     //~| NOTE: in this expansion of desugaring of `impl Trait`
     |bar| {
         //~^ NOTE: found signature of `fn(bool) -> _`
-        //~| NOTE: found signature of `fn(bool) -> _`
         if bar {
             yield bar;
         }

src/test/ui/generator/issue-88653.stderr

@@ -7,22 +7,6 @@ LL | fn foo(bar: bool) -> impl Generator<(bool,)> {
 LL |     |bar| {
    |     ----- found signature of `fn(bool) -> _`
 
-error[E0631]: type mismatch in generator arguments
-  --> $DIR/issue-88653.rs:8:46
-   |
-LL |   fn foo(bar: bool) -> impl Generator<(bool,)> {
-   |  ______________________________________________^
-LL | |
-LL | |
-LL | |
-...  |
-LL | |     |bar| {
-   | |     ----- found signature of `fn(bool) -> _`
-...  |
-LL | |     }
-LL | | }
-   | |_^ expected signature of `fn((bool,)) -> _`
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0631`.

src/test/ui/generator/type-mismatch-signature-deduction.rs

@@ -4,7 +4,6 @@ use std::ops::Generator;
 
 fn foo() -> impl Generator<Return = i32> {
     //~^ ERROR type mismatch
-    //~| ERROR type mismatch
     || {
         if false {
             return Ok(6);