Revert "Auto merge of #93893 - oli-obk:sad_revert, r=oli-obk"

This reverts commit 6499c5e7fc173a3f55b7a3bd1e6a50e9edef782d, reversing
changes made to 78450d2d602b06d9b94349aaf8cece1a4acaf3a8.

2 files changed, 117 insertions, 44 deletions

diff --git a/src/test/ui/parser/fn-header-semantic-fail.rs b/src/test/ui/parser/fn-header-semantic-fail.rs
index 91a63bafd99..cf5d3dab4aa 100644
--- a/src/test/ui/parser/fn-header-semantic-fail.rs
+++ b/src/test/ui/parser/fn-header-semantic-fail.rs
@@ -9,8 +9,9 @@ fn main() {
     unsafe fn ff2() {} // OK.
     const fn ff3() {} // OK.
     extern "C" fn ff4() {} // OK.
-    const async unsafe extern "C" fn ff5() {} // OK.
+    const async unsafe extern "C" fn ff5() {}
     //~^ ERROR functions cannot be both `const` and `async`
+    //~| ERROR cycle detected
 
     trait X {
         async fn ft1(); //~ ERROR functions in traits cannot be declared `async`
@@ -26,15 +27,14 @@ fn main() {
     struct Y;
     impl X for Y {
         async fn ft1() {} //~ ERROR functions in traits cannot be declared `async`
-        //~^ ERROR method `ft1` has an incompatible type for trait
         unsafe fn ft2() {} // OK.
         const fn ft3() {} //~ ERROR functions in traits cannot be declared const
         extern "C" fn ft4() {}
         const async unsafe extern "C" fn ft5() {}
         //~^ ERROR functions in traits cannot be declared `async`
         //~| ERROR functions in traits cannot be declared const
-        //~| ERROR method `ft5` has an incompatible type for trait
         //~| ERROR functions cannot be both `const` and `async`
+        //~| ERROR cycle detected
     }
 
     impl Y {
@@ -44,6 +44,7 @@ fn main() {
         extern "C" fn fi4() {} // OK.
         const async unsafe extern "C" fn fi5() {}
         //~^ ERROR functions cannot be both `const` and `async`
+        //~| ERROR cycle detected
     }
 
     extern "C" {
diff --git a/src/test/ui/parser/fn-header-semantic-fail.stderr b/src/test/ui/parser/fn-header-semantic-fail.stderr
index 8eaba559a62..1d7460b8d36 100644
--- a/src/test/ui/parser/fn-header-semantic-fail.stderr
+++ b/src/test/ui/parser/fn-header-semantic-fail.stderr
@@ -1,14 +1,14 @@
 error: functions cannot be both `const` and `async`
   --> $DIR/fn-header-semantic-fail.rs:12:5
    |
-LL |     const async unsafe extern "C" fn ff5() {} // OK.
+LL |     const async unsafe extern "C" fn ff5() {}
    |     ^^^^^-^^^^^------------------------------
    |     |     |
    |     |     `async` because of this
    |     `const` because of this
 
 error[E0706]: functions in traits cannot be declared `async`
-  --> $DIR/fn-header-semantic-fail.rs:16:9
+  --> $DIR/fn-header-semantic-fail.rs:17:9
    |
 LL |         async fn ft1();
    |         -----^^^^^^^^^^
@@ -19,19 +19,19 @@ LL |         async fn ft1();
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
 
 error[E0379]: functions in traits cannot be declared const
-  --> $DIR/fn-header-semantic-fail.rs:18:9
+  --> $DIR/fn-header-semantic-fail.rs:19:9
    |
 LL |         const fn ft3();
    |         ^^^^^ functions in traits cannot be const
 
 error[E0379]: functions in traits cannot be declared const
-  --> $DIR/fn-header-semantic-fail.rs:20:9
+  --> $DIR/fn-header-semantic-fail.rs:21:9
    |
 LL |         const async unsafe extern "C" fn ft5();
    |         ^^^^^ functions in traits cannot be const
 
 error[E0706]: functions in traits cannot be declared `async`
-  --> $DIR/fn-header-semantic-fail.rs:20:9
+  --> $DIR/fn-header-semantic-fail.rs:21:9
    |
 LL |         const async unsafe extern "C" fn ft5();
    |         ^^^^^^-----^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -42,7 +42,7 @@ LL |         const async unsafe extern "C" fn ft5();
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
 
 error: functions cannot be both `const` and `async`
-  --> $DIR/fn-header-semantic-fail.rs:20:9
+  --> $DIR/fn-header-semantic-fail.rs:21:9
    |
 LL |         const async unsafe extern "C" fn ft5();
    |         ^^^^^-^^^^^----------------------------
@@ -51,7 +51,7 @@ LL |         const async unsafe extern "C" fn ft5();
    |         `const` because of this
 
 error[E0706]: functions in traits cannot be declared `async`
-  --> $DIR/fn-header-semantic-fail.rs:28:9
+  --> $DIR/fn-header-semantic-fail.rs:29:9
    |
 LL |         async fn ft1() {}
    |         -----^^^^^^^^^^^^
@@ -103,7 +103,7 @@ LL |         const async unsafe extern "C" fn fi5() {}
    |         `const` because of this
 
 error: functions in `extern` blocks cannot have qualifiers
-  --> $DIR/fn-header-semantic-fail.rs:50:18
+  --> $DIR/fn-header-semantic-fail.rs:51:18
    |
 LL |     extern "C" {
    |     ---------- in this `extern` block
@@ -116,7 +116,7 @@ LL |         fn fe1();
    |         ~~
 
 error: functions in `extern` blocks cannot have qualifiers
-  --> $DIR/fn-header-semantic-fail.rs:51:19
+  --> $DIR/fn-header-semantic-fail.rs:52:19
    |
 LL |     extern "C" {
    |     ---------- in this `extern` block
@@ -130,7 +130,7 @@ LL |         fn fe2();
    |         ~~
 
 error: functions in `extern` blocks cannot have qualifiers
-  --> $DIR/fn-header-semantic-fail.rs:52:18
+  --> $DIR/fn-header-semantic-fail.rs:53:18
    |
 LL |     extern "C" {
    |     ---------- in this `extern` block
@@ -144,7 +144,7 @@ LL |         fn fe3();
    |         ~~
 
 error: functions in `extern` blocks cannot have qualifiers
-  --> $DIR/fn-header-semantic-fail.rs:53:23
+  --> $DIR/fn-header-semantic-fail.rs:54:23
    |
 LL |     extern "C" {
    |     ---------- in this `extern` block
@@ -158,7 +158,7 @@ LL |         fn fe4();
    |         ~~
 
 error: functions in `extern` blocks cannot have qualifiers
-  --> $DIR/fn-header-semantic-fail.rs:54:42
+  --> $DIR/fn-header-semantic-fail.rs:55:42
    |
 LL |     extern "C" {
    |     ---------- in this `extern` block
@@ -172,7 +172,7 @@ LL |         fn fe5();
    |         ~~
 
 error: functions cannot be both `const` and `async`
-  --> $DIR/fn-header-semantic-fail.rs:54:9
+  --> $DIR/fn-header-semantic-fail.rs:55:9
    |
 LL |         const async unsafe extern "C" fn fe5();
    |         ^^^^^-^^^^^----------------------------
@@ -180,43 +180,115 @@ LL |         const async unsafe extern "C" fn fe5();
    |         |     `async` because of this
    |         `const` because of this
 
-error[E0053]: method `ft1` has an incompatible type for trait
-  --> $DIR/fn-header-semantic-fail.rs:28:24
+error[E0391]: cycle detected when computing type of `main::ff5::{opaque#0}`
+  --> $DIR/fn-header-semantic-fail.rs:12:44
    |
-LL |         async fn ft1() {}
-   |                        ^
-   |                        |
-   |                        checked the `Output` of this `async fn`, found opaque type
-   |                        expected `()`, found opaque type
+LL |     const async unsafe extern "C" fn ff5() {}
+   |                                            ^
    |
-   = note: while checking the return type of the `async fn`
-note: type in trait
-  --> $DIR/fn-header-semantic-fail.rs:16:23
+note: ...which requires borrow-checking `main::ff5`...
+  --> $DIR/fn-header-semantic-fail.rs:12:5
    |
-LL |         async fn ft1();
-   |                       ^
-   = note: expected fn pointer `fn()`
-              found fn pointer `fn() -> impl Future<Output = ()>`
+LL |     const async unsafe extern "C" fn ff5() {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires processing `main::ff5`...
+  --> $DIR/fn-header-semantic-fail.rs:12:5
+   |
+LL |     const async unsafe extern "C" fn ff5() {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires const checking `main::ff5`...
+  --> $DIR/fn-header-semantic-fail.rs:12:5
+   |
+LL |     const async unsafe extern "C" fn ff5() {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: ...which requires computing whether `impl core::future::future::Future<Output = ()>` is freeze...
+   = note: ...which requires evaluating trait selection obligation `impl core::future::future::Future<Output = ()>: core::marker::Freeze`...
+   = note: ...which again requires computing type of `main::ff5::{opaque#0}`, completing the cycle
+note: cycle used when checking item types in top-level module
+  --> $DIR/fn-header-semantic-fail.rs:5:1
+   |
+LL | / #![feature(const_extern_fn)]
+LL | |
+LL | | fn main() {
+LL | |     async fn ff1() {} // OK.
+...  |
+LL | |     }
+LL | | }
+   | |_^
 
-error[E0053]: method `ft5` has an incompatible type for trait
+error[E0391]: cycle detected when computing type of `main::<impl at $DIR/fn-header-semantic-fail.rs:28:5: 38:6>::ft5::{opaque#0}`
   --> $DIR/fn-header-semantic-fail.rs:33:48
    |
 LL |         const async unsafe extern "C" fn ft5() {}
    |                                                ^
-   |                                                |
-   |                                                checked the `Output` of this `async fn`, found opaque type
-   |                                                expected `()`, found opaque type
    |
-   = note: while checking the return type of the `async fn`
-note: type in trait
-  --> $DIR/fn-header-semantic-fail.rs:20:47
+note: ...which requires borrow-checking `main::<impl at $DIR/fn-header-semantic-fail.rs:28:5: 38:6>::ft5`...
+  --> $DIR/fn-header-semantic-fail.rs:33:9
    |
-LL |         const async unsafe extern "C" fn ft5();
-   |                                               ^
-   = note: expected fn pointer `unsafe extern "C" fn()`
-              found fn pointer `unsafe extern "C" fn() -> impl Future<Output = ()>`
+LL |         const async unsafe extern "C" fn ft5() {}
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires processing `main::<impl at $DIR/fn-header-semantic-fail.rs:28:5: 38:6>::ft5`...
+  --> $DIR/fn-header-semantic-fail.rs:33:9
+   |
+LL |         const async unsafe extern "C" fn ft5() {}
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires const checking `main::<impl at $DIR/fn-header-semantic-fail.rs:28:5: 38:6>::ft5`...
+  --> $DIR/fn-header-semantic-fail.rs:33:9
+   |
+LL |         const async unsafe extern "C" fn ft5() {}
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: ...which requires computing whether `impl core::future::future::Future<Output = ()>` is freeze...
+   = note: ...which requires evaluating trait selection obligation `impl core::future::future::Future<Output = ()>: core::marker::Freeze`...
+   = note: ...which again requires computing type of `main::<impl at $DIR/fn-header-semantic-fail.rs:28:5: 38:6>::ft5::{opaque#0}`, completing the cycle
+note: cycle used when checking item types in top-level module
+  --> $DIR/fn-header-semantic-fail.rs:5:1
+   |
+LL | / #![feature(const_extern_fn)]
+LL | |
+LL | | fn main() {
+LL | |     async fn ff1() {} // OK.
+...  |
+LL | |     }
+LL | | }
+   | |_^
+
+error[E0391]: cycle detected when computing type of `main::<impl at $DIR/fn-header-semantic-fail.rs:40:5: 48:6>::fi5::{opaque#0}`
+  --> $DIR/fn-header-semantic-fail.rs:45:48
+   |
+LL |         const async unsafe extern "C" fn fi5() {}
+   |                                                ^
+   |
+note: ...which requires borrow-checking `main::<impl at $DIR/fn-header-semantic-fail.rs:40:5: 48:6>::fi5`...
+  --> $DIR/fn-header-semantic-fail.rs:45:9
+   |
+LL |         const async unsafe extern "C" fn fi5() {}
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires processing `main::<impl at $DIR/fn-header-semantic-fail.rs:40:5: 48:6>::fi5`...
+  --> $DIR/fn-header-semantic-fail.rs:45:9
+   |
+LL |         const async unsafe extern "C" fn fi5() {}
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires const checking `main::<impl at $DIR/fn-header-semantic-fail.rs:40:5: 48:6>::fi5`...
+  --> $DIR/fn-header-semantic-fail.rs:45:9
+   |
+LL |         const async unsafe extern "C" fn fi5() {}
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: ...which requires computing whether `impl core::future::future::Future<Output = ()>` is freeze...
+   = note: ...which requires evaluating trait selection obligation `impl core::future::future::Future<Output = ()>: core::marker::Freeze`...
+   = note: ...which again requires computing type of `main::<impl at $DIR/fn-header-semantic-fail.rs:40:5: 48:6>::fi5::{opaque#0}`, completing the cycle
+note: cycle used when checking item types in top-level module
+  --> $DIR/fn-header-semantic-fail.rs:5:1
+   |
+LL | / #![feature(const_extern_fn)]
+LL | |
+LL | | fn main() {
+LL | |     async fn ff1() {} // OK.
+...  |
+LL | |     }
+LL | | }
+   | |_^
 
-error: aborting due to 20 previous errors
+error: aborting due to 21 previous errors
 
-Some errors have detailed explanations: E0053, E0379, E0706.
-For more information about an error, try `rustc --explain E0053`.
+Some errors have detailed explanations: E0379, E0391, E0706.
+For more information about an error, try `rustc --explain E0379`.