Auto merge of #125289 - WaffleLapkin:never-obligations, r=compiler-errors

Implement lint for obligations broken by never type fallback change

This is the second (and probably last major?) lint required for the never type fallback change.

The idea is to check if the code errors with `fallback = ()` and if it errors with `fallback = !` and if it went from "ok" to "error", lint.

I'm not happy with the diagnostic, ideally we'd highlight what bound is the problem. But I'm really unsure how to do that  (cc `@jackh726,` iirc you had some ideas?)

r? `@compiler-errors`

Thanks `@BoxyUwU` with helping with trait solver stuff when I was implementing the initial version of this lint.

Tracking:
- rust-lang/rust#123748

Author: bors

tests/ui/new_ret_no_self.rs

@@ -390,9 +390,7 @@ mod issue7344 {
 
     impl<T> RetImplTraitSelf2<T> {
         // should not trigger lint
-        fn new(t: T) -> impl Trait2<(), Self> {
-            unimplemented!()
-        }
+        fn new(t: T) -> impl Trait2<(), Self> {}
     }
 
     struct RetImplTraitNoSelf2<T>(T);
@@ -401,7 +399,6 @@ mod issue7344 {
         // should trigger lint
         fn new(t: T) -> impl Trait2<(), i32> {
             //~^ ERROR: methods called `new` usually return `Self`
-            unimplemented!()
         }
     }
 

tests/ui/new_ret_no_self.stderr

@@ -96,11 +96,10 @@ LL | |         }
    | |_________^
 
 error: methods called `new` usually return `Self`
-  --> tests/ui/new_ret_no_self.rs:402:9
+  --> tests/ui/new_ret_no_self.rs:400:9
    |
 LL | /         fn new(t: T) -> impl Trait2<(), i32> {
 LL | |
-LL | |             unimplemented!()
 LL | |         }
    | |_________^