Fix type checking of return expressions outside of function bodies

[FabianWolff]

This pull request fixes #86188. The problem is that the current code for type-checking return expressions stops if the return occurs outside of a function body, while the correct behavior is to continue type-checking the return value expression (otherwise an ICE happens later on because variables declared in the return value expression don't have a type).

Also, I have noticed that it is sometimes not obvious why a return is outside of a function body; for instance, in the example from #86188 (which currently causes an ICE):

fn main() {
    [(); return || {
        let tx;
    }]
}

I have changed the error message to also explain why the return is considered outside of the function body:

error[E0572]: return statement outside of function body
 --> ice0.rs:2:10
  |
1 |  / fn main() {
2 |  |     [(); return || {
  |  |__________^
3 | ||         let tx;
4 | ||     }]
  | ||_____^ the return is part of this body...
5 |  | }
  |  |_- ...not the enclosing function body

[rust-highfive]

r? @varkor

(rust-highfive has picked a reviewer for you, use r? to override)

[Dylan-DPC-zz]

r? @Mark-Simulacrum

[tlyu]

I found the new error messages puzzling until I saw this comment. Maybe there is some way to make the new error messages be more specific, e.g., "this expression is an array repeat/length count, which is compile-time evaluated in a const context outside the function body"?

[Mark-Simulacrum]

@bors r+ rollup

I agree that improving the error message to more clearly describe the situation, but this seems like an improvement in the meantime.

[bors]

📌 Commit bdddaeb has been approved by Mark-Simulacrum