Regression (?) in name resolution of items defined in functions/blocks

[petrochenkov]

This code compiles on nightly, but is rejected by stable and beta compilers.
On nightly items defined in a block are visible from all modules defined in this block.
Such "pervasive" visibility is quite unusual and I suspect it was introduced unintentionally.

fn f() {
    struct S;

    fn g() {
        let s = S; // <- beta/stable/nightly - ok
    }
    mod m {
        fn h() {
            let s = S; // <- beta/stable - error: unresolved name `S` [E0425], nightly - ok
        }

        mod n {
            fn k() {
                let s = S; // <- beta/stable - error: unresolved name `S` [E0425], nightly - ok
            }
        }
    }
}

fn main() {
}

cc @jseyfried

[jseyfried]

This was introduced in #31105 while fixing #23880.
This bug existed earlier (in some sense) but was benign. Before #31105, only type parameters and local variables leaked, but using these "captured" locals and parameters is already a separate error.

[jseyfried]

@petrochenkov this can be closed -- I think bors didn't close it because the magic words were only in the title.