Add tests for issues with the E-needstest label

[topecongiro]

Fixes #19181.
Fixes #29516.
Fixes #29798.
Fixes #33504.
Fixes #34780.
Fixes #39211.
Fixes #39467.
Fixes #39720.

[rust-highfive]

r? @aturon

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[Mark-Simulacrum]

Could you add Fixes #xxx for each issue you've fixed by adding a test for it to the description? That way GitHub will automatically close them.

[topecongiro]

@Mark-Simulacrum Thanks, updated the description.

[Mark-Simulacrum]

@bors r+ rollup

[bors]

📌 Commit 04aa5c1 has been approved by Mark-Simulacrum

[RalfJung]

This adds a test checking that

trait VecN {
    const DIM: usize;
}
trait Mat {
    type Row: VecN;
}

fn m<M: Mat>() {
    let a = [3; M::Row::DIM]; //~ ERROR associated type `Row` not found for `M`
}
fn main() {
}

It seems that this used to ICE, and the test makes sure that doesn't happen. Okay. However, the error message doesn't make any sense, so I am somewhat surprised that it should be "enforced" by a test.

[Mark-Simulacrum]

Once #34344 is fixed, I'd imagine we'd remove that test. I think it's better to have it and avoid regressing on the ICE. Tests in general to me don't imply cemented behavior, just current behavior -- if you're fixing something and it fixes a test, consider removing the test; it's just somewhat unusual that we have tests like this.