Check FromIterator trait impl in prelude collision check.

[m-ou-se]

Fixes #86902.

[rust-highfive]

r? @jackh726

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[m-ou-se]

Oh, looks like this actually explodes if the type is generic. :(

[m-ou-se]

struct X<T>(T);

trait Hey {
    fn from_iter(_: i32) -> Self;
}

impl Hey for X<i32> {
    fn from_iter(x: i32) -> Self {
        X(x)
    }
}

fn main() {
    X::from_iter(1);
}

It works for X::<i32>::from_iter(1). But it doesn't like X::from_iter(1):

thread 'rustc' panicked at 'index out of bounds: the len is 0 but the index is 0', /home/mara/.cargo/registry/src/github.com-1ecc6299db9ec823/ena-0.14.0/src/snapshot_vec.rs:199:10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

error: internal compiler error: unexpected panic

I'm guessing I need to fill something in for the T of X.

[bors]

☔ The latest upstream changes (presumably #86920) made this pull request unmergeable. Please resolve the merge conflicts.

[m-ou-se]

Updated! Let's see if it works now.. :)

Also found a bug in the suggestion for Z::from_iter() when Z is generic. Filing an issue for that. (Edit: #86940)

[nikomatsakis]

Looks good, one nit

[nikomatsakis]

@bors r+

[bors]

📌 Commit 2083207 has been approved by nikomatsakis

[bors]

⌛ Testing commit 2083207 with merge 95fb131...

[bors]

☀️ Test successful - checks-actions
Approved by: nikomatsakis
Pushing 95fb131 to master...