Mir: casting &str to *const str will cast twice

[nagisa]

#[rustc_mir(graphviz="foo.gv")]
fn mir(){
    "hello" as *const str;
}

will result in

Note the repeated cast to *const str.

[arielb1]

This is semi-expected. One of the casts is a coercion and the other is the identity cast. We shouldn't however be emitting Misc instructions for identity casts.

[pczarn]

What kind of instruction should be emitted for identity casts?

[arielb1]

We should not be emitting any instructions for an identity cast. I think that this could be best done in a post-processing pass.

[eddyb]

I believe my "mir: Ignore noop casts (e.g. when as used for coercion)." commit from #32080 fixes this.

[nagisa]

Seems to be fixed indeed.