derive-new errors

[nikomatsakis]

So @wycats pointed out to me that some recent changes in name resolution seem to have broken derive_new. They pointed me at this commit from nushell dealing with the fallout -- @jonathandturner maybe you can leave a few notes as to what errors you were seeing before this commit?

cc @petrochenkov -- could this be the recent changes around name resolution of helper attributes in macros? It seems the relevant code is stuff like:

#[derive(new)]
struct Foo {
    #[new(value = "Arc::new(22)")]
    bar: Arc<i32>
}

[nikomatsakis]

(Nominated for @rust-lang/lang discussion around the breakage policy/implications here.)

[petrochenkov]

Duplicate of #66508, I'll prepare a fix.

[petrochenkov]

fix

Or rather a "tiebreaker" for the ambiguity, since both names are indeed in scope and create ambiguity, which is correctly detected after #64694.

[petrochenkov]

Fixed in #66529.