Fix split index calculation in check_type_alias_where_clause_location

[stuuupidcat]

Fix the calculation of the split index in check_type_alias_where_clause_location by ignoring before-equals-sign-clauses with attributes.

Fix #138010.

cc @frank-king.

[rustbot]

r? @BoxyUwU

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[rustbot]

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

[BoxyUwU]

Filtering things which have attributes applied to them isn't really correct because it's not necessarily the case that all cfgd where clauses will be removed during macro expansion, e.g.

#![feature(where_clause_attrs, cfg_boolean_literals, lazy_type_alias)]
#![expect(incomplete_features)]

struct Foo;
trait Trait {}

impl Trait for Foo {}

type MixedWhereBounds
where
    #[cfg(true)]
    Foo: Trait,
= Foo
where
    (): Sized;

Here we have a where clause before the = that has an attribute applied to it, yet the where clause will still be present after macro expansion.

If you run the compiler on this example with your changes, and then remove the cfg, you'll get different diagnostics output even though post-expansion it'll be the same set of where clauses. That's caused by this before_with_attr_count being 1 and then when we split the where clauses into before/after you wind up splitting at 0 so both clauses end up in the "after" bucket.

Can you add both version as a test (with/without cfg(true))?

[BoxyUwU]

I'm not really sure what the correct way to implement this is 🤔

[stuuupidcat]

Thank you for your detailed explanation, I will add a few tests first.

[wesleywiser]

Hi @BoxyUwU, have you had a chance to think about this more? Or if you know of another expert in this area, we could see if they have any ideas? 🙂

[stuuupidcat]

Some tests have been added (the results are in ba719b9#diff-9daeebc4c3e4cd13b6240809d45a7c6ebb58972f6c7c032df74a9eeb53b52300), and while the error messages look good, the suggestions are a bit inaccurate.
This might be because when generating suggestions, the compiler no longer has access to all of the original where clause information.

[stuuupidcat]

@rustbot ready

[bors]

☔ The latest upstream changes (presumably #140388) made this pull request unmergeable. Please resolve the merge conflicts.

[BoxyUwU]

I think this is still waiting on figuring out how to implement it correctly

[cjgillot]

There is no clean way to implement it while keeping a single predicate list. cfg-expansion does not track which is before/after the equal sign.
My recommendation is to drop the TyAliasWhereClauses shenanigans, and go for simplicity. Add post_equals_where: WhereClause to TyAlias. cfg expansion will do what we expect. And AST->HIR lowering will have to merge these clauses, which is its purpose.

[BoxyUwU]

seems reasonable to me

[Dylan-DPC]

@stuuupidcat any updates on this? thanks

[stuuupidcat]

Sorry, no progress yet. The proposed fix is a little complex for me to understand and implement.

[fmease]

I'm closing this in favor of #147713. Thanks for your contribution regardless!