Mitigate `#[align]` name resolution ambiguity regression with a rename

From `#[align]` -> `#[rustc_align]`. Attributes starting with `rustc`
are always perma-unstable and feature-gated by `feature(rustc_attrs)`.

See regression RUST-143834.

For the underlying problem where even introducing new feature-gated
unstable built-in attributes can break user code such as

```rs
macro_rules! align {
    () => {
        /* .. */
    };
}

pub(crate) use align; // `use` here becomes ambiguous
```

refer to RUST-134963.

Since the `#[align]` attribute is still feature-gated by
`feature(fn_align)`, we can rename it as a mitigation. Note that
`#[rustc_align]` will obviously mean that current unstable user code
using `feature(fn_aling)` will need additionally `feature(rustc_attrs)`,
but this is a short-term mitigation to buy time, and is expected to be
changed to a better name with less collision potential.

See
<https://rust-lang.zulipchat.com/#narrow/channel/238009-t-compiler.2Fmeetings/topic/.5Bweekly.5D.202025-07-17/near/529290371>
where mitigation options were considered.

1 files changed, 3 insertions, 1 deletions

diff --git a/compiler/rustc_parse/src/validate_attr.rs b/compiler/rustc_parse/src/validate_attr.rs
index a476f0db37e..cca621103b5 100644
--- a/compiler/rustc_parse/src/validate_attr.rs
+++ b/compiler/rustc_parse/src/validate_attr.rs
@@ -294,7 +294,9 @@ pub fn check_builtin_meta_item(
                 | sym::rustc_paren_sugar
                 | sym::type_const
                 | sym::repr
-                | sym::align
+                // FIXME(#82232, #143834): temporarily renamed to mitigate `#[align]` nameres
+                // ambiguity
+                | sym::rustc_align
                 | sym::deprecated
                 | sym::optimize
                 | sym::pointee