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.

Author: jieyouxu

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -234,6 +234,7 @@ pub enum CfgEntry {
 pub enum AttributeKind {
     // tidy-alphabetical-start
     /// Represents `#[align(N)]`.
+    // FIXME(#82232, #143834): temporarily renamed to mitigate `#[align]` nameres ambiguity
     Align { align: Align, span: Span },
 
     /// Represents `#[rustc_allow_const_fn_unstable]`.

compiler/rustc_attr_parsing/src/attributes/codegen_attrs.rs

@@ -177,7 +177,8 @@ impl<S: Stage> AttributeParser<S> for NakedParser {
             sym::instruction_set,
             sym::repr,
             sym::rustc_std_internal_symbol,
-            sym::align,
+            // FIXME(#82232, #143834): temporarily renamed to mitigate `#[align]` nameres ambiguity
+            sym::rustc_align,
             // obviously compatible with self
             sym::naked,
             // documentation

compiler/rustc_attr_parsing/src/attributes/repr.rs

@@ -274,7 +274,7 @@ fn parse_alignment(node: &LitKind) -> Result<Align, &'static str> {
 pub(crate) struct AlignParser(Option<(Align, Span)>);
 
 impl AlignParser {
-    const PATH: &'static [Symbol] = &[sym::align];
+    const PATH: &'static [Symbol] = &[sym::rustc_align];
     const TEMPLATE: AttributeTemplate = template!(List: "<alignment in bytes>");
 
     fn parse<'c, S: Stage>(

compiler/rustc_feature/src/builtin_attrs.rs

@@ -490,7 +490,8 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
     ungated!(no_link, Normal, template!(Word), WarnFollowing, EncodeCrossCrate::No),
     ungated!(repr, Normal, template!(List: "C"), DuplicatesOk, EncodeCrossCrate::No),
-    gated!(align, Normal, template!(List: "alignment"), DuplicatesOk, EncodeCrossCrate::No, fn_align, experimental!(align)),
+    // FIXME(#82232, #143834): temporarily renamed to mitigate `#[align]` nameres ambiguity
+    gated!(rustc_align, Normal, template!(List: "alignment"), DuplicatesOk, EncodeCrossCrate::No, fn_align, experimental!(rustc_align)),
     ungated!(unsafe(Edition2024) export_name, Normal, template!(NameValueStr: "name"), FutureWarnPreceding, EncodeCrossCrate::No),
     ungated!(unsafe(Edition2024) link_section, Normal, template!(NameValueStr: "name"), FutureWarnPreceding, EncodeCrossCrate::No),
     ungated!(unsafe(Edition2024) no_mangle, Normal, template!(Word), WarnFollowing, EncodeCrossCrate::No),

compiler/rustc_middle/src/middle/codegen_fn_attrs.rs

@@ -70,6 +70,7 @@ pub struct CodegenFnAttrs {
     /// switching between multiple instruction sets.
     pub instruction_set: Option<InstructionSetAttr>,
     /// The `#[align(...)]` attribute. Determines the alignment of the function body.
+    // FIXME(#82232, #143834): temporarily renamed to mitigate `#[align]` nameres ambiguity
     pub alignment: Option<Align>,
     /// The `#[patchable_function_entry(...)]` attribute. Indicates how many nops should be around
     /// the function entry.

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

compiler/rustc_passes/messages.ftl

@@ -14,15 +14,15 @@ passes_abi_of =
     fn_abi_of({$fn_name}) = {$fn_abi}
 
 passes_align_attr_application =
-    `#[align(...)]` should be applied to a function item
+    `#[rustc_align(...)]` should be applied to a function item
     .label = not a function item
 
 passes_align_on_fields =
     attribute should be applied to a function or method
     .warn = {-passes_previously_accepted}
 
 passes_align_should_be_repr_align =
-    `#[align(...)]` is not supported on {$item} items
+    `#[rustc_align(...)]` is not supported on {$item} items
     .suggestion = use `#[repr(align(...))]` instead
 
 passes_allow_incoherent_impl =
@@ -604,7 +604,7 @@ passes_repr_align_greater_than_target_max =
 
 passes_repr_align_should_be_align =
     `#[repr(align(...))]` is not supported on {$item} items
-    .help = use `#[align(...)]` instead
+    .help = use `#[rustc_align(...)]` instead
 
 passes_repr_conflicting =
     conflicting representation hints

compiler/rustc_passes/src/check_attr.rs

@@ -1957,6 +1957,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
     }
 
     /// Checks if the `#[align]` attributes on `item` are valid.
+    // FIXME(#82232, #143834): temporarily renamed to mitigate `#[align]` nameres ambiguity
     fn check_align(
         &self,
         span: Span,

compiler/rustc_span/src/symbol.rs

@@ -1809,6 +1809,8 @@ symbols! {
         rust_out,
         rustc,
         rustc_abi,
+        // FIXME(#82232, #143834): temporary name to mitigate `#[align]` nameres ambiguity
+        rustc_align,
         rustc_allocator,
         rustc_allocator_zeroed,
         rustc_allow_const_fn_unstable,

src/tools/miri/tests/pass/fn_align.rs

@@ -1,15 +1,18 @@
 //@compile-flags: -Zmin-function-alignment=8
 #![feature(fn_align)]
 
+// FIXME(rust-lang/rust#82232, rust-lang/rust#143834): temporarily renamed to mitigate `#[align]`
+// nameres ambiguity
+
 // When a function uses `align(N)`, the function address should be a multiple of `N`.
 
-#[align(256)]
+#[xxxxx_temp_mitigation_rename_of_align_xxxxx(256)]
 fn foo() {}
 
-#[align(16)]
+#[xxxxx_temp_mitigation_rename_of_align_xxxxx(16)]
 fn bar() {}
 
-#[align(4)]
+#[xxxxx_temp_mitigation_rename_of_align_xxxxx(4)]
 fn baz() {}
 
 fn main() {