Rollup merge of #144080 - jieyouxu:realign, r=BoxyUwU

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

Mitigates beta regression rust-lang/rust#143834 after a beta backport.

### Background on the beta regression

The name resolution regression arises due to rust-lang/rust#142507 adding a new feature-gated built-in attribute named `#[align]`. However, unfortunately even [introducing new feature-gated unstable built-in attributes can break user code](https://www.github.com/rust-lang/rust/issues/134963) such as

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

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

### Mitigation approach

This PR renames `#[align]` to `#[rustc_align]` to mitigate the beta regression by:

1. Undoing the introduction of a new built-in attribute with a common name, i.e. `#[align]`.
2. Renaming `#[align]` to `#[rustc_align]`. The renamed attribute being `rustc_align` will not introduce new stable breakages, as attributes beginning with `rustc` are reserved and perma-unstable. This does mean existing nightly code using `fn_align` feature will additionally need to specify `#![feature(rustc_attrs)]`.

This PR is very much a short-term mitigation to alleviate time pressure from having to fully fix the current limitation of inevitable name resolution regressions that would arise from adding any built-in attributes. Long-term solutions are discussed in [#t-lang > namespacing macro attrs to reduce conflicts with new adds](https://rust-lang.zulipchat.com/#narrow/channel/213817-t-lang/topic/namespacing.20macro.20attrs.20to.20reduce.20conflicts.20with.20new.20adds/with/529249622).

### Alternative mitigation options

[Various mitigation options were considered during the compiler triage meeting](https://github.com/rust-lang/rust/issues/143834#issuecomment-3084415277), and those consideration are partly reproduced here:

- Reverting the PR doesn't seem very minimal/trivial, and carries risks of its own.
- Rename to a less-common but aim-to-stabilization name is itself not safe nor convenient, because (1) that risks introducing new regressions (i.e. ambiguity against the new name), and (2) lang would have to FCP the new name hastily for the mitigation to land timely and have a chance to be backported. This also makes the path towards stabilization annoying.
- Rename the attribute to a rustc attribute, which will be perma-unstable and does not cause new ambiguities in stable code.
    - This alleviates the time pressure to address *this* regression, or for lang to have to rush an FCP for some new name that can still break user code.
    - This avoids backing out a whole implementation.

### Review advice

This PR is best reviewed commit-by-commit.

- Commit 1 adds a test `tests/ui/attributes/fn-align-nameres-ambiguity-143834.rs` which demonstrates the current name resolution regression re. `align`. This test fails against current master.
- Commit 2 carries out the renames and test reblesses. Notably, commit 2 will cause `tests/ui/attributes/fn-align-nameres-ambiguity-143834.rs` to change from fail (nameres regression) to pass.

This PR, if the approach still seems acceptable, will need a beta-backport to address the beta regression.

4 files changed, 40 insertions, 31 deletions

diff --git a/tests/codegen/align-fn.rs b/tests/codegen/align-fn.rs
index fd572910c28..cbc24e2ae2e 100644
--- a/tests/codegen/align-fn.rs
+++ b/tests/codegen/align-fn.rs
@@ -3,11 +3,13 @@
 //@ ignore-wasm32 aligning functions is not currently supported on wasm (#143368)
 
 #![crate_type = "lib"]
+// FIXME(#82232, #143834): temporarily renamed to mitigate `#[align]` nameres ambiguity
+#![feature(rustc_attrs)]
 #![feature(fn_align)]
 
 // CHECK: align 16
 #[unsafe(no_mangle)]
-#[align(16)]
+#[rustc_align(16)]
 pub fn fn_align() {}
 
 pub struct A;
@@ -15,12 +17,12 @@ pub struct A;
 impl A {
     // CHECK: align 16
     #[unsafe(no_mangle)]
-    #[align(16)]
+    #[rustc_align(16)]
     pub fn method_align(self) {}
 
     // CHECK: align 16
     #[unsafe(no_mangle)]
-    #[align(16)]
+    #[rustc_align(16)]
     pub fn associated_fn() {}
 }
 
@@ -29,18 +31,18 @@ trait T: Sized {
 
     fn trait_method(self) {}
 
-    #[align(8)]
+    #[rustc_align(8)]
     fn trait_method_inherit_low(self);
 
-    #[align(32)]
+    #[rustc_align(32)]
     fn trait_method_inherit_high(self);
 
-    #[align(32)]
+    #[rustc_align(32)]
     fn trait_method_inherit_default(self) {}
 
-    #[align(4)]
-    #[align(128)]
-    #[align(8)]
+    #[rustc_align(4)]
+    #[rustc_align(128)]
+    #[rustc_align(8)]
     fn inherit_highest(self) {}
 }
 
@@ -48,27 +50,27 @@ impl T for A {
     // CHECK-LABEL: trait_fn
     // CHECK-SAME: align 16
     #[unsafe(no_mangle)]
-    #[align(16)]
+    #[rustc_align(16)]
     fn trait_fn() {}
 
     // CHECK-LABEL: trait_method
     // CHECK-SAME: align 16
     #[unsafe(no_mangle)]
-    #[align(16)]
+    #[rustc_align(16)]
     fn trait_method(self) {}
 
     // The prototype's align is ignored because the align here is higher.
     // CHECK-LABEL: trait_method_inherit_low
     // CHECK-SAME: align 16
     #[unsafe(no_mangle)]
-    #[align(16)]
+    #[rustc_align(16)]
     fn trait_method_inherit_low(self) {}
 
     // The prototype's align is used because it is higher.
     // CHECK-LABEL: trait_method_inherit_high
     // CHECK-SAME: align 32
     #[unsafe(no_mangle)]
-    #[align(16)]
+    #[rustc_align(16)]
     fn trait_method_inherit_high(self) {}
 
     // The prototype's align inherited.
@@ -81,8 +83,8 @@ impl T for A {
     // CHECK-LABEL: inherit_highest
     // CHECK-SAME: align 128
     #[unsafe(no_mangle)]
-    #[align(32)]
-    #[align(64)]
+    #[rustc_align(32)]
+    #[rustc_align(64)]
     fn inherit_highest(self) {}
 }
 
@@ -90,7 +92,7 @@ trait HasDefaultImpl: Sized {
     // CHECK-LABEL: inherit_from_default_method
     // CHECK-LABEL: inherit_from_default_method
     // CHECK-SAME: align 32
-    #[align(32)]
+    #[rustc_align(32)]
     fn inherit_from_default_method(self) {}
 }
 
@@ -101,35 +103,35 @@ impl HasDefaultImpl for InstantiateDefaultMethods {}
 // CHECK-LABEL: align_specified_twice_1
 // CHECK-SAME: align 64
 #[unsafe(no_mangle)]
-#[align(32)]
-#[align(64)]
+#[rustc_align(32)]
+#[rustc_align(64)]
 pub fn align_specified_twice_1() {}
 
 // CHECK-LABEL: align_specified_twice_2
 // CHECK-SAME: align 128
 #[unsafe(no_mangle)]
-#[align(128)]
-#[align(32)]
+#[rustc_align(128)]
+#[rustc_align(32)]
 pub fn align_specified_twice_2() {}
 
 // CHECK-LABEL: align_specified_twice_3
 // CHECK-SAME: align 256
 #[unsafe(no_mangle)]
-#[align(32)]
-#[align(256)]
+#[rustc_align(32)]
+#[rustc_align(256)]
 pub fn align_specified_twice_3() {}
 
 const _: () = {
     // CHECK-LABEL: align_unmangled
     // CHECK-SAME: align 256
     #[unsafe(no_mangle)]
-    #[align(32)]
-    #[align(256)]
+    #[rustc_align(32)]
+    #[rustc_align(256)]
     extern "C" fn align_unmangled() {}
 };
 
 unsafe extern "C" {
-    #[align(256)]
+    #[rustc_align(256)]
     fn align_unmangled();
 }
 
@@ -137,5 +139,5 @@ unsafe extern "C" {
 // CHECK-LABEL: async_align
 // CHECK-SAME: align 64
 #[unsafe(no_mangle)]
-#[align(64)]
+#[rustc_align(64)]
 pub async fn async_align() {}
diff --git a/tests/codegen/min-function-alignment.rs b/tests/codegen/min-function-alignment.rs
index 6a3843b0f4f..ea5f957e81f 100644
--- a/tests/codegen/min-function-alignment.rs
+++ b/tests/codegen/min-function-alignment.rs
@@ -5,6 +5,8 @@
 //@ ignore-wasm32 aligning functions is not currently supported on wasm (#143368)
 
 #![crate_type = "lib"]
+// FIXME(#82232, #143834): temporarily renamed to mitigate `#[align]` nameres ambiguity
+#![feature(rustc_attrs)]
 #![feature(fn_align)]
 
 // Functions without explicit alignment use the global minimum.
@@ -21,7 +23,7 @@ pub fn no_explicit_align() {}
 // align16: align 16
 // align1024: align 1024
 #[no_mangle]
-#[align(8)]
+#[rustc_align(8)]
 pub fn lower_align() {}
 
 // the higher value of min-function-alignment and the align attribute wins out
@@ -30,7 +32,7 @@ pub fn lower_align() {}
 // align16: align 32
 // align1024: align 1024
 #[no_mangle]
-#[align(32)]
+#[rustc_align(32)]
 pub fn higher_align() {}
 
 // cold functions follow the same rules as other functions
diff --git a/tests/codegen/naked-fn/aligned.rs b/tests/codegen/naked-fn/aligned.rs
index 2648b0213ca..d7281c4219a 100644
--- a/tests/codegen/naked-fn/aligned.rs
+++ b/tests/codegen/naked-fn/aligned.rs
@@ -4,12 +4,15 @@
 //@ ignore-wasm32 aligning functions is not currently supported on wasm (#143368)
 
 #![crate_type = "lib"]
+// FIXME(#82232, #143834): temporarily renamed to mitigate `#[align]` nameres ambiguity
+#![feature(rustc_attrs)]
 #![feature(fn_align)]
+
 use std::arch::naked_asm;
 
 // CHECK: .balign 16
 // CHECK-LABEL: naked_empty:
-#[align(16)]
+#[rustc_align(16)]
 #[no_mangle]
 #[unsafe(naked)]
 pub extern "C" fn naked_empty() {
diff --git a/tests/codegen/naked-fn/min-function-alignment.rs b/tests/codegen/naked-fn/min-function-alignment.rs
index 4ebaacd3eff..406e9334fa5 100644
--- a/tests/codegen/naked-fn/min-function-alignment.rs
+++ b/tests/codegen/naked-fn/min-function-alignment.rs
@@ -3,6 +3,8 @@
 //@ ignore-arm no "ret" mnemonic
 //@ ignore-wasm32 aligning functions is not currently supported on wasm (#143368)
 
+// FIXME(#82232, #143834): temporarily renamed to mitigate `#[align]` nameres ambiguity
+#![feature(rustc_attrs)]
 #![feature(fn_align)]
 #![crate_type = "lib"]
 
@@ -17,7 +19,7 @@ pub extern "C" fn naked_no_explicit_align() {
 
 // CHECK: .balign 16
 #[no_mangle]
-#[align(8)]
+#[rustc_align(8)]
 #[unsafe(naked)]
 pub extern "C" fn naked_lower_align() {
     core::arch::naked_asm!("ret")
@@ -25,7 +27,7 @@ pub extern "C" fn naked_lower_align() {
 
 // CHECK: .balign 32
 #[no_mangle]
-#[align(32)]
+#[rustc_align(32)]
 #[unsafe(naked)]
 pub extern "C" fn naked_higher_align() {
     core::arch::naked_asm!("ret")