Adjust atomic-from-mut-not-available.rs

- Introduce two revisions: one for 32-bit x86 vs one for 64-bit x86_64
  and compare & contrast the errors.
- Document the test intention and note its limitations.

tests/ui/stdlib-unit-tests/atomic-from-mut-not-available.alignment_matches.stderr

@@ -0,0 +1,13 @@
+error[E0658]: use of unstable library feature `atomic_from_mut`
+  --> $DIR/atomic-from-mut-not-available.rs:24:5
+   |
+LL |     core::sync::atomic::AtomicU64::from_mut(&mut 0u64);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #76314 <https://github.com/rust-lang/rust/issues/76314> for more information
+   = help: add `#![feature(atomic_from_mut)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/stdlib-unit-tests/atomic-from-mut-not-available.alignment_mismatch.stderr

@@ -0,0 +1,17 @@
+error[E0599]: no function or associated item named `from_mut` found for struct `AtomicU64` in the current scope
+  --> $DIR/atomic-from-mut-not-available.rs:24:36
+   |
+LL |     core::sync::atomic::AtomicU64::from_mut(&mut 0u64);
+   |                                    ^^^^^^^^ function or associated item not found in `AtomicU64`
+   |
+note: if you're trying to build a new `AtomicU64`, consider using `AtomicU64::new` which returns `AtomicU64`
+  --> $SRC_DIR/core/src/sync/atomic.rs:LL:COL
+   = note: this error originates in the macro `atomic_int` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: there is an associated function `from` with a similar name
+   |
+LL |     core::sync::atomic::AtomicU64::from(&mut 0u64);
+   |                                    ~~~~
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0599`.

tests/ui/stdlib-unit-tests/atomic-from-mut-not-available.rs

@@ -1,7 +1,27 @@
-//@ only-x86
-//@ only-linux
+//! This test exercises the combined effect of the `cfg(target_has_atomic_equal_alignment = "...")`
+//! implementation in the compiler plus usage of said `cfg(target_has_atomic_equal_alignment)` in
+//! `core` for the `Atomic64::from_mut` API.
+//!
+//! This test is a basic smoke test: that `AtomicU64::from_mut` is gated by
+//! `#[cfg(target_has_atomic_equal_alignment = "8")]`, which is only available on platforms where
+//! `AtomicU64` has the same alignment as `u64`. This is notably *not* satisfied by `x86_32`, where
+//! they have differing alignments. Thus, `AtomicU64::from_mut` should *not* be available on
+//! `x86_32` linux and should report assoc item not found, if the `cfg` is working correctly.
+//! Conversely, `AtomicU64::from_mut` *should* be available on `x86_64` linux where the alignment
+//! matches.
+
+//@ revisions: alignment_mismatch alignment_matches
+
+// This should fail on 32-bit x86 linux...
+//@[alignment_mismatch] only-x86
+//@[alignment_mismatch] only-linux
+
+// ... but pass on 64-bit x86_64 linux.
+//@[alignment_matches] only-x86_64
+//@[alignment_matches] only-linux
 
 fn main() {
     core::sync::atomic::AtomicU64::from_mut(&mut 0u64);
-    //~^ ERROR: no function or associated item named `from_mut` found for struct `AtomicU64`
+    //[alignment_mismatch]~^ ERROR no function or associated item named `from_mut` found for struct `AtomicU64`
+    //[alignment_matches]~^^ ERROR use of unstable library feature `atomic_from_mut`
 }