ssa: implement #[collapse_debuginfo]

Debuginfo line information for macro invocations are collapsed by
default - line information are replaced by the line of the outermost
expansion site. Using `-Zdebug-macros` disables this behaviour.

When the `collapse_debuginfo` feature is enabled, the default behaviour
is reversed so that debuginfo is not collapsed by default. In addition,
the `#[collapse_debuginfo]` attribute is available and can be applied to
macro definitions which will then have their line information collapsed.

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_feature/src/active.rs

@@ -336,6 +336,8 @@ declare_features! (
     (active, closure_track_caller, "1.57.0", Some(87417), None),
     /// Allows to use the `#[cmse_nonsecure_entry]` attribute.
     (active, cmse_nonsecure_entry, "1.48.0", Some(75835), None),
+    /// Allows use of the `#[collapse_debuginfo]` attribute.
+    (active, collapse_debuginfo, "CURRENT_RUSTC_VERSION", Some(100758), None),
     /// Allows `async {}` expressions in const contexts.
     (active, const_async_blocks, "1.53.0", Some(85368), None),
     // Allows limiting the evaluation steps of const expressions

compiler/rustc_feature/src/builtin_attrs.rs

@@ -481,6 +481,12 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         experimental!(deprecated_safe),
     ),
 
+    // `#[collapse_debuginfo]`
+    gated!(
+        collapse_debuginfo, Normal, template!(Word), WarnFollowing,
+        experimental!(collapse_debuginfo)
+    ),
+
     // ==========================================================================
     // Internal attributes: Stability, deprecation, and unsafe:
     // ==========================================================================