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>

2 files changed, 23 insertions, 0 deletions

diff --git a/compiler/rustc_passes/src/check_attr.rs b/compiler/rustc_passes/src/check_attr.rs
index d0b46aa2c45..4f73f71f501 100644
--- a/compiler/rustc_passes/src/check_attr.rs
+++ b/compiler/rustc_passes/src/check_attr.rs
@@ -131,6 +131,7 @@ impl CheckAttrVisitor<'_> {
                 | sym::rustc_if_this_changed
                 | sym::rustc_then_this_would_need => self.check_rustc_dirty_clean(&attr),
                 sym::cmse_nonsecure_entry => self.check_cmse_nonsecure_entry(attr, span, target),
+                sym::collapse_debuginfo => self.check_collapse_debuginfo(attr, span, target),
                 sym::const_trait => self.check_const_trait(attr, span, target),
                 sym::must_not_suspend => self.check_must_not_suspend(&attr, span, target),
                 sym::must_use => self.check_must_use(hir_id, &attr, span, target),
@@ -431,6 +432,19 @@ impl CheckAttrVisitor<'_> {
         }
     }
 
+    /// Checks if `#[collapse_debuginfo]` is applied to a macro.
+    fn check_collapse_debuginfo(&self, attr: &Attribute, span: Span, target: Target) -> bool {
+        match target {
+            Target::MacroDef => true,
+            _ => {
+                self.tcx
+                    .sess
+                    .emit_err(errors::CollapseDebuginfo { attr_span: attr.span, defn_span: span });
+                false
+            }
+        }
+    }
+
     /// Checks if a `#[track_caller]` is applied to a non-naked function. Returns `true` if valid.
     fn check_track_caller(
         &self,
diff --git a/compiler/rustc_passes/src/errors.rs b/compiler/rustc_passes/src/errors.rs
index 901f56ad96d..96cc8ae988c 100644
--- a/compiler/rustc_passes/src/errors.rs
+++ b/compiler/rustc_passes/src/errors.rs
@@ -649,3 +649,12 @@ pub struct RustcLintOptDenyFieldAccess {
     #[label]
     pub span: Span,
 }
+
+#[derive(SessionDiagnostic)]
+#[diag(passes::collapse_debuginfo)]
+pub struct CollapseDebuginfo {
+    #[primary_span]
+    pub attr_span: Span,
+    #[label]
+    pub defn_span: Span,
+}