Builtin macros effectively have implicit #[collapse_debuginfo(yes)] a…

…ttribute

compiler/rustc_expand/src/base.rs

@@ -796,9 +796,15 @@ impl SyntaxExtension {
     /// | external      | no  | if-ext        | if-ext   | yes |
     /// | yes           | yes | yes           | yes      | yes |
     fn get_collapse_debuginfo(sess: &Session, attrs: &[ast::Attribute], is_local: bool) -> bool {
-        let collapse_debuginfo_attr = attr::find_by_name(attrs, sym::collapse_debuginfo)
+        let mut collapse_debuginfo_attr = attr::find_by_name(attrs, sym::collapse_debuginfo)
             .map(|v| Self::collapse_debuginfo_by_name(sess, v))
             .unwrap_or(CollapseMacroDebuginfo::Unspecified);
+        if collapse_debuginfo_attr == CollapseMacroDebuginfo::Unspecified
+            && attr::contains_name(attrs, sym::rustc_builtin_macro)
+        {
+            collapse_debuginfo_attr = CollapseMacroDebuginfo::Yes;
+        }
+
         let flag = sess.opts.unstable_opts.collapse_macro_debuginfo;
         let attr = collapse_debuginfo_attr;
         let ext = !is_local;

tests/debuginfo/collapse-debuginfo-builtin.rs

@@ -0,0 +1,32 @@
+// ignore-lldb
+#![feature(collapse_debuginfo)]
+
+use std::fmt;
+
+// Test that builtin macro debug info is collapsed.
+// Debug info for format_args must be #format_arg_external, not #format_arg_internal.
+// Because format_args is a builtin macro.
+// compile-flags:-g
+
+// === GDB TESTS ===================================================================================
+
+// gdb-command:run
+// gdb-command:next 2
+// gdb-command:frame
+// gdb-check:[...]#format_arg_external[...]
+// gdb-command:continue
+
+fn main() {
+    let ret = 0; // #break
+    let w = "world".to_string();
+    let s = fmt::format(
+        format_args!( // #format_arg_external
+            "hello {}", w // #format_arg_internal
+        )
+    ); // #format_callsite
+    println!(
+        "{}",
+        s
+    ); // #println_callsite
+    std::process::exit(ret);
+}