Add a new -Z force-full-debuginfo flag

Apparently firefox depends on the current behavior that adds too much
debuginfo, but that doesn't mean we need to force it on for rustc
itself. Add a new unstable flag for turning it off.

Author: jyn514

compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs

@@ -832,7 +832,7 @@ pub fn build_compile_unit_di_node<'ll, 'tcx>(
     .unwrap_or_default();
     let split_name = split_name.to_str().unwrap();
 
-    // FIXME(#60020):
+    // FIXME(#64405):
     //
     //    This should actually be
     //
@@ -847,7 +847,11 @@ pub fn build_compile_unit_di_node<'ll, 'tcx>(
     //    the emission kind as `FullDebug`.
     //
     //    See https://github.com/rust-lang/rust/issues/60020 for details.
-    let kind = DebugEmissionKind::FullDebug;
+    let kind = if tcx.sess.opts.unstable_opts.force_full_debuginfo {
+        DebugEmissionKind::FullDebug
+    } else {
+        DebugEmissionKind::from_generic(tcx.sess.opts.debuginfo)
+    };
     assert!(tcx.sess.opts.debuginfo != DebugInfo::None);
 
     unsafe {

compiler/rustc_session/src/options.rs

@@ -1425,6 +1425,8 @@ options! {
     flatten_format_args: bool = (false, parse_bool, [TRACKED],
         "flatten nested format_args!() and literals into a simplified format_args!() call \
         (default: no)"),
+    force_full_debuginfo: bool = (true, parse_bool, [TRACKED],
+        "force all codegen-units to have full debuginfo even for -C debuginfo=1. see #64405 (default: yes)"),
     force_unstable_if_unmarked: bool = (false, parse_bool, [TRACKED],
         "force all crates to be `rustc_private` unstable (default: no)"),
     fuel: Option<(String, u64)> = (None, parse_optimization_fuel, [TRACKED],