session: stabilize split debuginfo on linux

Stabilize the `-Csplit-debuginfo` flag...

- ...on Linux for all values of the flag. Split DWARF has been
  implemented for a few months, hasn't had any bug reports and has had
  some promising benchmarking for incremental debug build performance.
- ..on other platforms for the default value. It doesn't make any sense
  that `-Csplit-debuginfo=packed` is unstable on Windows MSVC when
  that's the default behaviour, but keep the other values unstable.

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

compiler/rustc_session/src/session.rs

@@ -31,7 +31,7 @@ use rustc_span::{sym, SourceFileHashAlgorithm, Symbol};
 use rustc_target::asm::InlineAsmArch;
 use rustc_target::spec::{CodeModel, PanicStrategy, RelocModel, RelroLevel};
 use rustc_target::spec::{
-    SanitizerSet, SplitDebuginfo, StackProtector, Target, TargetTriple, TlsModel,
+    DebuginfoKind, SanitizerSet, SplitDebuginfo, StackProtector, Target, TargetTriple, TlsModel,
 };
 
 use std::cell::{self, RefCell};
@@ -661,8 +661,9 @@ impl Session {
             )
     }
 
+    /// Returns `true` if the target can use the current split debuginfo configuration.
     pub fn target_can_use_split_dwarf(&self) -> bool {
-        !self.target.is_like_windows && !self.target.is_like_osx
+        self.target.debuginfo_kind == DebuginfoKind::Dwarf
     }
 
     pub fn generate_proc_macro_decls_symbol(&self, stable_crate_id: StableCrateId) -> String {
@@ -1543,6 +1544,15 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
             sess.err(&format!("requested DWARF version {} is greater than 5", dwarf_version));
         }
     }
+
+    if !sess.target.options.supported_split_debuginfo.contains(&sess.split_debuginfo())
+        && !sess.opts.unstable_opts.unstable_options
+    {
+        sess.err(&format!(
+            "`-Csplit-debuginfo={}` is unstable on this platform",
+            sess.split_debuginfo()
+        ));
+    }
 }
 
 /// Holds data on the current incremental compilation session, if there is one.