Protect error handler fields with single lock

This avoids concurrency-related bugs when locks are acquired for too
short a time and similar cases.

Author: Mark-Simulacrum

src/librustc/session/config/tests.rs

@@ -87,7 +87,7 @@ fn test_can_print_warnings() {
         let registry = errors::registry::Registry::new(&[]);
         let (sessopts, _) = build_session_options_and_crate_config(&matches);
         let sess = build_session(sessopts, None, registry);
-        assert!(!sess.diagnostic().flags.can_emit_warnings);
+        assert!(!sess.diagnostic().can_emit_warnings());
     });
 
     syntax::with_default_globals(|| {
@@ -97,15 +97,15 @@ fn test_can_print_warnings() {
         let registry = errors::registry::Registry::new(&[]);
         let (sessopts, _) = build_session_options_and_crate_config(&matches);
         let sess = build_session(sessopts, None, registry);
-        assert!(sess.diagnostic().flags.can_emit_warnings);
+        assert!(sess.diagnostic().can_emit_warnings());
     });
 
     syntax::with_default_globals(|| {
         let matches = optgroups().parse(&["-Adead_code".to_string()]).unwrap();
         let registry = errors::registry::Registry::new(&[]);
         let (sessopts, _) = build_session_options_and_crate_config(&matches);
         let sess = build_session(sessopts, None, registry);
-        assert!(sess.diagnostic().flags.can_emit_warnings);
+        assert!(sess.diagnostic().can_emit_warnings());
     });
 }