Stabilize --json unused-externs(-silent)

Implement rust-lang/compiler-team#674

Author: jsgf

compiler/rustc_session/src/config.rs

@@ -2630,13 +2630,6 @@ pub fn build_session_options(
 
     check_error_format_stability(handler, &unstable_opts, error_format, json_rendered);
 
-    if !unstable_opts.unstable_options && json_unused_externs.is_enabled() {
-        handler.early_error(
-            "the `-Z unstable-options` flag must also be passed to enable \
-            the flag `--json=unused-externs`",
-        );
-    }
-
     let output_types = parse_output_types(handler, &unstable_opts, matches);
 
     let mut cg = CodegenOptions::build(handler, matches);

tests/ui/unused-crate-deps/deny-cmdline-json-silent.rs

@@ -2,7 +2,7 @@
 
 // edition:2018
 // check-pass
-// compile-flags: -Dunused-crate-dependencies -Zunstable-options --json unused-externs-silent --error-format=json
+// compile-flags: -Dunused-crate-dependencies --json unused-externs-silent --error-format=json
 // aux-crate:bar=bar.rs
 
 fn main() {}

tests/ui/unused-crate-deps/deny-cmdline-json.rs

@@ -1,7 +1,7 @@
 // Check for unused crate dep, json event, deny, expect compile failure
 
 // edition:2018
-// compile-flags: -Dunused-crate-dependencies  -Zunstable-options --json unused-externs --error-format=json
+// compile-flags: -Dunused-crate-dependencies --json unused-externs --error-format=json
 // aux-crate:bar=bar.rs
 
 fn main() {}

tests/ui/unused-crate-deps/warn-cmdline-json.rs

@@ -2,7 +2,7 @@
 
 // edition:2018
 // check-pass
-// compile-flags: -Wunused-crate-dependencies -Zunstable-options --json unused-externs --error-format=json
+// compile-flags: -Wunused-crate-dependencies --json unused-externs --error-format=json
 // aux-crate:bar=bar.rs
 
 fn main() {}