Error when warnings lint group is used with force-warn

Author: rylev

compiler/rustc_error_codes/src/error_codes/E0602.md

@@ -1,4 +1,4 @@
-An unknown lint was used on the command line.
+An unknown or invalid lint was used on the command line.
 
 Erroneous code example:
 

compiler/rustc_lint/src/context.rs

@@ -332,7 +332,16 @@ impl LintStore {
         crate_attrs: &[ast::Attribute],
     ) {
         let (tool_name, lint_name_only) = parse_lint_and_tool_name(lint_name);
-
+        if lint_name_only == crate::WARNINGS.name_lower() && level == Level::ForceWarn {
+            return struct_span_err!(
+                sess,
+                DUMMY_SP,
+                E0602,
+                "`{}` lint group is not supported with ´--force-warn´",
+                crate::WARNINGS.name_lower()
+            )
+            .emit();
+        }
         let db = match self.check_lint_name(sess, lint_name_only, tool_name, crate_attrs) {
             CheckLintNameResult::Ok(_) => None,
             CheckLintNameResult::Warning(ref msg, _) => Some(sess.struct_warn(msg)),

src/test/ui/lint/force-warn/warnings-lint-group.rs

@@ -0,0 +1,5 @@
+// --force-warn warnings is an error
+// compile-flags: --force-warn warnings -Zunstable-options
+// error-pattern: `warnings` lint group is not supported
+
+fn main() {}

src/test/ui/lint/force-warn/warnings-lint-group.stderr

@@ -0,0 +1,9 @@
+error[E0602]: `warnings` lint group is not supported with ´--force-warn´
+
+error[E0602]: `warnings` lint group is not supported with ´--force-warn´
+
+error[E0602]: `warnings` lint group is not supported with ´--force-warn´
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0602`.