AstGen: error on unused switch label

Author: mlugg

lib/std/zig/AstGen.zig

@@ -7999,6 +7999,11 @@ fn switchExpr(
             appendBodyWithFixupsArrayList(astgen, payloads, case_slice);
         }
     }
+
+    if (switch_full.label_token) |label_token| if (!block_scope.label.?.used) {
+        try astgen.appendErrorTok(label_token, "unused switch label", .{});
+    };
+
     // Now that the item expressions are generated we can add this.
     try parent_gz.instructions.append(gpa, switch_block);
 

test/cases/compile_errors/duplicate-unused_labels.zig

@@ -22,6 +22,11 @@ comptime {
 comptime {
     blk: for (@as([0]void, undefined)) |_| {}
 }
+comptime {
+    blk: switch (true) {
+        else => {},
+    }
+}
 
 // error
 // target=native
@@ -35,3 +40,4 @@ comptime {
 // :17:5: error: unused block label
 // :20:5: error: unused while loop label
 // :23:5: error: unused for loop label
+// :26:5: error: unused switch label