std.mem.zeroes: Improve handling of extern struct and extern union

`extern struct` will now be recursively initialized, same as `struct`.
The only difference is that the `extern` variant is initially zeroed out
to ensure its padding is also set correctly.

`extern union` is still initialized to its first named member, but it is
also now initially zeroed out for the padding.

Fixes #17258

Author: jayschwa

lib/std/mem.zig

@@ -246,19 +246,16 @@ pub fn zeroes(comptime T: type) T {
         },
         .Struct => |struct_info| {
             if (@sizeOf(T) == 0) return undefined;
+            var structure: T = undefined;
             if (struct_info.layout == .Extern) {
-                var item: T = undefined;
-                @memset(asBytes(&item), 0);
-                return item;
-            } else {
-                var structure: T = undefined;
-                inline for (struct_info.fields) |field| {
-                    if (!field.is_comptime) {
-                        @field(structure, field.name) = zeroes(@TypeOf(@field(structure, field.name)));
-                    }
+                @memset(asBytes(&structure), 0); // Zero out padding.
+            }
+            inline for (struct_info.fields) |field| {
+                if (!field.is_comptime) {
+                    @field(structure, field.name) = zeroes(@TypeOf(@field(structure, field.name)));
                 }
-                return structure;
             }
+            return structure;
         },
         .Pointer => |ptr_info| {
             switch (ptr_info.size) {
@@ -292,13 +289,16 @@ pub fn zeroes(comptime T: type) T {
             return @splat(zeroes(info.child));
         },
         .Union => |info| {
-            if (comptime meta.containerLayout(T) == .Extern) {
-                // The C language specification states that (global) unions
-                // should be zero initialized to the first named member.
-                return @unionInit(T, info.fields[0].name, zeroes(info.fields[0].type));
+            if (comptime meta.containerLayout(T) != .Extern) {
+                @compileError("Can't set a " ++ @typeName(T) ++ " to zero.");
             }
-
-            @compileError("Can't set a " ++ @typeName(T) ++ " to zero.");
+            // The C language specification states that (global) unions
+            // have the first named member initialized (recursively),
+            // and any padding is initialized to zero bits.
+            var u: T = undefined;
+            @memset(asBytes(&u), 0); // Zero out padding.
+            @field(u, info.fields[0].name) = zeroes(info.fields[0].type);
+            return u;
         },
         .ErrorUnion,
         .ErrorSet,