checkUsed

fn checkUsed(gz: *GenZir, outer_scope: *Scope, inner_scope: *Scope) InnerError!void {
    const astgen = gz.astgen;

    var scope = inner_scope;
    while (scope != outer_scope) {
        switch (scope.tag) {
            .gen_zir => scope = scope.cast(GenZir).?.parent,
            .local_val => {
                const s = scope.cast(Scope.LocalVal).?;
                if (s.used == .none and s.discarded == .none) {
                    try astgen.appendErrorTok(s.token_src, "unused {s}", .{@tagName(s.id_cat)});
                } else if (s.used != .none and s.discarded != .none) {
                    try astgen.appendErrorTokNotes(s.discarded.unwrap().?, "pointless discard of {s}", .{@tagName(s.id_cat)}, &[_]u32{
                        try gz.astgen.errNoteTok(s.used.unwrap().?, "used here", .{}),
                    });
                }
                scope = s.parent;
            },
            .local_ptr => {
                const s = scope.cast(Scope.LocalPtr).?;
                if (s.used == .none and s.discarded == .none) {
                    try astgen.appendErrorTok(s.token_src, "unused {s}", .{@tagName(s.id_cat)});
                } else {
                    if (s.used != .none and s.discarded != .none) {
                        try astgen.appendErrorTokNotes(s.discarded.unwrap().?, "pointless discard of {s}", .{@tagName(s.id_cat)}, &[_]u32{
                            try astgen.errNoteTok(s.used.unwrap().?, "used here", .{}),
                        });
                    }
                    if (s.id_cat == .@"local variable" and !s.used_as_lvalue) {
                        try astgen.appendErrorTokNotes(s.token_src, "local variable is never mutated", .{}, &.{
                            try astgen.errNoteTok(s.token_src, "consider using 'const'", .{}),
                        });
                    }
                }

                scope = s.parent;
            },
            .defer_normal, .defer_error => scope = scope.cast(Scope.Defer).?.parent,
            .namespace => unreachable,
            .top => unreachable,
        }
    }
}