Fix the features macro.

[nnethercote]

The first rule of the features macro looks like this:

macro_rules! features {
    (
      @TARGET: $target:ident;
      @CFG: $cfg:meta;
      @MACRO_NAME: $macro_name:ident;
      @MACRO_ATTRS: $(#[$macro_attrs:meta])*
      $(@BIND_FEATURE_NAME: $bind_feature:tt; $feature_impl:tt; $(#[$deprecate_attr:meta];)?)*
      $(@NO_RUNTIME_DETECTION: $nort_feature:tt; )*
      $(@FEATURE: #[$stability_attr:meta] $feature:ident: $feature_lit:tt;
          $(without cfg check: $feature_cfg_check:literal;)?
          $(implied by target_features: [$($target_feature_lit:tt),*];)?
          $(#[$feature_comment:meta])*)*
    ) => {

Notice all the tt specifiers. They are used because they are forwarded to another macro. Only ident, lifetime, and tt specifiers can be forwarded this way.

But there is an exception: $feature_lit:tt, which was added recently. In theory it should cause an error like this:

error: no rules expected `literal` metavariable
   --> /home/njn/dev/rust3/library/stdarch/crates/std_detect/src/detect/macros.rs:54:91
    |
51  | /         macro_rules! $macro_name {
52  | |             $(
53  | |                 ($feature_lit) => {
54  | |                     $crate::detect_feature!($feature, $feature_lit $(, without cfg check: $feature_cfg_check)? ...
    | |                                                                                           ^^^^^^^^^^^^^^^^^^ no rules expected this token in macro call
...   |
88  | |             };
89  | |         }
    | |_________- in this expansion of `is_x86_feature_detected!`
    |
   ::: std/tests/run-time-detect.rs:145:27
    |
145 |       println!("tsc: {:?}", is_x86_feature_detected!("tsc"));
    |                             ------------------------------- in this macro invocation
    |
note: while trying to match keyword `true`
   --> /home/njn/dev/rust3/library/stdarch/crates/std_detect/src/detect/macros.rs:12:55
    |
12  |     ($feature:tt, $feature_lit:tt, without cfg check: true) => {
    |                                                       ^^^^
    = note: captured metavariables except for `:tt`, `:ident` and `:lifetime` cannot be compared to other tokens
    = note: see <https://doc.rust-lang.org/nightly/reference/macros-by-example.html#forwarding-a-matched-fragment> for more information

(The URL at the end of the error has more details about this forwarding limitation.)

In practice it doesn't cause this error. I'm not sure why, but the existing macro implementation in rustc is far from perfect, so it's believable that it does the wrong thing here.

Why does this matter? Because rust-lang/rust#124141 is modifying the macro implementation, and when that PR is applied the error does occur. (It's one of several cases I have found where the existing compiler accepts code it shouldn't, but #124141 causes that code to be rejected.)

Fortunately the fix is simple: replace the literal specifier with tt.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Amanieu (or someone else) some time within the next two weeks.

[nnethercote]

You can see the problem in action on the rustc CI at rust-lang/rust#124141 (comment)

[Amanieu]

CI should be fixed now, can you rebase?

[nnethercote]

I rebased.