From 5d5026be24e5d8892570bbe6eeab59ef5d94f251 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Mon, 25 Nov 2024 19:06:28 +1100
Subject: [PATCH] Fix the `features` macro.

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 https://github.com/rust-lang/rust/pull/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`.
---
 crates/std_detect/src/detect/macros.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/crates/std_detect/src/detect/macros.rs b/crates/std_detect/src/detect/macros.rs
index 23b238d1bc..6ce842b31e 100644
--- a/crates/std_detect/src/detect/macros.rs
+++ b/crates/std_detect/src/detect/macros.rs
@@ -38,7 +38,7 @@ macro_rules! features {
       $(@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;)?
+          $(without cfg check: $feature_cfg_check:tt;)?
           $(implied by target_features: [$($target_feature_lit:tt),*];)?
           $(#[$feature_comment:meta])*)*
     ) => {