Rollup merge of #85678 - lukas-code:matches2021, r=dtolnay

fix `matches!` and `assert_matches!` on edition 2021

Previously this code failed to compile on edition 2021. [(Playground)](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=53960f2f051f641777b9e458da747707)
```rust
fn main() {
    matches!((), ());
}
```
```
   Compiling playground v0.0.1 (/playground)
error: `$pattern:pat` may be followed by `|`, which is not allowed for `pat` fragments
    |
    = note: allowed there are: `=>`, `,`, `=`, `if` or `in`

error: aborting due to previous error

error: could not compile `playground`

To learn more, run the command again with --verbose.
```

Author: Dylan-DPC

library/core/src/lib.rs

@@ -168,6 +168,7 @@
 #![feature(no_coverage)] // rust-lang/rust#84605
 #![feature(int_error_matching)]
 #![deny(unsafe_op_in_unsafe_fn)]
+#![deny(or_patterns_back_compat)]
 
 // allow using `core::` in intra-doc links
 #[allow(unused_extern_crates)]

library/core/src/macros/mod.rs

@@ -138,7 +138,7 @@ macro_rules! assert_ne {
 #[unstable(feature = "assert_matches", issue = "82775")]
 #[allow_internal_unstable(core_panic)]
 macro_rules! assert_matches {
-    ($left:expr, $( $pattern:pat )|+ $( if $guard: expr )? $(,)?) => ({
+    ($left:expr, $( $pattern:pat_param )|+ $( if $guard: expr )? $(,)?) => ({
         match $left {
             $( $pattern )|+ $( if $guard )? => {}
             ref left_val => {
@@ -150,7 +150,7 @@ macro_rules! assert_matches {
             }
         }
     });
-    ($left:expr, $( $pattern:pat )|+ $( if $guard: expr )?, $($arg:tt)+) => ({
+    ($left:expr, $( $pattern:pat_param )|+ $( if $guard: expr )?, $($arg:tt)+) => ({
         match $left {
             $( $pattern )|+ $( if $guard )? => {}
             ref left_val => {
@@ -315,7 +315,7 @@ macro_rules! debug_assert_matches {
 #[macro_export]
 #[stable(feature = "matches_macro", since = "1.42.0")]
 macro_rules! matches {
-    ($expression:expr, $( $pattern:pat )|+ $( if $guard: expr )? $(,)?) => {
+    ($expression:expr, $( $pattern:pat_param )|+ $( if $guard: expr )? $(,)?) => {
         match $expression {
             $( $pattern )|+ $( if $guard )? => true,
             _ => false

src/test/ui/matches2021.rs

@@ -0,0 +1,12 @@
+// run-pass
+// edition:2021
+// compile-flags: -Zunstable-options
+
+// regression test for https://github.com/rust-lang/rust/pull/85678
+
+#![feature(assert_matches)]
+
+fn main() {
+    assert!(matches!((), ()));
+    assert_matches!((), ());
+}