Merge #6606

6606: Parse unsafe extern block r=lnicola a=dtolnay

`unsafe extern` block is parsed successfully by rustc, which means it is usable in attribute macro input.
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=6f805556f176d082d87255957f16b5f6

```rust
#[cfg(parse)]
unsafe extern "C++" {
    fn demo();
}
```

```diff
  [email protected]
-   [email protected]
+   [email protected]
      [email protected]
        [email protected] "#"
        [email protected] "["
        [email protected]
          [email protected]
            [email protected]
              [email protected] "cfg"
        [email protected]
          [email protected] "("
          [email protected] "parse"
          [email protected] ")"
        [email protected] "]"
      [email protected] "\n"
      [email protected] "unsafe"
      [email protected] " "
      [email protected]
        [email protected] "extern"
        [email protected] " "
        [email protected] "\"C++\""
-   [email protected] " "
-   [email protected]
-     [email protected] "{"
-     [email protected] "\n    "
-     [email protected]
-       [email protected] "fn"
-       [email protected] " "
-       [email protected]
-         [email protected] "demo"
-       [email protected]
-         [email protected] "("
-         [email protected] ")"
-       [email protected] ";"
-     [email protected] "\n"
-     [email protected] "}"
+     [email protected] " "
+     [email protected]
+       [email protected] "{"
+       [email protected] "\n    "
+       [email protected]
+         [email protected] "fn"
+         [email protected] " "
+         [email protected]
+           [email protected] "demo"
+         [email protected]
+           [email protected] "("
+           [email protected] ")"
+         [email protected] ";"
+       [email protected] "\n"
+       [email protected] "}"
```

This is of interest for https://github.com/dtolnay/cxx.

Co-authored-by: David Tolnay <[email protected]>

1 files changed, 2 insertions, 0 deletions

diff --git a/crates/syntax/test_data/parser/ok/0068_item_modifiers.rs b/crates/syntax/test_data/parser/ok/0068_item_modifiers.rs
index 8d697c04b..6d27a082c 100644
--- a/crates/syntax/test_data/parser/ok/0068_item_modifiers.rs
+++ b/crates/syntax/test_data/parser/ok/0068_item_modifiers.rs
@@ -14,3 +14,5 @@ unsafe auto trait T {}
 unsafe impl Foo {}
 default impl Foo {}
 unsafe default impl Foo {}
+
+unsafe extern "C++" {}