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]>

Author: bors[bot]

crates/parser/src/grammar/items.rs

@@ -112,7 +112,7 @@ pub(super) fn maybe_item(p: &mut Parser, m: Marker) -> Result<(), Marker> {
         has_mods = true;
     }
 
-    if p.at(T![extern]) {
+    if p.at(T![extern]) && p.nth(1) != T!['{'] && (p.nth(1) != STRING || p.nth(2) != T!['{']) {
         has_mods = true;
         abi(p);
     }
@@ -181,6 +181,14 @@ pub(super) fn maybe_item(p: &mut Parser, m: Marker) -> Result<(), Marker> {
         T![type] => {
             type_alias(p, m);
         }
+
+        // unsafe extern "C" {}
+        T![extern] => {
+            abi(p);
+            extern_item_list(p);
+            m.complete(p, EXTERN_BLOCK);
+        }
+
         _ => {
             if !has_visibility && !has_mods {
                 return Err(m);

crates/syntax/test_data/parser/ok/0068_item_modifiers.rast

@@ -1,4 +1,4 @@
-SOURCE_FILE@0..304
+SOURCE_FILE@0..328
   [email protected]
     [email protected] "async"
     [email protected] " "
@@ -215,4 +215,16 @@ [email protected]
     [email protected]
       [email protected] "{"
       [email protected] "}"
-  [email protected] "\n"
+  [email protected] "\n\n"
+  [email protected]
+    [email protected] "unsafe"
+    [email protected] " "
+    [email protected]
+      [email protected] "extern"
+      [email protected] " "
+      [email protected] "\"C++\""
+    [email protected] " "
+    [email protected]
+      [email protected] "{"
+      [email protected] "}"
+  [email protected] "\n"

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++" {}