Parse unsafe extern block

Author: dtolnay

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++" {}