GRAM: allow unsafe modifier for extern block item

Note, `unsafe` is semantically invalid in case of extern block item. The corresponding change is made to improve error message.
See the corresponding change in rustc: rust-lang/rust#75857

Author: Undin

src/main/grammars/RustParser.bnf

@@ -708,7 +708,7 @@ upper ModDeclItem ::= unsafe? mod identifier ';' {
   elementTypeFactory = "org.rust.lang.core.stubs.StubImplementationsKt.factory"
 }
 
-upper ForeignModItem ::= ExternAbi ForeignModBody {
+upper ForeignModItem ::= unsafe? ExternAbi ForeignModBody {
   name = ""
   implements = [ "org.rust.lang.core.psi.ext.RsItemElement"
                  "org.rust.lang.core.psi.ext.RsInnerAttributeOwner" ]

src/main/kotlin/org/rust/lang/core/parser/RustParserDefinition.kt

@@ -104,6 +104,6 @@ class RustParserDefinition : ParserDefinition {
         /**
          * Should be increased after any change of parser rules
          */
-        const val PARSER_VERSION: Int = LEXER_VERSION + 30
+        const val PARSER_VERSION: Int = LEXER_VERSION + 31
     }
 }

src/test/resources/org/rust/lang/core/parser/fixtures/complete/extern_block.rs

@@ -16,3 +16,6 @@ extern "C" {}
 extern "R\u{0075}st" {}
 extern r"system" {}
 extern 'C' {} // semantically invalid
+pub extern {} // semantically invalid
+unsafe extern {} // semantically invalid
+pub unsafe extern {} // semantically invalid

src/test/resources/org/rust/lang/core/parser/fixtures/complete/extern_block.txt

@@ -193,4 +193,41 @@ FILE
     PsiElement({)('{')
     PsiElement(})('}')
   PsiWhiteSpace(' ')
+  RsForeignModItemImpl(FOREIGN_MOD_ITEM)
+    PsiComment(<EOL_COMMENT>)('// semantically invalid')
+    PsiWhiteSpace('\n')
+    RsVisImpl(VIS)
+      PsiElement(pub)('pub')
+    PsiWhiteSpace(' ')
+    RsExternAbiImpl(EXTERN_ABI)
+      PsiElement(extern)('extern')
+    PsiWhiteSpace(' ')
+    PsiElement({)('{')
+    PsiElement(})('}')
+  PsiWhiteSpace(' ')
+  RsForeignModItemImpl(FOREIGN_MOD_ITEM)
+    PsiComment(<EOL_COMMENT>)('// semantically invalid')
+    PsiWhiteSpace('\n')
+    PsiElement(unsafe)('unsafe')
+    PsiWhiteSpace(' ')
+    RsExternAbiImpl(EXTERN_ABI)
+      PsiElement(extern)('extern')
+    PsiWhiteSpace(' ')
+    PsiElement({)('{')
+    PsiElement(})('}')
+  PsiWhiteSpace(' ')
+  RsForeignModItemImpl(FOREIGN_MOD_ITEM)
+    PsiComment(<EOL_COMMENT>)('// semantically invalid')
+    PsiWhiteSpace('\n')
+    RsVisImpl(VIS)
+      PsiElement(pub)('pub')
+    PsiWhiteSpace(' ')
+    PsiElement(unsafe)('unsafe')
+    PsiWhiteSpace(' ')
+    RsExternAbiImpl(EXTERN_ABI)
+      PsiElement(extern)('extern')
+    PsiWhiteSpace(' ')
+    PsiElement({)('{')
+    PsiElement(})('}')
+  PsiWhiteSpace(' ')
   PsiComment(<EOL_COMMENT>)('// semantically invalid')