Syntactically permit unsafety on mods

src/test/ui/parser/unsafe-foreign-mod.rs

@@ -0,0 +1,9 @@
+unsafe extern {
+    //~^ ERROR extern block cannot be declared unsafe
+}
+
+unsafe extern "C" {
+    //~^ ERROR extern block cannot be declared unsafe
+}
+
+fn main() {}

src/test/ui/parser/unsafe-foreign-mod.stderr

@@ -0,0 +1,14 @@
+error: extern block cannot be declared unsafe
+  --> $DIR/unsafe-foreign-mod.rs:1:1
+   |
+LL | unsafe extern {
+   | ^^^^^^
+
+error: extern block cannot be declared unsafe
+  --> $DIR/unsafe-foreign-mod.rs:5:1
+   |
+LL | unsafe extern "C" {
+   | ^^^^^^
+
+error: aborting due to 2 previous errors
+

src/test/ui/parser/unsafe-mod.rs

@@ -0,0 +1,9 @@
+unsafe mod m {
+    //~^ ERROR module cannot be declared unsafe
+}
+
+unsafe mod n;
+//~^ ERROR module cannot be declared unsafe
+//~^^ ERROR file not found for module `n`
+
+fn main() {}

src/test/ui/parser/unsafe-mod.stderr

@@ -0,0 +1,23 @@
+error[E0583]: file not found for module `n`
+  --> $DIR/unsafe-mod.rs:5:1
+   |
+LL | unsafe mod n;
+   | ^^^^^^^^^^^^^
+   |
+   = help: to create the module `n`, create file "$DIR/n.rs"
+
+error: module cannot be declared unsafe
+  --> $DIR/unsafe-mod.rs:1:1
+   |
+LL | unsafe mod m {
+   | ^^^^^^
+
+error: module cannot be declared unsafe
+  --> $DIR/unsafe-mod.rs:5:1
+   |
+LL | unsafe mod n;
+   | ^^^^^^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0583`.