Add parse fail test using safe trait/impl trait

Author: spastorino

Diff for: tests/ui/rust-2024/unsafe-extern-blocks/safe-impl-trait.gated.stderr

@@ -0,0 +1,8 @@
+error: expected one of `!` or `::`, found keyword `impl`
+  --> $DIR/safe-impl-trait.rs:5:6
+   |
+LL | safe impl Bar for () { }
+   |      ^^^^ expected one of `!` or `::`
+
+error: aborting due to 1 previous error
+

Diff for: tests/ui/rust-2024/unsafe-extern-blocks/safe-impl-trait.rs

@@ -0,0 +1,8 @@
+//@ revisions: gated ungated
+#![cfg_attr(gated, feature(unsafe_extern_blocks))]
+
+trait Bar {}
+safe impl Bar for () { }
+//~^ ERROR expected one of `!` or `::`, found keyword `impl`
+
+fn main() {}

Diff for: tests/ui/rust-2024/unsafe-extern-blocks/safe-impl-trait.ungated.stderr

@@ -0,0 +1,8 @@
+error: expected one of `!` or `::`, found keyword `impl`
+  --> $DIR/safe-impl-trait.rs:5:6
+   |
+LL | safe impl Bar for () { }
+   |      ^^^^ expected one of `!` or `::`
+
+error: aborting due to 1 previous error
+

Diff for: tests/ui/rust-2024/unsafe-extern-blocks/safe-trait.gated.stderr

@@ -0,0 +1,8 @@
+error: expected one of `!` or `::`, found keyword `trait`
+  --> $DIR/safe-trait.rs:4:6
+   |
+LL | safe trait Foo {}
+   |      ^^^^^ expected one of `!` or `::`
+
+error: aborting due to 1 previous error
+

Diff for: tests/ui/rust-2024/unsafe-extern-blocks/safe-trait.rs

@@ -0,0 +1,7 @@
+//@ revisions: gated ungated
+#![cfg_attr(gated, feature(unsafe_extern_blocks))]
+
+safe trait Foo {}
+//~^ ERROR expected one of `!` or `::`, found keyword `trait`
+
+fn main() {}

Diff for: tests/ui/rust-2024/unsafe-extern-blocks/safe-trait.ungated.stderr

@@ -0,0 +1,8 @@
+error: expected one of `!` or `::`, found keyword `trait`
+  --> $DIR/safe-trait.rs:4:6
+   |
+LL | safe trait Foo {}
+   |      ^^^^^ expected one of `!` or `::`
+
+error: aborting due to 1 previous error
+