tests: add tests for doc comments on expressions

tests/ui/feature-gates/feature-gate-stmt_expr_attributes.rs

@@ -1,4 +1,15 @@
 const X: i32 = #[allow(dead_code)] 8;
 //~^ ERROR attributes on expressions are experimental
 
+const Y: i32 =
+    /// foo
+//~^ ERROR attributes on expressions are experimental
+    8;
+
+const Z: i32 = {
+    //! foo
+//~^ ERROR attributes on expressions are experimental
+    8
+};
+
 fn main() {}

tests/ui/feature-gates/feature-gate-stmt_expr_attributes.stderr

@@ -8,6 +8,28 @@ LL | const X: i32 = #[allow(dead_code)] 8;
    = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: aborting due to 1 previous error
+error[E0658]: attributes on expressions are experimental
+  --> $DIR/feature-gate-stmt_expr_attributes.rs:5:5
+   |
+LL |     /// foo
+   |     ^^^^^^^
+   |
+   = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
+   = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = help: `///` is for documentation comments. For a plain comment, use `//`.
+
+error[E0658]: attributes on expressions are experimental
+  --> $DIR/feature-gate-stmt_expr_attributes.rs:10:5
+   |
+LL |     //! foo
+   |     ^^^^^^^
+   |
+   = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
+   = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = help: `///` is for documentation comments. For a plain comment, use `//`.
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0658`.