auto merge of #16891 : eddyb/rust/patlit-from-expr-macros, r=kballard

Enables any macros using `MacExpr` to be treated as patterns when
they produce a literal in the form `ExprLit` (e.g. `stringify!` or `line!`).

Fixes #16876.

src/test/run-pass/concat.rs

@@ -18,4 +18,9 @@ pub fn main() {
         concat!(1, 2i, 3u, 4f32, 4.0, 'a', true, ()),
         "12344.0atrue"
     );
+
+    assert!(match "12344.0atrue" {
+        concat!(1, 2i, 3u, 4f32, 4.0, 'a', true, ()) => true,
+        _ => false
+    })
 }

src/test/run-pass/syntax-extension-source-utils.rs

@@ -43,4 +43,9 @@ pub fn main() {
         [1] == (42 as u8)); // '*'
     // The Windows tests are wrapped in an extra module for some reason
     assert!((m1::m2::where_am_i().as_slice().ends_with("m1::m2")));
+
+    assert!(match (47, "( 2 * 3 ) + 5") {
+        (line!(), stringify!((2*3) + 5)) => true,
+        _ => false
+    })
 }