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.

1 files changed, 10 insertions, 0 deletions

diff --git a/src/libsyntax/ext/base.rs b/src/libsyntax/ext/base.rs
index b3b66a6a604..d59b20dfc4c 100644
--- a/src/libsyntax/ext/base.rs
+++ b/src/libsyntax/ext/base.rs
@@ -154,6 +154,16 @@ impl MacResult for MacExpr {
     fn make_expr(&self) -> Option<Gc<ast::Expr>> {
         Some(self.e)
     }
+    fn make_pat(&self) -> Option<Gc<ast::Pat>> {
+        match self.e.node {
+            ast::ExprLit(_) => Some(box(GC) ast::Pat {
+                id: ast::DUMMY_NODE_ID,
+                node: ast::PatLit(self.e),
+                span: self.e.span
+            }),
+            _ => None
+        }
+    }
 }
 /// A convenience type for macros that return a single pattern.
 pub struct MacPat {