Add a new concat metavar expr

tests/ui/macros/macro-metavar-expr-concat/syntax-errors.rs

@@ -0,0 +1,50 @@
+#![feature(macro_metavar_expr_concat)]
+
+macro_rules! wrong_concat_declarations {
+    ($ex:expr) => {
+        ${concat()}
+        //~^ ERROR expected identifier
+
+        ${concat(aaaa)}
+        //~^ ERROR `concat` must have at least two elements
+
+        ${concat(aaaa,)}
+        //~^ ERROR expected identifier
+
+        ${concat(aaaa, 1)}
+        //~^ ERROR expected identifier
+
+        ${concat(_, aaaa)}
+
+        ${concat(aaaa aaaa)}
+        //~^ ERROR expected comma
+
+        ${concat($ex)}
+        //~^ ERROR `concat` must have at least two elements
+
+        ${concat($ex, aaaa)}
+        //~^ ERROR `${concat(..)}` currently only accepts identifiers
+
+        ${concat($ex, aaaa 123)}
+        //~^ ERROR expected comma
+
+        ${concat($ex, aaaa,)}
+        //~^ ERROR expected identifier
+
+        ${concat($ex, aaaa, 123)}
+        //~^ ERROR expected identifier
+    };
+}
+
+macro_rules! dollar_sign_without_referenced_ident {
+    ($ident:ident) => {
+        const ${concat(FOO, $foo)}: i32 = 2;
+        //~^ ERROR variable `foo` is not recognized in meta-variable expression
+    };
+}
+
+fn main() {
+    wrong_concat_declarations!(1);
+
+    dollar_sign_without_referenced_ident!(VAR);
+}