mbe: Add parser test for macro attribute recovery

Author: joshtriplett

tests/ui/parser/macro/macro-attr-recovery.rs

@@ -0,0 +1,19 @@
+#![crate_type = "lib"]
+#![feature(macro_attr)]
+
+macro_rules! attr {
+    attr[$($args:tt)*] { $($body:tt)* } => {
+        //~^ ERROR: macro attribute argument matchers require parentheses
+        //~v ERROR: attr:
+        compile_error!(concat!(
+            "attr: args=\"",
+            stringify!($($args)*),
+            "\" body=\"",
+            stringify!($($body)*),
+            "\"",
+        ));
+    };
+}
+
+#[attr]
+struct S;

tests/ui/parser/macro/macro-attr-recovery.stderr

@@ -0,0 +1,31 @@
+error: macro attribute argument matchers require parentheses
+  --> $DIR/macro-attr-recovery.rs:5:9
+   |
+LL |     attr[$($args:tt)*] { $($body:tt)* } => {
+   |         ^^^^^^^^^^^^^^
+   |
+help: the delimiters should be `(` and `)`
+   |
+LL -     attr[$($args:tt)*] { $($body:tt)* } => {
+LL +     attr($($args:tt)*) { $($body:tt)* } => {
+   |
+
+error: attr: args="" body="struct S;"
+  --> $DIR/macro-attr-recovery.rs:8:9
+   |
+LL | /         compile_error!(concat!(
+LL | |             "attr: args=\"",
+LL | |             stringify!($($args)*),
+LL | |             "\" body=\"",
+LL | |             stringify!($($body)*),
+LL | |             "\"",
+LL | |         ));
+   | |__________^
+...
+LL |   #[attr]
+   |   ------- in this attribute macro expansion
+   |
+   = note: this error originates in the attribute macro `attr` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
+