add error message to string_enum!s string conversions

src/tools/compiletest/src/common.rs

@@ -39,12 +39,12 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         }
 
         impl FromStr for $name {
-            type Err = ();
+            type Err = String;
 
-            fn from_str(s: &str) -> Result<Self, ()> {
+            fn from_str(s: &str) -> Result<Self, Self::Err> {
                 match s {
                     $($repr => Ok(Self::$variant),)*
-                    _ => Err(()),
+                    _ => Err(format!(concat!("unknown `", stringify!($name), "` variant: `{}`"), s)),
                 }
             }
         }

src/tools/compiletest/src/tests.rs

@@ -91,5 +91,5 @@ enum Animal {
 
     // Invalid conversions
     let animal = "nya".parse::<Animal>();
-    assert!(matches!(animal, Err(())));
+    assert_eq!("unknown `Animal` variant: `nya`", animal.unwrap_err());
 }