fix(client-policy): correct ensure_nonempty! error message format (#4491)

The `ensure_nonempty!` macro in `Meta::try_from` produces malformed error
messages when a Resource identity field was empty. The original form
`concat!(stringify!($name, "must not be empty"))` passes both `$name`
and the literal `"must not be empty"` as a single token list to the
`stringify!` macro, which then renders them as source text. The result
is an error string like `<resource>, "must not be empty"` (comma and
double quotes included), instead of `<resource> must not be empty`. The
outer `concat!` wraps a single string, so it becomes a no-op.

Fix this by stringifying only the identifier, then concatenating the
suffix separately: `concat!(stringify!($name), " must not be empty")`.

Signed-off-by: Alejandro Martinez Ruiz <amr@buoyant.io>

Author: unleashed

linkerd/proxy/client-policy/src/lib.rs

@@ -564,7 +564,7 @@ pub mod proto {
                         ($($name:ident),+) => {
                             $(
                                 if $name.is_empty() {
-                                    return Err(InvalidMeta(concat!(stringify!($name, "must not be empty"))));
+                                    return Err(InvalidMeta(concat!(stringify!($name), " must not be empty")));
                                 }
                             )+
                         }