Rewrite parse_meta_item.

It can't use `maybe_whole`, but it can match `maybe_whole` more closely.

Also add a test for a case that wasn't previously covered.

compiler/rustc_parse/src/parser/attr.rs

@@ -362,22 +362,18 @@ impl<'a> Parser<'a> {
     /// meta_item_inner : (meta_item | UNSUFFIXED_LIT) (',' meta_item_inner)? ;
     /// ```
     pub fn parse_meta_item(&mut self) -> PResult<'a, ast::MetaItem> {
-        let nt_meta = match &self.token.kind {
-            token::Interpolated(nt) => match &nt.0 {
-                token::NtMeta(e) => Some(e.clone()),
-                _ => None,
-            },
-            _ => None,
-        };
-
-        if let Some(item) = nt_meta {
-            match item.meta(item.path.span) {
+        // We can't use `maybe_whole` here because it would bump in the `None`
+        // case, which we don't want.
+        if let token::Interpolated(nt) = &self.token.kind
+            && let token::NtMeta(attr_item) = &nt.0
+        {
+            match attr_item.meta(attr_item.path.span) {
                 Some(meta) => {
                     self.bump();
                     return Ok(meta);
                 }
                 None => self.unexpected()?,
-            };
+            }
         }
 
         let lo = self.token.span;

tests/ui/parser/attribute/attr-bad-meta-4.rs

@@ -0,0 +1,12 @@
+macro_rules! mac {
+    ($attr_item: meta) => {
+        #[cfg($attr_item)]
+        //~^ ERROR expected unsuffixed literal or identifier, found `an(arbitrary token stream)`
+        //~| ERROR expected unsuffixed literal or identifier, found `an(arbitrary token stream)`
+        struct S;
+    }
+}
+
+mac!(an(arbitrary token stream));
+
+fn main() {}

tests/ui/parser/attribute/attr-bad-meta-4.stderr

@@ -0,0 +1,25 @@
+error: expected unsuffixed literal or identifier, found `an(arbitrary token stream)`
+  --> $DIR/attr-bad-meta-4.rs:3:15
+   |
+LL |         #[cfg($attr_item)]
+   |               ^^^^^^^^^^
+...
+LL | mac!(an(arbitrary token stream));
+   | -------------------------------- in this macro invocation
+   |
+   = note: this error originates in the macro `mac` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: expected unsuffixed literal or identifier, found `an(arbitrary token stream)`
+  --> $DIR/attr-bad-meta-4.rs:3:15
+   |
+LL |         #[cfg($attr_item)]
+   |               ^^^^^^^^^^
+...
+LL | mac!(an(arbitrary token stream));
+   | -------------------------------- in this macro invocation
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+   = note: this error originates in the macro `mac` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
+