rfc: cfg_attr-multiple-attrs - Disallow cfg_attr(predicate)

Author: Havvy

text/0000-cfg_attr-multiple-attrs.md

@@ -61,15 +61,18 @@ configuration predicate.
 It is written as `cfg_attr` followed by `(`, a comma separated metaitem
 sequence, and then `)` The metaitem sequence contains one or more metaitems.
 The first is a conditional predicate. The rest are metaitems that are also
-attributes. Trailing commas are permitted. The following list are all allowed:
+attributes. Trailing commas after attributes are permitted. The following list
+are all allowed:
 
 * `cfg_attr(predicate, attr)`
 * `cfg_attr(predicate, attr_1, attr_2)`
 * `cfg_attr(predicate, attr,)`
 * `cfg_attr(predicate, attr_1, attr_2,)`
-* `cfg_attr(predicate)`
 * `cfg_attr(predicate,)`
 
+> Note: `cfg_attr(predicate)` is not allowed. That comma is semantically
+> distinct from the commas following attributes, so we require it.
+
 When the configuration predicate is true, this attribute expands out to be an
 attribute for each attribute metaitem. For example, the following module will
 either be found at `linux.rs` or `windows.rs` based on the target.