Merge #6934

6934: Implement `cfg_attr` handling r=jonas-schievink a=jonas-schievink

Part of https://github.com/rust-analyzer/rust-analyzer/issues/5548

Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>

crates/syntax/src/lib.rs

@@ -205,6 +205,13 @@ impl ast::Type {
     }
 }
 
+impl ast::Attr {
+    /// Returns `text`, parsed as an attribute, but only if it has no errors.
+    pub fn parse(text: &str) -> Result<Self, ()> {
+        parsing::parse_text_fragment(text, parser::FragmentKind::Attr)
+    }
+}
+
 /// Matches a `SyntaxNode` against an `ast` type.
 ///
 /// # Example: