Optimize the codegen for Span::from_expansion

Author: Jarcho

compiler/rustc_span/src/span_encoding.rs

@@ -306,8 +306,16 @@ impl Span {
     /// Returns `true` if this span comes from any kind of macro, desugaring or inlining.
     #[inline]
     pub fn from_expansion(self) -> bool {
-        // If the span is fully inferred then ctxt > MAX_CTXT
-        self.inline_ctxt().map_or(true, |ctxt| !ctxt.is_root())
+      // Optimizes much better than `inline_ctxt` since it avoids comparing against `lo_or_index`.
+      let ctxt = match_span_kind! {
+          self,
+          InlineCtxt(span) => SyntaxContext::from_u16(span.ctxt),
+          InlineParent(_span) => SyntaxContext::root(),
+          PartiallyInterned(span) => SyntaxContext::from_u16(span.ctxt),
+          // Must be the value stored in `ctxt_or_parent_or_marker` to optimize properly.
+          Interned(_span) => SyntaxContext::from_u16(CTXT_INTERNED_MARKER),
+      };
+      !ctxt.is_root()
     }
 
     /// Returns `true` if this is a dummy span with any hygienic context.