This is achieving a similar goal as #3849, using placeholders instead of
an ambiguous start node to clarify structure and incrementally simplify
checking. The benefit isn't quite as big here because both paths are
structs, and so checking is more consistent than paren exprs versus
tuples. But I think this removes the only other multi-purpose parse
node.
This uses StructLiteral/StructTypeLiteral naming, reflecting equivalent
SemIR naming. Note, I would lean towards renaming StructLiteral to
StructValueLiteral, but I think consistency in naming takes precedence.
Any renames of StructLiteral might be better in a separate PR.
StructFieldType/StructFieldValue -> StructTypeField/StructField is
trying to making the reading more consistent with
StructTypeLiteral/StructLiteral. SemIR has StructTypeField but not a
value equivalent.
On the parsing side, we treat `a.(b)` as a member access whose second
operand is a `ParenExpr` rather than a `MemberName`. A new node category
is added for the union of `MemberName` and `ParenExpr` to support this.
Checking is mostly reusing the same pieces we already have for simple
member access. Compound member access is in most ways a simplified form
of simple member access because it doesn't need to do any lookup.
Jon Ross-Perkins
Richard Smith