This enforces the new constraint that templates expanding into an `Expr`
context must only inline a single `Expr`.
Perhaps in the future we'll support explicit splicing, like `,@` in
Lisp. But this new restriction is intended for two purposes:
- To make templates more predictable (if you have a list of expressions
inlined then they will act differently depending on the type of
expression that they are inlined into, which means that more defensive
programming would otherwise be required); and
- To make expansion easier, since we're going to have to set aside an
expansion workspace ahead of time to ensure ordering (Petgraph can't
replace edges in-place). If we support multi-expansion, we'd have to
handle associativity in all expression contexts.
This'll become more clear in future commits.
It's nice to see all this hard work coming together now, though; it's easy
now to perform static analysis on the system, and any part of the graph
construction can throw errors with rich diagnostic information and still
recover properly. And, importantly, the system enforces its own state, and
the compiler helps us with that (the previous commits).
DEV-13163
Mike Gerwitz
c19ecba6ef