It's hard to track 2-space indent levels across large vertical gaps, and
it can be hard to suppress the instinct to read the dump as if it were
preorder. This change introduces a new dump mode that addresses both
problems by using box-drawing characters to explicitly represent the
parent-child edges of the tree. This new mode is the default, but the
old behavior remains available with
`--parse-dump-format=yaml-postorder`.
When parsing a pattern, if we encounter something that isn't pattern
syntax, try parsing as an expression instead. We only need one-token
lookahead to distinguish pattern syntax from expression syntax.
Track a precedence group through pattern parsing so that we can allow
different kinds of expressions in a top-level pattern (such as the
operand of `let`) and in a nested pattern (such as a subpattern of a
tuple pattern or within grouping parens). For example, we do not allow
`case if ...`, and for now I've chosen to also not allow logical or
relational operators at the top level of a pattern, so `case 1 + 1` is
OK, but `case 1 == 1` and `case true and false` require parentheses.
This decision should be ratified or revisited by a design proposal.
Very basic check support is also provided, only sufficient to form an
`ExprPattern` instruction and nothing beyond that. For now, all pattern
matching against an `ExprPattern` fails with a TODO error. To support
that, I've switched from calling `BeginSubpattern` in the parent handler
of a pattern and `EndSubpatternAs*` in the pattern handler itself to
calling both functions in parent handlers, with `EndSubpattern`
converting an expression into an expression pattern where needed.
Depends on #6976.
Assisted-by: Gemini via Google Antigravity
Geoff Romer
Richard Smith