In order to desugar `dbg` in a pipeline we need to allow a bare `dbg`
node in desugaring and only report it as an error if the bare node
survives to the next step of canonicalization. This means we move the
error code out of `desugar_expr` and into `canonicalize_expr`. This is
much simpler to do now that these functions use the same `env` struct,
since previously we would have had to pass down extra args to
`canonicalize_expr`. Sharing the `env` struct means that we also don't
have to worry about calculating `line_info` more than once.
Instead of parsing dbg with an expression block, parse the dbg keyword
with no additional arguments. This way the parser treats dbg just like a
variable in function application. We desugar by pattern matching on
`Apply(Dbg, args, called_via)` nodes. This changes the output of syntax
tests since the initial AST is different, but does not change the output
of can or mono.
Add two new errors for dbg in expression position with either no args or
too many args. This is similar to the error behavior of `crash`.
Continue to parse dbg statements with an expression block, as before.
Add dbg parsing logic everywhere we parse normal expressions. Add
special case to statement parsing to handle a series of statements
ending in a `dbg` in expression position.
Rename existing `dbg_help` function to `dbg_stmt_help`, similarly rename
syntax snapshot test files to specify which ones are for dbg statements.
Make `Expr::Dbg` node specific to dbg expressions, add `Expr::DbgStmt`
for dbg statements. Debug expressions don't have a continuation branch.
Support formatting both kinds of `Dbg` nodes.
