Committing to a conversion

[CAD97]

In order to aid the creation of correct ASTs, it's sometimes useful to know when things go wrong when you don't expect. I propose adding a #[pest_ast(commit)] or similar, after which conversions in the sequence which would otherwise cause a Err(NoMatch) to propagate, instead cause a panic detailing where the error is.

[CAD97]

This is probably a bit more involved than I'd wish, as it adds state to the conversion where it didn't exist before. However, here's a rough outline of what needs to change to support this:

• The attribute grammar needs to be adjusted to add the attribute
• The conversion of sequential fields needs to be adjusted for the commit toggle.

Best guess: replace the FromPest::from_pest(inner)? with FromPest::from_pest(inner).map_err(|e| if commit { panic!(..) } else { e })?, and add code to manage the commit variable. We can rely on optimizing it out if it's never set.

ast/derive/src/from_pest/field.rs

Lines 58 to 70 in db9e2b4

	// TODO: Should this be panicking instead?
	// panic!(
	// concat!(
	// "in ",
	// stringify!(#name),
	// ".",
	// stringify!(#member),
	// ", expected `",
	// stringify!(#rule),
	// "` but found `{:?}`"
	// ),
	// pair.as_rule(),
	// )