Implement GATs, step 1 #118
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chalk-parse/src/parser.lalrpop
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| parameter_kinds: p | ||
| "type" <name:Id> <p:Angle<ParameterKind>> <b:(":" <Id> <Angle<Parameter>>)?> | ||
| <w:QuantifiedWhereClauses> ";" => | ||
| { |
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Ok so currently we do not handle at all the + separator which expresses multiple bounds as in where T: Clone + Sized. This is not a problem for e.g. where clauses on traits because you can still write where T: Clone, T: Sized but for associated types since there is no other way to express these multiple bounds, we need to be able to parse them with the + acting as a separator.
chalk-parse/src/ast.rs
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| @@ -54,6 +54,8 @@ pub struct TraitFlags { | |||
| pub struct AssocTyDefn { | |||
| pub name: Identifier, | |||
| pub parameter_kinds: Vec<ParameterKind>, | |||
| pub bound: Option<TraitRef>, | |||
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We would need multiple bounds there (see below), and using TraitRef is too rigid as we cannot parse things like type Foo<T>: Iterator<Item = T>.
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I'm thinking this would maybe need some slight refactoring for how we handle where clauses in the grammar. Something which may work would be to split existing where clauses like TraitRef into two parts: a left part (which would be T in T: Foo<K>) and a right part (which would be Foo<K>), separated by the : symbol. This would prevent code duplication I think because in the case of associated types, you would only ask for the right part and then build the whole TraitRef by hand as you did. It would be especially useful when we add lifetime requirements on types as well.
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Yep, what do you think of something like TraitBound and ProjectionEqBound that I added to ast? (Currently they're only being used for GATs.)
These can contain methods to convert them to TraitRef, WhereClause::ProjectionEq, etc. if the proper information is obtained.
chalk-parse/src/ast.rs
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| @@ -177,6 +177,11 @@ impl PolarizedTraitRef { | |||
| } | |||
| } | |||
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| pub struct TraitBound { | |||
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I think it would be useful to add a comment here explaining what this represents and how it is used.
chalk-parse/src/ast.rs
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| @@ -68,6 +68,23 @@ pub enum Parameter { | |||
| Lifetime(Lifetime), | |||
| } | |||
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| pub struct TraitBound { | |||
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Maybe this should not be called TraitBound? I would think something like TypeParameterBound
This adds parsing rules for bounds and where clauses on associated type definitions for #116.
It also removes where clauses on associated type values.
The way I handled bounds directly in the parser felt a bit messy, open to suggestions. Not sure if using the associated type name directly in the
TraitRefis going to work downstream.