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yeast: Implement IntoIterator for Id as a singleton
There's an awkward divide in yeast between returning a list of nodes or optional node (both of which are iterable), and returning a single node (which is not). In practice, we would like all of these cases to be handled transparently: if a single node is returned, it behaves as if it were a singleton list containing that node. This gives us a uniform interface during translation -- no matter what is returned, it will be an iterable of nodes. To facilitate this, we make the slightly unorthodox choice of implementing IntoIterator for Id, with the behaviour detailed above.
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Lines changed: 18 additions & 9 deletions

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‎shared/yeast/src/lib.rs‎

Lines changed: 18 additions & 9 deletions
Original file line numberDiff line numberDiff line change
@@ -26,6 +26,12 @@ use query::QueryNode;
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/// without colliding with the impls for plain integers.
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///
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/// Use `id.0` (or `id.into()`) to obtain the raw arena index.
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///
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/// Implements [`IntoIterator`] as a singleton (`iter::once(self)`)
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/// so that a bare `Id` can be used interchangeably with `Option<Id>`
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/// / `Vec<Id>` in places that expect an iterable of ids (e.g.
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/// [`crate::build::BuildCtx::translate`] and the field-splice
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/// interpolation via [`IntoFieldIds`]).
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#[repr(transparent)]
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#[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Debug, Hash, Serialize)]
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pub struct Id(pub usize);
@@ -42,16 +48,25 @@ impl From<Id> for usize {
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}
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}
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impl IntoIterator for Id {
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type Item = Id;
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type IntoIter = std::iter::Once<Id>;
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fn into_iter(self) -> Self::IntoIter {
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std::iter::once(self)
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}
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}
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/// Field and Kind ids are provided by tree-sitter
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type FieldId = u16;
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type KindId = u16;
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/// Trait for values that can be appended to a field's id list inside a
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/// `tree!`/`trees!`/`rule!` template (in `{expr}` placeholders).
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///
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/// `Id` pushes a single id; the blanket impl for
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/// `IntoIterator<Item: Into<Id>>` handles `Vec<Id>`, `Option<Id>`,
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/// arbitrary iterators yielding `Id`, etc.
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/// The blanket impl for `IntoIterator<Item: Into<Id>>` handles all
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/// current shapes: `Vec<Id>`, `Option<Id>`, arbitrary iterators
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/// yielding `Id`, and a bare `Id` itself (which is `IntoIterator`
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/// via a singleton).
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///
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/// This lets `{expr}` interpolate any of these shapes without a
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/// dedicated splice syntax — the macro emits the same trait-dispatched
@@ -60,12 +75,6 @@ pub trait IntoFieldIds {
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fn extend_into(self, out: &mut Vec<Id>);
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}
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impl IntoFieldIds for Id {
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fn extend_into(self, out: &mut Vec<Id>) {
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out.push(self);
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}
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}
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impl<I, T> IntoFieldIds for I
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where
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I: IntoIterator<Item = T>,

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