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use super::*;
use crate::fold::shift::Shift;
/// Substitution used during folding
#[derive(FallibleTypeFolder)]
pub struct Subst<'s, I: Interner> {
/// Values to substitute. A reference to a free variable with
/// index `i` will be mapped to `parameters[i]` -- if `i >
/// parameters.len()`, then we will leave the variable untouched.
parameters: &'s [GenericArg<I>],
interner: I,
}
impl<I: Interner> Subst<'_, I> {
/// Applies the substitution by folding
pub fn apply<T: TypeFoldable<I>>(interner: I, parameters: &[GenericArg<I>], value: T) -> T {
value
.try_fold_with(
&mut Subst {
parameters,
interner,
},
DebruijnIndex::INNERMOST,
)
.unwrap()
}
}
impl<I: Interner> TypeFolder<I> for Subst<'_, I> {
fn as_dyn(&mut self) -> &mut dyn TypeFolder<I> {
self
}
/// We are eliminating one binder, but binders outside of that get preserved.
///
/// So e.g. consider this:
///
/// ```notrust
/// for<A, B> { for<C> { [A, C] } }
/// // ^ the binder we are substituing with `[u32]`
/// ```
///
/// Here, `A` would be `^1.0` and `C` would be `^0.0`. We will replace `^0.0` with the
/// 0th index from the list (`u32`). We will convert `^1.0` (A) to `^0.0` -- i.e., shift
/// it **out** of one level of binder (the `for<C>` binder we are eliminating).
///
/// This gives us as a result:
///
/// ```notrust
/// for<A, B> { [A, u32] }
/// ^ represented as `^0.0`
/// ```
fn fold_free_var_ty(&mut self, bound_var: BoundVar, outer_binder: DebruijnIndex) -> Ty<I> {
if let Some(index) = bound_var.index_if_innermost() {
match self.parameters[index].data(TypeFolder::interner(self)) {
GenericArgData::Ty(t) => t
.clone()
.shifted_in_from(TypeFolder::interner(self), outer_binder),
_ => panic!("mismatched kinds in substitution"),
}
} else {
bound_var
.shifted_out()
.expect("cannot fail because this is not the innermost")
.shifted_in_from(outer_binder)
.to_ty(TypeFolder::interner(self))
}
}
/// see `fold_free_var_ty`
fn fold_free_var_lifetime(
&mut self,
bound_var: BoundVar,
outer_binder: DebruijnIndex,
) -> Lifetime<I> {
if let Some(index) = bound_var.index_if_innermost() {
match self.parameters[index].data(TypeFolder::interner(self)) {
GenericArgData::Lifetime(l) => l
.clone()
.shifted_in_from(TypeFolder::interner(self), outer_binder),
_ => panic!("mismatched kinds in substitution"),
}
} else {
bound_var
.shifted_out()
.unwrap()
.shifted_in_from(outer_binder)
.to_lifetime(TypeFolder::interner(self))
}
}
/// see `fold_free_var_ty`
fn fold_free_var_const(
&mut self,
ty: Ty<I>,
bound_var: BoundVar,
outer_binder: DebruijnIndex,
) -> Const<I> {
if let Some(index) = bound_var.index_if_innermost() {
match self.parameters[index].data(TypeFolder::interner(self)) {
GenericArgData::Const(c) => c
.clone()
.shifted_in_from(TypeFolder::interner(self), outer_binder),
_ => panic!("mismatched kinds in substitution"),
}
} else {
bound_var
.shifted_out()
.unwrap()
.shifted_in_from(outer_binder)
.to_const(TypeFolder::interner(self), ty)
}
}
fn interner(&self) -> I {
self.interner
}
}
|