struct SubstFolder<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
tcx: TyCtxt<'a, 'gcx, 'tcx>,
substs: &'a [Kind<'tcx>],
span: Option<Span>,
root_ty: Option<Ty<'tcx>>,
ty_stack_depth: usize,
region_binders_passed: u32,
}
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🔬 This is a nightly-only experimental API. (rustc_private
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🔬 This is a nightly-only experimental API. (rustc_private
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🔬 This is a nightly-only experimental API. (rustc_private
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🔬 This is a nightly-only experimental API. (rustc_private
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🔬 This is a nightly-only experimental API. (rustc_private
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🔬 This is a nightly-only experimental API. (rustc_private
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🔬 This is a nightly-only experimental API. (rustc_private
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It is sometimes necessary to adjust the debruijn indices during substitution. This occurs
when we are substituting a type with escaping regions into a context where we have passed
through region binders. That's quite a mouthful. Let's see an example:
type Func<A> = fn(A);
type MetaFunc = for<'a> fn(Func<&'a int>)
The type MetaFunc
, when fully expanded, will be
for<'a> fn(fn(&'a int))
^~ ^~ ^~~
| | |
| | DebruijnIndex of 2
Binders
Here the 'a
lifetime is bound in the outer function, but appears as an argument of the
inner one. Therefore, that appearance will have a DebruijnIndex of 2, because we must skip
over the inner binder (remember that we count Debruijn indices from 1). However, in the
definition of MetaFunc
, the binder is not visible, so the type &'a int
will have a
debruijn index of 1. It's only during the substitution that we can see we must increase the
depth by 1 to account for the binder that we passed through.
As a second example, consider this twist:
type FuncTuple<A> = (A,fn(A));
type MetaFuncTuple = for<'a> fn(FuncTuple<&'a int>)
Here the final type will be:
for<'a> fn((&'a int, fn(&'a int)))
^~~ ^~~
| |
DebruijnIndex of 1 |
DebruijnIndex of 2
As indicated in the diagram, here the same type &'a int
is substituted once, but in the
first case we do not increase the Debruijn index and in the second case we do. The reason
is that only in the second case have we passed through a fn binder.
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🔬 This is a nightly-only experimental API. (rustc_private
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🔬 This is a nightly-only experimental API. (rustc_private
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🔬 This is a nightly-only experimental API. (rustc_private
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🔬 This is a nightly-only experimental API. (rustc_private
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🔬 This is a nightly-only experimental API. (try_from
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The type returned in the event of a conversion error.
🔬 This is a nightly-only experimental API. (try_from
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Immutably borrows from an owned value. Read more
type Error = <U as TryFrom<T>>::Error
🔬 This is a nightly-only experimental API. (try_from
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The type returned in the event of a conversion error.
🔬 This is a nightly-only experimental API. (try_from
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Mutably borrows from an owned value. Read more
🔬 This is a nightly-only experimental API. (get_type_id
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this method will likely be replaced by an associated static
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Create an error for a missing method specialization. Defaults to panicking with type, trait & method names. S
is the encoder/decoder state type, T
is the type being encoded/decoded, and the arguments are the names of the trait and method that should've been overridden. Read more