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compare_predicate_entailment move comment
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‎compiler/rustc_typeck/src/check/compare_method.rs

+66-63
Original file line numberDiff line numberDiff line change
@@ -70,6 +70,72 @@ pub(crate) fn compare_impl_method<'tcx>(
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}
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}
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/// This function is best explained by example. Consider a trait:
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///
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/// trait Trait<'t, T> {
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/// // `trait_m`
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/// fn method<'a, M>(t: &'t T, m: &'a M) -> Self;
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/// }
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///
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/// And an impl:
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///
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/// impl<'i, 'j, U> Trait<'j, &'i U> for Foo {
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/// // `impl_m`
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/// fn method<'b, N>(t: &'j &'i U, m: &'b N) -> Foo;
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/// }
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///
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/// We wish to decide if those two method types are compatible.
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/// For this we have to show that, assuming the bounds of the impl hold, the
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/// bounds of `trait_m` imply the bounds of `impl_m`.
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///
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/// We start out with `trait_to_impl_substs`, that maps the trait
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/// type parameters to impl type parameters. This is taken from the
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/// impl trait reference:
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///
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/// trait_to_impl_substs = {'t => 'j, T => &'i U, Self => Foo}
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///
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/// We create a mapping `dummy_substs` that maps from the impl type
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/// parameters to fresh types and regions. For type parameters,
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/// this is the identity transform, but we could as well use any
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/// placeholder types. For regions, we convert from bound to free
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/// regions (Note: but only early-bound regions, i.e., those
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/// declared on the impl or used in type parameter bounds).
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///
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/// impl_to_placeholder_substs = {'i => 'i0, U => U0, N => N0 }
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///
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/// Now we can apply `placeholder_substs` to the type of the impl method
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/// to yield a new function type in terms of our fresh, placeholder
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/// types:
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///
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/// <'b> fn(t: &'i0 U0, m: &'b) -> Foo
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///
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/// We now want to extract and substitute the type of the *trait*
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/// method and compare it. To do so, we must create a compound
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/// substitution by combining `trait_to_impl_substs` and
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/// `impl_to_placeholder_substs`, and also adding a mapping for the method
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/// type parameters. We extend the mapping to also include
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/// the method parameters.
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///
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/// trait_to_placeholder_substs = { T => &'i0 U0, Self => Foo, M => N0 }
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///
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/// Applying this to the trait method type yields:
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///
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/// <'a> fn(t: &'i0 U0, m: &'a) -> Foo
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///
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/// This type is also the same but the name of the bound region (`'a`
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/// vs `'b`). However, the normal subtyping rules on fn types handle
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/// this kind of equivalency just fine.
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///
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/// We now use these substitutions to ensure that all declared bounds are
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/// satisfied by the implementation's method.
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///
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/// We do this by creating a parameter environment which contains a
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/// substitution corresponding to `impl_to_placeholder_substs`. We then build
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/// `trait_to_placeholder_substs` and use it to convert the predicates contained
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/// in the `trait_m` generics to the placeholder form.
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///
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/// Finally we register each of these predicates as an obligation and check that
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/// they hold.
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fn compare_predicate_entailment<'tcx>(
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tcx: TyCtxt<'tcx>,
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impl_m: &ty::AssocItem,
@@ -96,69 +162,6 @@ fn compare_predicate_entailment<'tcx>(
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},
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);
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// This code is best explained by example. Consider a trait:
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//
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// trait Trait<'t, T> {
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// fn method<'a, M>(t: &'t T, m: &'a M) -> Self;
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// }
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//
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// And an impl:
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//
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// impl<'i, 'j, U> Trait<'j, &'i U> for Foo {
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// fn method<'b, N>(t: &'j &'i U, m: &'b N) -> Foo;
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// }
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//
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// We wish to decide if those two method types are compatible.
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//
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// We start out with trait_to_impl_substs, that maps the trait
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// type parameters to impl type parameters. This is taken from the
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// impl trait reference:
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//
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// trait_to_impl_substs = {'t => 'j, T => &'i U, Self => Foo}
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//
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// We create a mapping `dummy_substs` that maps from the impl type
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// parameters to fresh types and regions. For type parameters,
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// this is the identity transform, but we could as well use any
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// placeholder types. For regions, we convert from bound to free
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// regions (Note: but only early-bound regions, i.e., those
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// declared on the impl or used in type parameter bounds).
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//
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// impl_to_placeholder_substs = {'i => 'i0, U => U0, N => N0 }
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//
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// Now we can apply placeholder_substs to the type of the impl method
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// to yield a new function type in terms of our fresh, placeholder
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// types:
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//
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// <'b> fn(t: &'i0 U0, m: &'b) -> Foo
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//
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// We now want to extract and substitute the type of the *trait*
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// method and compare it. To do so, we must create a compound
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// substitution by combining trait_to_impl_substs and
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// impl_to_placeholder_substs, and also adding a mapping for the method
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// type parameters. We extend the mapping to also include
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// the method parameters.
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//
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// trait_to_placeholder_substs = { T => &'i0 U0, Self => Foo, M => N0 }
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//
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// Applying this to the trait method type yields:
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//
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// <'a> fn(t: &'i0 U0, m: &'a) -> Foo
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//
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// This type is also the same but the name of the bound region ('a
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// vs 'b). However, the normal subtyping rules on fn types handle
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// this kind of equivalency just fine.
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//
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// We now use these substitutions to ensure that all declared bounds are
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// satisfied by the implementation's method.
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//
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// We do this by creating a parameter environment which contains a
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// substitution corresponding to impl_to_placeholder_substs. We then build
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// trait_to_placeholder_substs and use it to convert the predicates contained
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// in the trait_m.generics to the placeholder form.
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//
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// Finally we register each of these predicates as an obligation in
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// a fresh FulfillmentCtxt, and invoke select_all_or_error.
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// Create mapping from impl to placeholder.
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let impl_to_placeholder_substs = InternalSubsts::identity_for_item(tcx, impl_m.def_id);
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