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update select docs

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This commit is contained in:
Bastian Kauschke 2020-04-02 14:05:04 +02:00
parent 5943351d0e
commit aab144f6d1
1 changed files with 18 additions and 14 deletions
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32
src/librustc_middle/traits/select.rs
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@ -34,7 +34,7 @@ impl<'tcx> SelectionCache<'tcx> {
/// clauses, and so forth that might resolve an obligation. Sometimes
/// we'll be able to say definitively that (e.g.) an impl does not
/// apply to the obligation: perhaps it is defined for `usize` but the
/// obligation is for `int`. In that case, we drop the impl out of the
/// obligation is for `i32`. In that case, we drop the impl out of the
/// list. But the other cases are considered *candidates*.
///
/// For selection to succeed, there must be exactly one matching
@ -54,12 +54,14 @@ impl<'tcx> SelectionCache<'tcx> {
/// will always be satisfied) picking the blanket impl will be wrong
/// for at least *some* substitutions. To make this concrete, if we have
///
/// trait AsDebug { type Out : fmt::Debug; fn debug(self) -> Self::Out; }
/// impl<T: fmt::Debug> AsDebug for T {
/// type Out = T;
/// fn debug(self) -> fmt::Debug { self }
/// }
/// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }
/// ```rust, ignore
/// trait AsDebug { type Out: fmt::Debug; fn debug(self) -> Self::Out; }
/// impl<T: fmt::Debug> AsDebug for T {
/// type Out = T;
/// fn debug(self) -> fmt::Debug { self }
/// }
/// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }
/// ```
///
/// we can't just use the impl to resolve the `<T as AsDebug>` obligation
/// -- a type from another crate (that doesn't implement `fmt::Debug`) could
@ -79,14 +81,16 @@ impl<'tcx> SelectionCache<'tcx> {
/// inference variables. The can lead to inference making "leaps of logic",
/// for example in this situation:
///
/// pub trait Foo<T> { fn foo(&self) -> T; }
/// impl<T> Foo<()> for T { fn foo(&self) { } }
/// impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }
/// ```rust, ignore
/// pub trait Foo<T> { fn foo(&self) -> T; }
/// impl<T> Foo<()> for T { fn foo(&self) { } }
/// impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }
///
/// pub fn foo<T>(t: T) where T: Foo<bool> {
/// println!("{:?}", <T as Foo<_>>::foo(&t));
/// }
/// fn main() { foo(false); }
/// pub fn foo<T>(t: T) where T: Foo<bool> {
/// println!("{:?}", <T as Foo<_>>::foo(&t));
/// }
/// fn main() { foo(false); }
/// ```
///
/// Here the obligation `<T as Foo<$0>>` can be matched by both the blanket
/// impl and the where-clause. We select the where-clause and unify `$0=bool`,