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update coherence docs, fix opaque type + generator ice

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This commit is contained in:
lcnr 2022-04-12 16:02:57 +02:00
parent 1491e5cc14
commit 58781edc54
7 changed files with 120 additions and 22 deletions
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30
compiler/rustc_trait_selection/src/traits/select/mod.rs
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@ -103,22 +103,31 @@ pub struct SelectionContext<'cx, 'tcx> {
/// require themselves.
freshener: TypeFreshener<'cx, 'tcx>,
/// If `true`, indicates that the evaluation should be conservative
/// and consider the possibility of types outside this crate.
/// During coherence we have to assume that other crates may add
/// additional impls which we currently don't know about.
///
/// To deal with this evaluation should be conservative
/// and consider the possibility of impls from outside this crate.
/// This comes up primarily when resolving ambiguity. Imagine
/// there is some trait reference `$0: Bar` where `$0` is an
/// inference variable. If `intercrate` is true, then we can never
/// say for sure that this reference is not implemented, even if
/// there are *no impls at all for `Bar`*, because `$0` could be
/// bound to some type that in a downstream crate that implements
/// `Bar`. This is the suitable mode for coherence. Elsewhere,
/// though, we set this to false, because we are only interested
/// in types that the user could actually have written --- in
/// other words, we consider `$0: Bar` to be unimplemented if
/// `Bar`.
///
/// Outside of coherence we set this to false because we are only
/// interested in types that the user could actually have written.
/// In other words, we consider `$0: Bar` to be unimplemented if
/// there is no type that the user could *actually name* that
/// would satisfy it. This avoids crippling inference, basically.
intercrate: bool,
/// If `intercrate` is set, we remember predicates which were
/// considered ambiguous because of impls potentially added in other crates.
/// This is used in coherence to give improved diagnostics.
/// We don't do his until we detect a coherence error because it can
/// lead to false overflow results (#47139) and because always
/// computing it may negatively impact performance.
intercrate_ambiguity_causes: Option<Vec<IntercrateAmbiguityCause>>,
/// The mode that trait queries run in, which informs our error handling
@ -240,11 +249,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
}
}
/// Enables tracking of intercrate ambiguity causes. These are
/// used in coherence to give improved diagnostics. We don't do
/// this until we detect a coherence error because it can lead to
/// false overflow results (#47139) and because it costs
/// computation time.
/// Enables tracking of intercrate ambiguity causes. See
/// the documentation of [`Self::intercrate_ambiguity_causes`] for more.
pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
assert!(self.intercrate);
assert!(self.intercrate_ambiguity_causes.is_none());