Rollup merge of #135639 - lqd:trivial-builtin-impls, r=lcnr
new solver: prefer trivial builtin impls As discussed [on zulip](https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/needs_help.3A.20trivial.20builtin.20impls), this PR: - adds a new `BuiltinImplSource::Trivial` source, and marks the `Sized` builtin impls as trivial - prefers these trivial builtin impls in `merge_trait_candidates` The comments can likely be wordsmithed a bit better, and I ~stole~ was inspired by the old solver ones. Let me know how you want them improved. When enabling the new solver for tests, 3 UI tests now pass: - `regions/issue-26448-1.rs` and its sibling `regions/issue-26448-2.rs` were rejected by the new solver but accepted by the old one - and `issues/issue-42796.rs` where the old solver emitted some overflow errors in addition to the expected error (For some reason one of these tests is run-pass, but I can take care of that another day) r? lcnr
This commit is contained in:
commit
862a17cd17
8 changed files with 41 additions and 6 deletions
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@ -4259,6 +4259,7 @@ dependencies = [
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"rustc_serialize",
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"rustc_type_ir",
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"rustc_type_ir_macros",
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"smallvec",
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"tracing",
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]
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@ -13,6 +13,7 @@ rustc_macros = { path = "../rustc_macros", optional = true }
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rustc_serialize = { path = "../rustc_serialize", optional = true }
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rustc_type_ir = { path = "../rustc_type_ir", default-features = false }
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rustc_type_ir_macros = { path = "../rustc_type_ir_macros" }
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smallvec = "1.8.1"
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tracing = "0.1"
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# tidy-alphabetical-end
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@ -8,6 +8,7 @@ use rustc_type_ir::lang_items::TraitSolverLangItem;
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use rustc_type_ir::solve::CanonicalResponse;
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use rustc_type_ir::visit::TypeVisitableExt as _;
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use rustc_type_ir::{self as ty, Interner, TraitPredicate, TypingMode, Upcast as _, elaborate};
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use smallvec::SmallVec;
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use tracing::{instrument, trace};
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use crate::delegate::SolverDelegate;
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@ -225,7 +226,7 @@ where
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}
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ecx.probe_and_evaluate_goal_for_constituent_tys(
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CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
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CandidateSource::BuiltinImpl(BuiltinImplSource::Trivial),
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goal,
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structural_traits::instantiate_constituent_tys_for_sized_trait,
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)
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@ -1194,7 +1195,30 @@ where
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};
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}
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// FIXME: prefer trivial builtin impls
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// We prefer trivial builtin candidates, i.e. builtin impls without any
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// nested requirements, over all others. This is a fix for #53123 and
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// prevents where-bounds from accidentally extending the lifetime of a
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// variable.
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if candidates
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.iter()
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.any(|c| matches!(c.source, CandidateSource::BuiltinImpl(BuiltinImplSource::Trivial)))
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{
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let trivial_builtin_impls: SmallVec<[_; 1]> = candidates
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.iter()
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.filter(|c| {
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matches!(c.source, CandidateSource::BuiltinImpl(BuiltinImplSource::Trivial))
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})
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.map(|c| c.result)
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.collect();
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// There should only ever be a single trivial builtin candidate
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// as they would otherwise overlap.
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assert_eq!(trivial_builtin_impls.len(), 1);
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return if let Some(response) = self.try_merge_responses(&trivial_builtin_impls) {
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Ok((response, Some(TraitGoalProvenVia::Misc)))
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} else {
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Ok((self.bail_with_ambiguity(&trivial_builtin_impls), None))
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};
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}
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// If there are non-global where-bounds, prefer where-bounds
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// (including global ones) over everything else.
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@ -991,7 +991,7 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
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Err(ErrorGuaranteed { .. }) => true,
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}
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}
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ImplSource::Builtin(BuiltinImplSource::Misc, _) => {
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ImplSource::Builtin(BuiltinImplSource::Misc | BuiltinImplSource::Trivial, _) => {
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// While a builtin impl may be known to exist, the associated type may not yet
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// be known. Any type with multiple potential associated types is therefore
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// not eligible.
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@ -1296,7 +1296,7 @@ fn confirm_select_candidate<'cx, 'tcx>(
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) -> Progress<'tcx> {
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match impl_source {
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ImplSource::UserDefined(data) => confirm_impl_candidate(selcx, obligation, data),
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ImplSource::Builtin(BuiltinImplSource::Misc, data) => {
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ImplSource::Builtin(BuiltinImplSource::Misc | BuiltinImplSource::Trivial, data) => {
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let tcx = selcx.tcx();
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let trait_def_id = obligation.predicate.trait_def_id(tcx);
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if tcx.is_lang_item(trait_def_id, LangItem::Coroutine) {
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@ -248,7 +248,7 @@ fn resolve_associated_item<'tcx>(
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})
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}
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}
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traits::ImplSource::Builtin(BuiltinImplSource::Misc, _) => {
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traits::ImplSource::Builtin(BuiltinImplSource::Misc | BuiltinImplSource::Trivial, _) => {
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if tcx.is_lang_item(trait_ref.def_id, LangItem::Clone) {
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// FIXME(eddyb) use lang items for methods instead of names.
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let name = tcx.item_name(trait_item_id);
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@ -169,6 +169,9 @@ pub enum CandidateSource<I: Interner> {
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#[derive(Clone, Copy, Hash, PartialEq, Eq, Debug)]
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#[cfg_attr(feature = "nightly", derive(HashStable_NoContext, TyEncodable, TyDecodable))]
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pub enum BuiltinImplSource {
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/// A built-in impl that is considered trivial, without any nested requirements. They
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/// are preferred over where-clauses, and we want to track them explicitly.
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Trivial,
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/// Some built-in impl we don't need to differentiate. This should be used
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/// unless more specific information is necessary.
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Misc,
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@ -1,4 +1,7 @@
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//@ run-pass
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//@ revisions: current next
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//@ [next] compile-flags: -Znext-solver
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//@ ignore-compare-mode-next-solver (explicit revisions)
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//@ check-pass
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pub trait Foo<T> {
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fn foo(self) -> T;
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@ -1,3 +1,6 @@
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//@ revisions: current next
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//@ [next] compile-flags: -Znext-solver
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//@ ignore-compare-mode-next-solver (explicit revisions)
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//@ check-pass
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pub struct Bar<T> {
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