Rollup merge of #79287 - jonas-schievink:const-trait-impl, r=oli-obk

Allow using generic trait methods in `const fn`

Next step for https://github.com/rust-lang/rust/issues/67792, this now also allows code like the following:

```rust
struct S;

impl const PartialEq for S {
    fn eq(&self, _: &S) -> bool {
        true
    }
}

const fn equals_self<T: PartialEq>(t: &T) -> bool {
    *t == *t
}

pub const EQ: bool = equals_self(&S);
```

This works by threading const-ness of trait predicates through trait selection, in particular through `ParamCandidate`, and exposing it in the resulting `ImplSource`.

Since this change makes two bounds `T: Trait` and `T: ?const Trait` that only differ in their const-ness be treated like different bounds, candidate winnowing has been changed to drop the `?const` candidate in favor of the const candidate, to avoid ambiguities when both a const and a non-const bound is present.

compiler/rustc_infer/src/traits/util.rs

@@ -309,7 +309,7 @@ impl<'tcx, I: Iterator<Item = PredicateObligation<'tcx>>> Iterator for FilterToT
     fn next(&mut self) -> Option<ty::PolyTraitRef<'tcx>> {
         while let Some(obligation) = self.base_iterator.next() {
             if let Some(data) = obligation.predicate.to_opt_poly_trait_ref() {
-                return Some(data);
+                return Some(data.value);
             }
         }
         None

compiler/rustc_middle/src/traits/mod.rs

@@ -16,6 +16,7 @@ use crate::ty::{self, AdtKind, Ty, TyCtxt};
 use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
+use rustc_hir::Constness;
 use rustc_span::symbol::Symbol;
 use rustc_span::{Span, DUMMY_SP};
 use smallvec::SmallVec;
@@ -457,7 +458,7 @@ pub enum ImplSource<'tcx, N> {
     /// for some type parameter. The `Vec<N>` represents the
     /// obligations incurred from normalizing the where-clause (if
     /// any).
-    Param(Vec<N>),
+    Param(Vec<N>, Constness),
 
     /// Virtual calls through an object.
     Object(ImplSourceObjectData<'tcx, N>),
@@ -487,7 +488,7 @@ impl<'tcx, N> ImplSource<'tcx, N> {
     pub fn nested_obligations(self) -> Vec<N> {
         match self {
             ImplSource::UserDefined(i) => i.nested,
-            ImplSource::Param(n) => n,
+            ImplSource::Param(n, _) => n,
             ImplSource::Builtin(i) => i.nested,
             ImplSource::AutoImpl(d) => d.nested,
             ImplSource::Closure(c) => c.nested,
@@ -502,7 +503,7 @@ impl<'tcx, N> ImplSource<'tcx, N> {
     pub fn borrow_nested_obligations(&self) -> &[N] {
         match &self {
             ImplSource::UserDefined(i) => &i.nested[..],
-            ImplSource::Param(n) => &n[..],
+            ImplSource::Param(n, _) => &n[..],
             ImplSource::Builtin(i) => &i.nested[..],
             ImplSource::AutoImpl(d) => &d.nested[..],
             ImplSource::Closure(c) => &c.nested[..],
@@ -524,7 +525,7 @@ impl<'tcx, N> ImplSource<'tcx, N> {
                 substs: i.substs,
                 nested: i.nested.into_iter().map(f).collect(),
             }),
-            ImplSource::Param(n) => ImplSource::Param(n.into_iter().map(f).collect()),
+            ImplSource::Param(n, ct) => ImplSource::Param(n.into_iter().map(f).collect(), ct),
             ImplSource::Builtin(i) => ImplSource::Builtin(ImplSourceBuiltinData {
                 nested: i.nested.into_iter().map(f).collect(),
             }),

compiler/rustc_middle/src/traits/select.rs

@@ -101,7 +101,7 @@ pub enum SelectionCandidate<'tcx> {
         /// `false` if there are no *further* obligations.
         has_nested: bool,
     },
-    ParamCandidate(ty::PolyTraitRef<'tcx>),
+    ParamCandidate(ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>),
     ImplCandidate(DefId),
     AutoImplCandidate(DefId),
 

compiler/rustc_middle/src/traits/structural_impls.rs

@@ -21,7 +21,9 @@ impl<'tcx, N: fmt::Debug> fmt::Debug for traits::ImplSource<'tcx, N> {
 
             super::ImplSource::Object(ref d) => write!(f, "{:?}", d),
 
-            super::ImplSource::Param(ref n) => write!(f, "ImplSourceParamData({:?})", n),
+            super::ImplSource::Param(ref n, ct) => {
+                write!(f, "ImplSourceParamData({:?}, {:?})", n, ct)
+            }
 
             super::ImplSource::Builtin(ref d) => write!(f, "{:?}", d),
 

compiler/rustc_middle/src/ty/context.rs

@@ -42,7 +42,9 @@ use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, LocalDefId, LOCAL_CRATE};
 use rustc_hir::definitions::{DefPathHash, Definitions};
 use rustc_hir::intravisit::Visitor;
 use rustc_hir::lang_items::LangItem;
-use rustc_hir::{HirId, ItemKind, ItemLocalId, ItemLocalMap, ItemLocalSet, Node, TraitCandidate};
+use rustc_hir::{
+    Constness, HirId, ItemKind, ItemLocalId, ItemLocalMap, ItemLocalSet, Node, TraitCandidate,
+};
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_macros::HashStable;
 use rustc_session::config::{BorrowckMode, CrateType, OutputFilenames};
@@ -1635,6 +1637,8 @@ nop_list_lift! {projs; ProjectionKind => ProjectionKind}
 // This is the impl for `&'a InternalSubsts<'a>`.
 nop_list_lift! {substs; GenericArg<'a> => GenericArg<'tcx>}
 
+CloneLiftImpls! { for<'tcx> { Constness, } }
+
 pub mod tls {
     use super::{ptr_eq, GlobalCtxt, TyCtxt};
 

compiler/rustc_middle/src/ty/mod.rs

@@ -1503,9 +1503,11 @@ impl<'tcx> ToPredicate<'tcx> for PolyProjectionPredicate<'tcx> {
 }
 
 impl<'tcx> Predicate<'tcx> {
-    pub fn to_opt_poly_trait_ref(self) -> Option<PolyTraitRef<'tcx>> {
+    pub fn to_opt_poly_trait_ref(self) -> Option<ConstnessAnd<PolyTraitRef<'tcx>>> {
         match self.skip_binders() {
-            PredicateAtom::Trait(t, _) => Some(ty::Binder::bind(t.trait_ref)),
+            PredicateAtom::Trait(t, constness) => {
+                Some(ConstnessAnd { constness, value: ty::Binder::bind(t.trait_ref) })
+            }
             PredicateAtom::Projection(..)
             | PredicateAtom::Subtype(..)
             | PredicateAtom::RegionOutlives(..)
@@ -1947,7 +1949,7 @@ impl<'tcx> ParamEnv<'tcx> {
     }
 }
 
-#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TypeFoldable)]
 pub struct ConstnessAnd<T> {
     pub constness: Constness,
     pub value: T,

compiler/rustc_mir/src/transform/check_consts/validation.rs

@@ -4,6 +4,7 @@ use rustc_errors::{struct_span_err, Applicability, Diagnostic, ErrorReported};
 use rustc_hir::def_id::DefId;
 use rustc_hir::{self as hir, HirId, LangItem};
 use rustc_infer::infer::TyCtxtInferExt;
+use rustc_infer::traits::{ImplSource, Obligation, ObligationCause};
 use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::cast::CastTy;
@@ -11,9 +12,10 @@ use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::{
     self, adjustment::PointerCast, Instance, InstanceDef, Ty, TyCtxt, TypeAndMut,
 };
+use rustc_middle::ty::{Binder, TraitPredicate, TraitRef};
 use rustc_span::{sym, Span, Symbol};
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
-use rustc_trait_selection::traits::{self, TraitEngine};
+use rustc_trait_selection::traits::{self, SelectionContext, TraitEngine};
 
 use std::mem;
 use std::ops::Deref;
@@ -765,9 +767,39 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
                     }
                 };
 
-                // Resolve a trait method call to its concrete implementation, which may be in a
-                // `const` trait impl.
-                if self.tcx.features().const_trait_impl {
+                // Attempting to call a trait method?
+                if let Some(trait_id) = tcx.trait_of_item(callee) {
+                    if !self.tcx.features().const_trait_impl {
+                        self.check_op(ops::FnCallNonConst(callee));
+                        return;
+                    }
+
+                    let trait_ref = TraitRef::from_method(tcx, trait_id, substs);
+                    let obligation = Obligation::new(
+                        ObligationCause::dummy(),
+                        param_env,
+                        Binder::bind(TraitPredicate {
+                            trait_ref: TraitRef::from_method(tcx, trait_id, substs),
+                        }),
+                    );
+
+                    let implsrc = tcx.infer_ctxt().enter(|infcx| {
+                        let mut selcx = SelectionContext::new(&infcx);
+                        selcx.select(&obligation).unwrap()
+                    });
+
+                    // If the method is provided via a where-clause that does not use the `?const`
+                    // opt-out, the call is allowed.
+                    if let Some(ImplSource::Param(_, hir::Constness::Const)) = implsrc {
+                        debug!(
+                            "const_trait_impl: provided {:?} via where-clause in {:?}",
+                            trait_ref, param_env
+                        );
+                        return;
+                    }
+
+                    // Resolve a trait method call to its concrete implementation, which may be in a
+                    // `const` trait impl.
                     let instance = Instance::resolve(tcx, param_env, callee, substs);
                     debug!("Resolving ({:?}) -> {:?}", callee, instance);
                     if let Ok(Some(func)) = instance {

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -350,11 +350,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         // Micro-optimization: filter out predicates relating to different traits.
         let matching_bounds =
-            all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id());
+            all_bounds.filter(|p| p.value.def_id() == stack.obligation.predicate.def_id());
 
         // Keep only those bounds which may apply, and propagate overflow if it occurs.
         for bound in matching_bounds {
-            let wc = self.evaluate_where_clause(stack, bound)?;
+            let wc = self.evaluate_where_clause(stack, bound.value)?;
             if wc.may_apply() {
                 candidates.vec.push(ParamCandidate(bound));
             }

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -8,6 +8,7 @@
 //! https://rustc-dev-guide.rust-lang.org/traits/resolution.html#confirmation
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hir::lang_items::LangItem;
+use rustc_hir::Constness;
 use rustc_index::bit_set::GrowableBitSet;
 use rustc_infer::infer::InferOk;
 use rustc_infer::infer::LateBoundRegionConversionTime::HigherRankedType;
@@ -55,8 +56,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
 
             ParamCandidate(param) => {
-                let obligations = self.confirm_param_candidate(obligation, param);
-                Ok(ImplSource::Param(obligations))
+                let obligations = self.confirm_param_candidate(obligation, param.value);
+                Ok(ImplSource::Param(obligations, param.constness))
             }
 
             ImplCandidate(impl_def_id) => {
@@ -70,7 +71,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
             ProjectionCandidate(idx) => {
                 let obligations = self.confirm_projection_candidate(obligation, idx)?;
-                Ok(ImplSource::Param(obligations))
+                // FIXME(jschievink): constness
+                Ok(ImplSource::Param(obligations, Constness::NotConst))
             }
 
             ObjectCandidate(idx) => {
@@ -106,7 +108,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 // This indicates something like `Trait + Send: Send`. In this case, we know that
                 // this holds because that's what the object type is telling us, and there's really
                 // no additional obligations to prove and no types in particular to unify, etc.
-                Ok(ImplSource::Param(Vec::new()))
+                Ok(ImplSource::Param(Vec::new(), Constness::NotConst))
             }
 
             BuiltinUnsizeCandidate => {
@@ -151,7 +153,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             obligations.extend(self.infcx.commit_if_ok(|_| {
                 self.infcx
                     .at(&obligation.cause, obligation.param_env)
-                    .sup(placeholder_trait_predicate.trait_ref.to_poly_trait_ref(), candidate)
+                    .sup(placeholder_trait_predicate.trait_ref.to_poly_trait_ref(), candidate.value)
                     .map(|InferOk { obligations, .. }| obligations)
                     .map_err(|_| Unimplemented)
             })?);

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -31,6 +31,7 @@ use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_errors::ErrorReported;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
+use rustc_hir::Constness;
 use rustc_middle::dep_graph::{DepKind, DepNodeIndex};
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::ty::fast_reject;
@@ -1335,7 +1336,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             (BuiltinCandidate { has_nested: false } | DiscriminantKindCandidate, _) => true,
             (_, BuiltinCandidate { has_nested: false } | DiscriminantKindCandidate) => false,
 
-            (ParamCandidate(..), ParamCandidate(..)) => false,
+            (ParamCandidate(other), ParamCandidate(victim)) => {
+                if other.value == victim.value && victim.constness == Constness::NotConst {
+                    // Drop otherwise equivalent non-const candidates in favor of const candidates.
+                    true
+                } else {
+                    false
+                }
+            }
 
             // Global bounds from the where clause should be ignored
             // here (see issue #50825). Otherwise, we have a where
@@ -1354,11 +1362,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | TraitAliasCandidate(..)
                 | ObjectCandidate(_)
                 | ProjectionCandidate(_),
-            ) => !is_global(cand),
+            ) => !is_global(&cand.value),
             (ObjectCandidate(_) | ProjectionCandidate(_), ParamCandidate(ref cand)) => {
                 // Prefer these to a global where-clause bound
                 // (see issue #50825).
-                is_global(cand)
+                is_global(&cand.value)
             }
             (
                 ImplCandidate(_)
@@ -1373,7 +1381,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             ) => {
                 // Prefer these to a global where-clause bound
                 // (see issue #50825).
-                is_global(cand) && other.evaluation.must_apply_modulo_regions()
+                is_global(&cand.value) && other.evaluation.must_apply_modulo_regions()
             }
 
             (ProjectionCandidate(i), ProjectionCandidate(j))

compiler/rustc_trait_selection/src/traits/specialize/mod.rs

@@ -498,8 +498,8 @@ fn to_pretty_impl_header(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Option<String>
 
     for (p, _) in predicates {
         if let Some(poly_trait_ref) = p.to_opt_poly_trait_ref() {
-            if Some(poly_trait_ref.def_id()) == sized_trait {
-                types_without_default_bounds.remove(poly_trait_ref.self_ty().skip_binder());
+            if Some(poly_trait_ref.value.def_id()) == sized_trait {
+                types_without_default_bounds.remove(poly_trait_ref.value.self_ty().skip_binder());
                 continue;
             }
         }

compiler/rustc_trait_selection/src/traits/util.rs

@@ -125,7 +125,7 @@ impl<'tcx> TraitAliasExpander<'tcx> {
         let items = predicates.predicates.iter().rev().filter_map(|(pred, span)| {
             pred.subst_supertrait(tcx, &trait_ref)
                 .to_opt_poly_trait_ref()
-                .map(|trait_ref| item.clone_and_push(trait_ref, *span))
+                .map(|trait_ref| item.clone_and_push(trait_ref.value, *span))
         });
         debug!("expand_trait_aliases: items={:?}", items.clone());
 
@@ -182,7 +182,7 @@ impl Iterator for SupertraitDefIds<'tcx> {
                 .predicates
                 .iter()
                 .filter_map(|(pred, _)| pred.to_opt_poly_trait_ref())
-                .map(|trait_ref| trait_ref.def_id())
+                .map(|trait_ref| trait_ref.value.def_id())
                 .filter(|&super_def_id| visited.insert(super_def_id)),
         );
         Some(def_id)

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -294,7 +294,7 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
             let mut cause = cause.clone();
             if let Some(parent_trait_ref) = obligation.predicate.to_opt_poly_trait_ref() {
                 let derived_cause = traits::DerivedObligationCause {
-                    parent_trait_ref,
+                    parent_trait_ref: parent_trait_ref.value,
                     parent_code: Rc::new(obligation.cause.code.clone()),
                 };
                 cause.make_mut().code =

compiler/rustc_typeck/src/astconv/mod.rs

@@ -1364,7 +1364,9 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             || {
                 traits::transitive_bounds(
                     tcx,
-                    predicates.iter().filter_map(|(p, _)| p.to_opt_poly_trait_ref()),
+                    predicates.iter().filter_map(|(p, _)| {
+                        p.to_opt_poly_trait_ref().map(|trait_ref| trait_ref.value)
+                    }),
                 )
             },
             || param_name.to_string(),

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-chain.rs

@@ -0,0 +1,27 @@
+//! Basic test for calling methods on generic type parameters in `const fn`.
+
+// check-pass
+
+#![feature(const_fn)]
+#![feature(const_trait_impl)]
+#![allow(incomplete_features)]
+
+struct S;
+
+impl const PartialEq for S {
+    fn eq(&self, _: &S) -> bool {
+        true
+    }
+}
+
+const fn equals_self<T: PartialEq>(t: &T) -> bool {
+    *t == *t
+}
+
+const fn equals_self_wrapper<T: PartialEq>(t: &T) -> bool {
+    equals_self(t)
+}
+
+pub const EQ: bool = equals_self_wrapper(&S);
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-dup-bound.rs

@@ -0,0 +1,24 @@
+// check-pass
+
+#![feature(const_fn)]
+#![feature(const_trait_impl)]
+#![feature(const_trait_bound_opt_out)]
+#![allow(incomplete_features)]
+
+struct S;
+
+impl const PartialEq for S {
+    fn eq(&self, _: &S) -> bool {
+        true
+    }
+}
+
+// This duplicate bound should not result in ambiguities. It should be equivalent to a single const
+// bound.
+const fn equals_self<T: PartialEq + ?const PartialEq>(t: &T) -> bool {
+    *t == *t
+}
+
+pub const EQ: bool = equals_self(&S);
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-fail.rs

@@ -0,0 +1,11 @@
+#![feature(const_fn)]
+#![feature(const_trait_impl)]
+#![feature(const_trait_bound_opt_out)]
+#![allow(incomplete_features)]
+
+pub const fn equals_self<T: ?const PartialEq>(t: &T) -> bool {
+    *t == *t
+    //~^ ERROR calls in constant functions are limited to constant functions
+}
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-fail.stderr

@@ -0,0 +1,9 @@
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/call-generic-method-fail.rs:7:5
+   |
+LL |     *t == *t
+   |     ^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0015`.

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-nonconst-opt-out.rs

@@ -0,0 +1,24 @@
+// check-pass
+
+#![feature(const_fn)]
+#![feature(const_trait_impl)]
+#![feature(const_trait_bound_opt_out)]
+#![allow(incomplete_features)]
+
+struct S;
+
+impl PartialEq for S {
+    fn eq(&self, _: &S) -> bool {
+        true
+    }
+}
+
+const fn equals_self<T: ?const PartialEq>(t: &T) -> bool {
+    true
+}
+
+pub const EQ: bool = equals_self(&S);
+
+// Calling `equals_self` with a type that only has a non-const impl is fine, because we opted out.
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-nonconst.rs

@@ -0,0 +1,26 @@
+// FIXME(jschievink): this is not rejected correctly (only when the non-const impl is actually used)
+// ignore-test
+
+#![feature(const_fn)]
+#![feature(const_trait_impl)]
+#![allow(incomplete_features)]
+
+struct S;
+
+impl PartialEq for S {
+    fn eq(&self, _: &S) -> bool {
+        true
+    }
+}
+
+const fn equals_self<T: PartialEq>(t: &T) -> bool {
+    true
+}
+
+// Calling `equals_self` with something that has a non-const impl should throw an error, despite
+// it not using the impl.
+
+pub const EQ: bool = equals_self(&S);
+//~^ ERROR
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-pass.rs

@@ -0,0 +1,23 @@
+//! Basic test for calling methods on generic type parameters in `const fn`.
+
+// check-pass
+
+#![feature(const_fn)]
+#![feature(const_trait_impl)]
+#![allow(incomplete_features)]
+
+struct S;
+
+impl const PartialEq for S {
+    fn eq(&self, _: &S) -> bool {
+        true
+    }
+}
+
+const fn equals_self<T: PartialEq>(t: &T) -> bool {
+    *t == *t
+}
+
+pub const EQ: bool = equals_self(&S);
+
+fn main() {}

src/tools/clippy/clippy_lints/src/future_not_send.rs

@@ -68,7 +68,7 @@ impl<'tcx> LateLintPass<'tcx> for FutureNotSend {
             for &(p, _span) in preds {
                 let p = p.subst(cx.tcx, subst);
                 if let Some(trait_ref) = p.to_opt_poly_trait_ref() {
-                    if Some(trait_ref.def_id()) == cx.tcx.lang_items().future_trait() {
+                    if Some(trait_ref.value.def_id()) == cx.tcx.lang_items().future_trait() {
                         is_future = true;
                         break;
                     }