const fn: allow use of trait impls from bounds

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 {

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() {}