Auto merge of rust-lang#119934 - compiler-errors:could-impl, r=jackh726

Make `InferCtxtExt::could_impl_trait` more precise, less ICEy The implementation for `InferCtxtExt::could_impl_trait` was very wrong. Along with being pretty poorly named, way too specific to ADTs, it was also doing impl substitution wrong -- this caused an ICE (rust-lang#119915). This PR generalizes that code, gives it a clearer name, makes it stop using the new trait solver (lol), and fixes some fallout bad suggestions that are made worse with the code fix. Fixes rust-lang#119915

Author: bors

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -1178,9 +1178,11 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
  } else {
  vec![(move_span.shrink_to_hi(), ".clone()".to_string())]
  };
- if let Some(errors) =
- self.infcx.could_impl_trait(clone_trait, ty, self.param_env)
- && !has_sugg
+ if let Some(errors) = self.infcx.type_implements_trait_shallow(
+ clone_trait,
+ ty,
+ self.param_env,
+ ) && !has_sugg
  {
  let msg = match &errors[..] {
  [] => "you can `clone` the value and consume it, but this \

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -1217,19 +1217,22 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
  {
  match self
  .infcx
- .could_impl_trait(clone_trait, ty.peel_refs(), self.param_env)
+ .type_implements_trait_shallow(
+ clone_trait,
+ ty.peel_refs(),
+ self.param_env,
+ )
  .as_deref()
  {
  Some([]) => {
- // The type implements Clone.
- err.span_help(
- expr.span,
- format!(
- "you can `clone` the `{}` value and consume it, but this \
- might not be your desired behavior",
- ty.peel_refs(),
- ),
- );
+ // FIXME: This error message isn't useful, since we're just
+ // vaguely suggesting to clone a value that already
+ // implements `Clone`.
+ //
+ // A correct suggestion here would take into account the fact
+ // that inference may be affected by missing types on bindings,
+ // etc., to improve "tests/ui/borrowck/issue-91206.stderr", for
+ // example.
  }
  None => {
  if let hir::ExprKind::MethodCall(segment, _rcvr, [], span) =

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -1623,7 +1623,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  );
  } else {
  if let Some(errors) =
- self.could_impl_trait(clone_trait_did, expected_ty, self.param_env)
+ self.type_implements_trait_shallow(clone_trait_did, expected_ty, self.param_env)
  {
  match &errors[..] {
  [] => {}

compiler/rustc_trait_selection/src/infer.rs

@@ -1,13 +1,14 @@
-use crate::solve::FulfillmentCtxt;
 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
-use crate::traits::{self, DefiningAnchor, ObligationCtxt};
+use crate::traits::{self, DefiningAnchor, ObligationCtxt, SelectionContext};
 
+use crate::traits::TraitEngineExt as _;
 use rustc_hir::def_id::DefId;
 use rustc_hir::lang_items::LangItem;
-use rustc_infer::traits::{TraitEngine, TraitEngineExt};
+use rustc_infer::traits::{Obligation, TraitEngine, TraitEngineExt as _};
 use rustc_middle::arena::ArenaAllocatable;
 use rustc_middle::infer::canonical::{Canonical, CanonicalQueryResponse, QueryResponse};
 use rustc_middle::traits::query::NoSolution;
+use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeVisitableExt};
 use rustc_middle::ty::{GenericArg, ToPredicate};
 use rustc_span::DUMMY_SP;
@@ -21,24 +22,36 @@ pub trait InferCtxtExt<'tcx> {
 
  fn type_is_sized_modulo_regions(&self, param_env: ty::ParamEnv<'tcx>, ty: Ty<'tcx>) -> bool;
 
- /// Check whether a `ty` implements given trait(trait_def_id).
+ /// Check whether a `ty` implements given trait(trait_def_id) without side-effects.
+ ///
  /// The inputs are:
  ///
  /// - the def-id of the trait
  /// - the type parameters of the trait, including the self-type
  /// - the parameter environment
  ///
  /// Invokes `evaluate_obligation`, so in the event that evaluating
- /// `Ty: Trait` causes overflow, EvaluatedToErrStackDependent (or EvaluatedToAmbigStackDependent)
- /// will be returned.
+ /// `Ty: Trait` causes overflow, EvaluatedToErrStackDependent
+ /// (or EvaluatedToAmbigStackDependent) will be returned.
  fn type_implements_trait(
  &self,
  trait_def_id: DefId,
  params: impl IntoIterator<Item: Into<GenericArg<'tcx>>>,
  param_env: ty::ParamEnv<'tcx>,
  ) -> traits::EvaluationResult;
 
- fn could_impl_trait(
+ /// Returns `Some` if a type implements a trait shallowly, without side-effects,
+ /// along with any errors that would have been reported upon further obligation
+ /// processing.
+ ///
+ /// - If this returns `Some([])`, then the trait holds modulo regions.
+ /// - If this returns `Some([errors..])`, then the trait has an impl for
+ /// the self type, but some nested obligations do not hold.
+ /// - If this returns `None`, no implementation that applies could be found.
+ ///
+ /// FIXME(-Znext-solver): Due to the recursive nature of the new solver,
+ /// this will probably only ever return `Some([])` or `None`.
+ fn type_implements_trait_shallow(
  &self,
  trait_def_id: DefId,
  ty: Ty<'tcx>,
@@ -86,64 +99,26 @@ impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
  self.evaluate_obligation(&obligation).unwrap_or(traits::EvaluationResult::EvaluatedToErr)
  }
 
- fn could_impl_trait(
+ fn type_implements_trait_shallow(
  &self,
  trait_def_id: DefId,
  ty: Ty<'tcx>,
  param_env: ty::ParamEnv<'tcx>,
  ) -> Option<Vec<traits::FulfillmentError<'tcx>>> {
  self.probe(|_snapshot| {
- if let ty::Adt(def, args) = ty.kind()
- && let Some((impl_def_id, _)) = self
- .tcx
- .all_impls(trait_def_id)
- .filter_map(|impl_def_id| {
- self.tcx.impl_trait_ref(impl_def_id).map(|r| (impl_def_id, r))
- })
- .map(|(impl_def_id, imp)| (impl_def_id, imp.skip_binder()))
- .find(|(_, imp)| match imp.self_ty().peel_refs().kind() {
- ty::Adt(i_def, _) if i_def.did() == def.did() => true,
- _ => false,
- })
- {
- let mut fulfill_cx = FulfillmentCtxt::new(self);
- // We get all obligations from the impl to talk about specific
- // trait bounds.
- let obligations = self
- .tcx
- .predicates_of(impl_def_id)
- .instantiate(self.tcx, args)
- .into_iter()
- .map(|(clause, span)| {
- traits::Obligation::new(
- self.tcx,
- traits::ObligationCause::dummy_with_span(span),
- param_env,
- clause,
- )
- })
- .collect::<Vec<_>>();
- fulfill_cx.register_predicate_obligations(self, obligations);
- let trait_ref = ty::TraitRef::new(self.tcx, trait_def_id, [ty]);
- let obligation = traits::Obligation::new(
- self.tcx,
- traits::ObligationCause::dummy(),
- param_env,
- trait_ref,
- );
- fulfill_cx.register_predicate_obligation(self, obligation);
- let mut errors = fulfill_cx.select_all_or_error(self);
- // We remove the last predicate failure, which corresponds to
- // the top-level obligation, because most of the type we only
- // care about the other ones, *except* when it is the only one.
- // This seems to only be relevant for arbitrary self-types.
- // Look at `tests/ui/moves/move-fn-self-receiver.rs`.
- if errors.len() > 1 {
- errors.truncate(errors.len() - 1);
+ let mut selcx = SelectionContext::new(self);
+ match selcx.select(&Obligation::new(
+ self.tcx,
+ ObligationCause::dummy(),
+ param_env,
+ ty::TraitRef::new(self.tcx, trait_def_id, [ty]),
+ )) {
+ Ok(Some(selection)) => {
+ let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(self);
+ fulfill_cx.register_predicate_obligations(self, selection.nested_obligations());
+ Some(fulfill_cx.select_all_or_error(self))
  }
- Some(errors)
- } else {
- None
+ Ok(None) | Err(_) => None,
  }
  })
  }

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -36,6 +36,11 @@ pub struct FulfillmentCtxt<'tcx> {
 
 impl<'tcx> FulfillmentCtxt<'tcx> {
  pub fn new(infcx: &InferCtxt<'tcx>) -> FulfillmentCtxt<'tcx> {
+ assert!(
+ infcx.next_trait_solver(),
+ "new trait solver fulfillment context created when \
+ infcx is set up for old trait solver"
+ );
  FulfillmentCtxt { obligations: Vec::new(), usable_in_snapshot: infcx.num_open_snapshots() }
  }
 }

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -80,6 +80,11 @@ static_assert_size!(PendingPredicateObligation<'_>, 72);
 impl<'tcx> FulfillmentContext<'tcx> {
  /// Creates a new fulfillment context.
  pub(super) fn new(infcx: &InferCtxt<'tcx>) -> FulfillmentContext<'tcx> {
+ assert!(
+ !infcx.next_trait_solver(),
+ "old trait solver fulfillment context created when \
+ infcx is set up for new trait solver"
+ );
  FulfillmentContext {
  predicates: ObligationForest::new(),
  usable_in_snapshot: infcx.num_open_snapshots(),

tests/ui/borrowck/issue-119915-bad-clone-suggestion.rs

@@ -0,0 +1,28 @@
+use std::marker::PhantomData;
+
+struct Example<E, FakeParam>(PhantomData<(fn(E), fn(FakeParam))>);
+
+struct NoLifetime;
+struct Immutable<'a>(PhantomData<&'a ()>);
+
+impl<'a, E: 'a> Copy for Example<E, Immutable<'a>> {}
+impl<'a, E: 'a> Clone for Example<E, Immutable<'a>> {
+ fn clone(&self) -> Self {
+ *self
+ }
+}
+
+impl<E, FakeParam> Example<E, FakeParam> {
+ unsafe fn change<NewFakeParam>(self) -> Example<E, NewFakeParam> {
+ Example(PhantomData)
+ }
+}
+
+impl<E> Example<E, NoLifetime> {
+ fn the_ice(&mut self) -> Example<E, Immutable<'_>> {
+ unsafe { self.change() }
+ //~^ ERROR cannot move out of `*self` which is behind a mutable reference
+ }
+}
+
+fn main() {}

tests/ui/borrowck/issue-119915-bad-clone-suggestion.stderr

@@ -0,0 +1,17 @@
+error[E0507]: cannot move out of `*self` which is behind a mutable reference
+ --> $DIR/issue-119915-bad-clone-suggestion.rs:23:18
+ |
+LL | unsafe { self.change() }
+ | ^^^^ -------- `*self` moved due to this method call
+ | |
+ | move occurs because `*self` has type `Example<E, NoLifetime>`, which does not implement the `Copy` trait
+ |
+note: `Example::<E, FakeParam>::change` takes ownership of the receiver `self`, which moves `*self`
+ --> $DIR/issue-119915-bad-clone-suggestion.rs:16:36
+ |
+LL | unsafe fn change<NewFakeParam>(self) -> Example<E, NewFakeParam> {
+ | ^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0507`.

tests/ui/borrowck/issue-85765-closure.rs

@@ -3,7 +3,6 @@ fn main() {
  let mut test = Vec::new();
  let rofl: &Vec<Vec<i32>> = &mut test;
  //~^ HELP consider changing this binding's type
- //~| HELP you can `clone` the `Vec<Vec<i32>>` value and consume it, but this might not be your desired behavior
  rofl.push(Vec::new());
  //~^ ERROR cannot borrow `*rofl` as mutable, as it is behind a `&` reference
  //~| NOTE `rofl` is a `&` reference, so the data it refers to cannot be borrowed as mutable

tests/ui/borrowck/issue-85765-closure.stderr

@@ -1,21 +1,16 @@
 error[E0596]: cannot borrow `*rofl` as mutable, as it is behind a `&` reference
- --> $DIR/issue-85765-closure.rs:7:9
+ --> $DIR/issue-85765-closure.rs:6:9
  |
 LL | rofl.push(Vec::new());
  | ^^^^ `rofl` is a `&` reference, so the data it refers to cannot be borrowed as mutable
  |
-help: you can `clone` the `Vec<Vec<i32>>` value and consume it, but this might not be your desired behavior
- --> $DIR/issue-85765-closure.rs:4:36
- |
-LL | let rofl: &Vec<Vec<i32>> = &mut test;
- | ^^^^^^^^^
 help: consider changing this binding's type
  |
 LL | let rofl: &mut Vec<Vec<i32>> = &mut test;
  | ~~~~~~~~~~~~~~~~~~
 
 error[E0594]: cannot assign to `*r`, which is behind a `&` reference
- --> $DIR/issue-85765-closure.rs:14:9
+ --> $DIR/issue-85765-closure.rs:13:9
  |
 LL | *r = 0;
  | ^^^^^^ `r` is a `&` reference, so the data it refers to cannot be written
@@ -26,7 +21,7 @@ LL | let r = &mut mutvar;
  | +++
 
 error[E0594]: cannot assign to `*x`, which is behind a `&` reference
- --> $DIR/issue-85765-closure.rs:21:9
+ --> $DIR/issue-85765-closure.rs:20:9
  |
 LL | *x = 1;
  | ^^^^^^ `x` is a `&` reference, so the data it refers to cannot be written
@@ -37,7 +32,7 @@ LL | let x: &mut usize = &mut{0};
  | ~~~~~~~~~~
 
 error[E0594]: cannot assign to `*y`, which is behind a `&` reference
- --> $DIR/issue-85765-closure.rs:28:9
+ --> $DIR/issue-85765-closure.rs:27:9
  |
 LL | *y = 1;
  | ^^^^^^ `y` is a `&` reference, so the data it refers to cannot be written