Try to normalize associated types before processing obligations

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -346,6 +346,8 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
 
         let obligation = &mut pending_obligation.obligation;
 
+        debug!(?obligation, "process_obligation pre-resolve");
+
         if obligation.predicate.has_infer_types_or_consts() {
             obligation.predicate =
                 self.selcx.infcx().resolve_vars_if_possible(obligation.predicate);
@@ -355,6 +357,21 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
 
         let infcx = self.selcx.infcx();
 
+        if obligation.predicate.has_projections() {
+            let mut obligations = Vec::new();
+            let predicate = crate::traits::project::try_normalize_with_depth_to(
+                self.selcx,
+                obligation.param_env,
+                obligation.cause.clone(),
+                obligation.recursion_depth + 1,
+                obligation.predicate,
+                &mut obligations,
+            );
+            if predicate != obligation.predicate {
+                obligations.push(obligation.with(predicate));
+                return ProcessResult::Changed(mk_pending(obligations));
+            }
+        }
         let binder = obligation.predicate.kind();
         match binder.no_bound_vars() {
             None => match binder.skip_binder() {

compiler/rustc_trait_selection/src/traits/project.rs

@@ -295,6 +295,32 @@ where
     result
 }
 
+#[instrument(level = "info", skip(selcx, param_env, cause, obligations))]
+pub fn try_normalize_with_depth_to<'a, 'b, 'tcx, T>(
+    selcx: &'a mut SelectionContext<'b, 'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    cause: ObligationCause<'tcx>,
+    depth: usize,
+    value: T,
+    obligations: &mut Vec<PredicateObligation<'tcx>>,
+) -> T
+where
+    T: TypeFoldable<'tcx>,
+{
+    debug!(obligations.len = obligations.len());
+    let mut normalizer = AssocTypeNormalizer::new_without_eager_inference_replacement(
+        selcx,
+        param_env,
+        cause,
+        depth,
+        obligations,
+    );
+    let result = ensure_sufficient_stack(|| normalizer.fold(value));
+    debug!(?result, obligations.len = normalizer.obligations.len());
+    debug!(?normalizer.obligations,);
+    result
+}
+
 pub(crate) fn needs_normalization<'tcx, T: TypeFoldable<'tcx>>(value: &T, reveal: Reveal) -> bool {
     match reveal {
         Reveal::UserFacing => value
@@ -314,6 +340,10 @@ struct AssocTypeNormalizer<'a, 'b, 'tcx> {
     obligations: &'a mut Vec<PredicateObligation<'tcx>>,
     depth: usize,
     universes: Vec<Option<ty::UniverseIndex>>,
+    /// If true, when a projection is unable to be completed, an inference
+    /// variable will be created and an obligation registered to project to that
+    /// inference variable. Also, constants will be eagerly evaluated.
+    eager_inference_replacement: bool,
 }
 
 impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
@@ -324,7 +354,33 @@ impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
         depth: usize,
         obligations: &'a mut Vec<PredicateObligation<'tcx>>,
     ) -> AssocTypeNormalizer<'a, 'b, 'tcx> {
-        AssocTypeNormalizer { selcx, param_env, cause, obligations, depth, universes: vec![] }
+        AssocTypeNormalizer {
+            selcx,
+            param_env,
+            cause,
+            obligations,
+            depth,
+            universes: vec![],
+            eager_inference_replacement: true,
+        }
+    }
+
+    fn new_without_eager_inference_replacement(
+        selcx: &'a mut SelectionContext<'b, 'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        cause: ObligationCause<'tcx>,
+        depth: usize,
+        obligations: &'a mut Vec<PredicateObligation<'tcx>>,
+    ) -> AssocTypeNormalizer<'a, 'b, 'tcx> {
+        AssocTypeNormalizer {
+            selcx,
+            param_env,
+            cause,
+            obligations,
+            depth,
+            universes: vec![],
+            eager_inference_replacement: false,
+        }
     }
 
     fn fold<T: TypeFoldable<'tcx>>(&mut self, value: T) -> T {
@@ -428,14 +484,28 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
                 // there won't be bound vars there.
 
                 let data = data.super_fold_with(self);
-                let normalized_ty = normalize_projection_type(
-                    self.selcx,
-                    self.param_env,
-                    data,
-                    self.cause.clone(),
-                    self.depth,
-                    &mut self.obligations,
-                );
+                let normalized_ty = if self.eager_inference_replacement {
+                    normalize_projection_type(
+                        self.selcx,
+                        self.param_env,
+                        data,
+                        self.cause.clone(),
+                        self.depth,
+                        &mut self.obligations,
+                    )
+                } else {
+                    opt_normalize_projection_type(
+                        self.selcx,
+                        self.param_env,
+                        data,
+                        self.cause.clone(),
+                        self.depth,
+                        &mut self.obligations,
+                    )
+                    .ok()
+                    .flatten()
+                    .unwrap_or_else(|| ty::Term::Ty(ty.super_fold_with(self)))
+                };
                 debug!(
                     ?self.depth,
                     ?ty,
@@ -501,7 +571,7 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
     }
 
     fn fold_const(&mut self, constant: ty::Const<'tcx>) -> ty::Const<'tcx> {
-        if self.selcx.tcx().lazy_normalization() {
+        if self.selcx.tcx().lazy_normalization() || !self.eager_inference_replacement {
             constant
         } else {
             let constant = constant.super_fold_with(self);

src/test/ui/associated-types/issue-59324.rs

@@ -15,9 +15,9 @@ pub trait ThriftService<Bug: NotFoo>:
 {
     fn get_service(
     //~^ ERROR the trait bound `Bug: Foo` is not satisfied
-    //~| ERROR the trait bound `Bug: Foo` is not satisfied
         &self,
     ) -> Self::AssocType;
+    //~^ the trait bound `Bug: Foo` is not satisfied
 }
 
 fn with_factory<H>(factory: dyn ThriftService<()>) {}

src/test/ui/associated-types/issue-59324.stderr

@@ -6,7 +6,7 @@ LL | |
 LL | |
 LL | |     Service<AssocType = <Bug as Foo>::OnlyFoo>
 ...  |
-LL | |     ) -> Self::AssocType;
+LL | |
 LL | | }
    | |_^ the trait `Foo` is not implemented for `Bug`
    |
@@ -23,7 +23,7 @@ LL | |
 LL | |
 LL | |     Service<AssocType = <Bug as Foo>::OnlyFoo>
 ...  |
-LL | |     ) -> Self::AssocType;
+LL | |
 LL | | }
    | |_^ the trait `Foo` is not implemented for `Bug`
    |
@@ -37,7 +37,6 @@ error[E0277]: the trait bound `Bug: Foo` is not satisfied
    |
 LL | /     fn get_service(
 LL | |
-LL | |
 LL | |         &self,
 LL | |     ) -> Self::AssocType;
    | |_________________________^ the trait `Foo` is not implemented for `Bug`
@@ -48,10 +47,10 @@ LL | pub trait ThriftService<Bug: NotFoo + Foo>:
    |                                     +++++
 
 error[E0277]: the trait bound `Bug: Foo` is not satisfied
-  --> $DIR/issue-59324.rs:16:8
+  --> $DIR/issue-59324.rs:19:10
    |
-LL |     fn get_service(
-   |        ^^^^^^^^^^^ the trait `Foo` is not implemented for `Bug`
+LL |     ) -> Self::AssocType;
+   |          ^^^^^^^^^^^^^^^ the trait `Foo` is not implemented for `Bug`
    |
 help: consider further restricting this bound
    |

src/test/ui/generic-associated-types/issue-90729.rs

@@ -0,0 +1,38 @@
+// check-pass
+
+#![feature(generic_associated_types)]
+
+use std::marker::PhantomData;
+
+pub trait Type {
+    type Ref<'a>;
+}
+
+pub trait AsBytes {}
+
+impl AsBytes for &str {}
+
+pub struct Utf8;
+
+impl Type for Utf8 {
+    type Ref<'a> = &'a str;
+}
+
+pub struct Bytes<T: Type> {
+    _marker: PhantomData<T>,
+}
+
+impl<T: Type> Bytes<T>
+where
+    for<'a> T::Ref<'a>: AsBytes,
+{
+    pub fn new() -> Self {
+        Self {
+            _marker: PhantomData,
+        }
+    }
+}
+
+fn main() {
+    let _b = Bytes::<Utf8>::new();
+}

src/test/ui/generic-associated-types/issue-91139.migrate.stderr

@@ -0,0 +1,10 @@
+error[E0311]: the parameter type `T` may not live long enough
+  --> $DIR/issue-91139.rs:27:12
+   |
+LL | fn foo<T>() {
+   |        - help: consider adding an explicit lifetime bound...: `T: 'a`
+LL |     let _: for<'a> fn(<() as Foo<T>>::Type<'a>, &'a T) = |_, _| ();
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds
+
+error: aborting due to previous error
+

src/test/ui/generic-associated-types/issue-91139.rs

@@ -1,4 +1,13 @@
-// check-pass
+// revisions: migrate nll
+//[nll]compile-flags: -Z borrowck=mir
+
+// Since we are testing nll (and migration) explicitly as a separate
+// revisions, don't worry about the --compare-mode=nll on this test.
+
+// ignore-compare-mode-nll
+
+//[nll] check-pass
+//[migrate] check-fail
 
 #![feature(generic_associated_types)]
 
@@ -16,6 +25,7 @@ impl<T> Foo<T> for () {
 
 fn foo<T>() {
     let _: for<'a> fn(<() as Foo<T>>::Type<'a>, &'a T) = |_, _| ();
+    //[migrate]~^ the parameter type `T` may not live long enough
 }
 
 pub fn main() {}

src/test/ui/generic-associated-types/issue-93341.rs

@@ -0,0 +1,55 @@
+// check-pass
+
+#![feature(generic_associated_types)]
+use std::marker::PhantomData;
+
+pub struct Id<'id>(PhantomData<fn(&'id ()) -> &'id ()>);
+
+fn new_id() -> Id<'static> {
+    Id(PhantomData)
+}
+
+pub trait HasLifetime where {
+    type AtLifetime<'a>;
+}
+
+pub struct ExistentialLifetime<S: HasLifetime>(S::AtLifetime<'static>);
+
+impl<S: HasLifetime> ExistentialLifetime<S> {
+    pub fn new<F>(f: F) -> ExistentialLifetime<S>
+        where for<'id> F: FnOnce(Id<'id>) -> S::AtLifetime<'id> {
+        ExistentialLifetime(f(new_id()))
+    }
+}
+
+
+struct ExampleS<'id>(Id<'id>);
+
+struct ExampleMarker;
+
+impl HasLifetime for ExampleMarker {
+    type AtLifetime<'id> = ExampleS<'id>;
+}
+
+
+fn broken0() -> ExistentialLifetime<ExampleMarker> {
+    fn new_helper<'id>(id: Id<'id>) -> ExampleS<'id> {
+        ExampleS(id)
+    }
+
+    ExistentialLifetime::<ExampleMarker>::new(new_helper)
+}
+
+fn broken1() -> ExistentialLifetime<ExampleMarker> {
+    fn new_helper<'id>(id: Id<'id>) -> <ExampleMarker as HasLifetime>::AtLifetime<'id> {
+        ExampleS(id)
+    }
+
+    ExistentialLifetime::<ExampleMarker>::new(new_helper)
+}
+
+fn broken2() -> ExistentialLifetime<ExampleMarker> {
+    ExistentialLifetime::<ExampleMarker>::new(|id| ExampleS(id))
+}
+
+fn main() {}

src/test/ui/generic-associated-types/issue-93342.rs

@@ -0,0 +1,57 @@
+// check-pass
+
+#![feature(generic_associated_types)]
+
+use std::marker::PhantomData;
+
+pub trait Scalar: 'static {
+    type RefType<'a>: ScalarRef<'a>;
+}
+
+pub trait ScalarRef<'a>: 'a {}
+
+impl Scalar for i32 {
+    type RefType<'a> = i32;
+}
+
+impl Scalar for String {
+    type RefType<'a> = &'a str;
+}
+
+impl Scalar for bool {
+    type RefType<'a> = i32;
+}
+
+impl<'a> ScalarRef<'a> for bool {}
+
+impl<'a> ScalarRef<'a> for i32 {}
+
+impl<'a> ScalarRef<'a> for &'a str {}
+
+fn str_contains(a: &str, b: &str) -> bool {
+    a.contains(b)
+}
+
+pub struct BinaryExpression<A: Scalar, B: Scalar, O: Scalar, F>
+where
+    F: Fn(A::RefType<'_>, B::RefType<'_>) -> O,
+{
+    f: F,
+    _phantom: PhantomData<(A, B, O)>,
+}
+
+impl<A: Scalar, B: Scalar, O: Scalar, F> BinaryExpression<A, B, O, F>
+where
+    F: Fn(A::RefType<'_>, B::RefType<'_>) -> O,
+{
+    pub fn new(f: F) -> Self {
+        Self {
+            f,
+            _phantom: PhantomData,
+        }
+    }
+}
+
+fn main() {
+    BinaryExpression::<String, String, bool, _>::new(str_contains);
+}