Report better WF obligation leaf obligations in new solver

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -384,39 +384,64 @@ impl<'tcx> ProofTreeVisitor<'tcx> for BestObligation<'tcx> {
             return ControlFlow::Break(self.obligation.clone());
         }
 
-        // FIXME: Could we extract a trait ref from a projection here too?
+        let tcx = goal.infcx().tcx;
         // FIXME: Also, what about considering >1 layer up the stack? May be necessary
         // for normalizes-to.
-        let Some(parent_trait_pred) = goal.goal().predicate.to_opt_poly_trait_pred() else {
-            return ControlFlow::Break(self.obligation.clone());
+        let pred_kind = goal.goal().predicate.kind();
+        let child_mode = match pred_kind.skip_binder() {
+            ty::PredicateKind::Clause(ty::ClauseKind::Trait(parent_trait_pred)) => {
+                ChildMode::Trait(pred_kind.rebind(parent_trait_pred))
+            }
+            ty::PredicateKind::NormalizesTo(normalizes_to)
+                if matches!(
+                    normalizes_to.alias.kind(tcx),
+                    ty::AliasTermKind::ProjectionTy | ty::AliasTermKind::ProjectionConst
+                ) =>
+            {
+                ChildMode::Trait(pred_kind.rebind(ty::TraitPredicate {
+                    trait_ref: normalizes_to.alias.trait_ref(tcx),
+                    polarity: ty::PredicatePolarity::Positive,
+                }))
+            }
+            ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(_)) => {
+                ChildMode::WellFormedObligation
+            }
+            _ => {
+                return ControlFlow::Break(self.obligation.clone());
+            }
         };
 
-        let tcx = goal.infcx().tcx;
         let mut impl_where_bound_count = 0;
         for nested_goal in candidate.instantiate_nested_goals(self.span()) {
+            let make_obligation = |cause| Obligation {
+                cause,
+                param_env: nested_goal.goal().param_env,
+                predicate: nested_goal.goal().predicate,
+                recursion_depth: self.obligation.recursion_depth + 1,
+            };
+
             let obligation;
-            match nested_goal.source() {
-                GoalSource::Misc => {
+            match (child_mode, nested_goal.source()) {
+                (ChildMode::Trait(_), GoalSource::Misc) => {
                     continue;
                 }
-                GoalSource::ImplWhereBound => {
-                    obligation = Obligation {
-                        cause: derive_cause(
-                            tcx,
-                            candidate.kind(),
-                            self.obligation.cause.clone(),
-                            impl_where_bound_count,
-                            parent_trait_pred,
-                        ),
-                        param_env: nested_goal.goal().param_env,
-                        predicate: nested_goal.goal().predicate,
-                        recursion_depth: self.obligation.recursion_depth + 1,
-                    };
+                (ChildMode::Trait(parent_trait_pred), GoalSource::ImplWhereBound) => {
+                    obligation = make_obligation(derive_cause(
+                        tcx,
+                        candidate.kind(),
+                        self.obligation.cause.clone(),
+                        impl_where_bound_count,
+                        parent_trait_pred,
+                    ));
                     impl_where_bound_count += 1;
                 }
-                GoalSource::InstantiateHigherRanked => {
+                // Skip over a higher-ranked predicate.
+                (_, GoalSource::InstantiateHigherRanked) => {
                     obligation = self.obligation.clone();
                 }
+                (ChildMode::WellFormedObligation, _) => {
+                    obligation = make_obligation(self.obligation.cause.clone());
+                }
             }
 
             // Skip nested goals that aren't the *reason* for our goal's failure.
@@ -436,6 +461,18 @@ impl<'tcx> ProofTreeVisitor<'tcx> for BestObligation<'tcx> {
     }
 }
 
+#[derive(Copy, Clone)]
+enum ChildMode<'tcx> {
+    // Try to derive an `ObligationCause::{ImplDerived,BuiltinDerived}`,
+    // and skip all `GoalSource::Misc`, which represent useless obligations
+    // such as alias-eq which may not hold.
+    Trait(ty::PolyTraitPredicate<'tcx>),
+    // Skip trying to derive an `ObligationCause` from this obligation, and
+    // report *all* sub-obligations as if they came directly from the parent
+    // obligation.
+    WellFormedObligation,
+}
+
 fn derive_cause<'tcx>(
     tcx: TyCtxt<'tcx>,
     candidate_kind: ProbeKind<'tcx>,

tests/ui/associated-inherent-types/bugs/wf-check-skipped.next.stderr

@@ -1,8 +1,11 @@
-error: the type `Foo::Bar<Vec<[u32]>>` is not well-formed
-  --> $DIR/wf-check-skipped.rs:17:14
+error[E0277]: the size for values of type `[u32]` cannot be known at compilation time
+  --> $DIR/wf-check-skipped.rs:17:25
    |
 LL | fn main() -> Foo::Bar::<Vec<[u32]>> {}
-   |              ^^^^^^^^^^^^^^^^^^^^^^
+   |                         ^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[u32]`
 
 error: aborting due to 1 previous error
 
+For more information about this error, try `rustc --explain E0277`.

tests/ui/associated-inherent-types/bugs/wf-check-skipped.rs

@@ -15,4 +15,4 @@ impl Foo {
 }
 
 fn main() -> Foo::Bar::<Vec<[u32]>> {}
-//[next]~^ ERROR the type `Foo::Bar<Vec<[u32]>>` is not well-formed
+//[next]~^ ERROR the size for values of type `[u32]` cannot be known at compilation time

tests/ui/for/issue-20605.next.stderr

@@ -11,31 +11,13 @@ help: consider mutably borrowing here
 LL |     for item in &mut *things { *item = 0 }
    |                 ++++
 
-error: the type `<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter` is not well-formed
-  --> $DIR/issue-20605.rs:6:17
-   |
-LL |     for item in *things { *item = 0 }
-   |                 ^^^^^^^
-
-error: the type `&mut <dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter` is not well-formed
-  --> $DIR/issue-20605.rs:6:17
-   |
-LL |     for item in *things { *item = 0 }
-   |                 ^^^^^^^
-
-error: the type `Option<<<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter as Iterator>::Item>` is not well-formed
-  --> $DIR/issue-20605.rs:6:17
-   |
-LL |     for item in *things { *item = 0 }
-   |                 ^^^^^^^
-
 error[E0614]: type `<dyn Iterator<Item = &'a mut u8> as IntoIterator>::Item` cannot be dereferenced
   --> $DIR/issue-20605.rs:6:27
    |
 LL |     for item in *things { *item = 0 }
    |                           ^^^^^
 
-error: aborting due to 5 previous errors
+error: aborting due to 2 previous errors
 
 Some errors have detailed explanations: E0277, E0614.
 For more information about an error, try `rustc --explain E0277`.

tests/ui/for/issue-20605.rs

@@ -6,9 +6,6 @@ fn changer<'a>(mut things: Box<dyn Iterator<Item=&'a mut u8>>) {
     for item in *things { *item = 0 }
     //[current]~^ ERROR `dyn Iterator<Item = &'a mut u8>` is not an iterator
     //[next]~^^ ERROR `dyn Iterator<Item = &'a mut u8>` is not an iterator
-    //[next]~| ERROR the type `<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter` is not well-formed
-    //[next]~| ERROR the type `&mut <dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter` is not well-formed
-    //[next]~| ERROR the type `Option<<<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter as Iterator>::Item>` is not well-formed
     //[next]~| ERROR type `<dyn Iterator<Item = &'a mut u8> as IntoIterator>::Item` cannot be dereferenced
 
     // FIXME(-Znext-solver): these error messages are horrible and have to be

tests/ui/impl-trait/in-trait/alias-bounds-when-not-wf.rs

@@ -14,6 +14,6 @@ struct W<T>(T);
 // `usize: Foo` doesn't hold. Therefore we ICE, because we don't expect to still
 // encounter weak types in `assemble_alias_bound_candidates_recur`.
 fn hello(_: W<A<usize>>) {}
-//~^ ERROR the type `W<A<usize>>` is not well-formed
+//~^ ERROR the size for values of type `A<usize>` cannot be known at compilation time
 
 fn main() {}

tests/ui/impl-trait/in-trait/alias-bounds-when-not-wf.stderr

@@ -7,11 +7,14 @@ LL | #![feature(lazy_type_alias)]
    = note: see issue #112792 <https://github.com./rust-lang/rust/issues/112792> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-error: the type `W<A<usize>>` is not well-formed
+error[E0277]: the size for values of type `A<usize>` cannot be known at compilation time
   --> $DIR/alias-bounds-when-not-wf.rs:16:13
    |
 LL | fn hello(_: W<A<usize>>) {}
-   |             ^^^^^^^^^^^
+   |             ^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `A<usize>`
 
 error: aborting due to 1 previous error; 1 warning emitted
 
+For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/next-solver/canonical/const-region-infer-to-static-in-binder.stderr

@@ -1,8 +1,8 @@
-error[E0282]: type annotations needed
+error[E0284]: type annotations needed: cannot satisfy `the constant `{ || {} }` can be evaluated`
   --> $DIR/const-region-infer-to-static-in-binder.rs:4:10
    |
 LL | struct X<const FN: fn() = { || {} }>;
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer the value of the constant `{ || {} }`
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot satisfy `the constant `{ || {} }` can be evaluated`
 
 error: using function pointers as const generic parameters is forbidden
   --> $DIR/const-region-infer-to-static-in-binder.rs:4:20
@@ -23,4 +23,4 @@ LL | struct X<const FN: fn() = { || {} }>;
 
 error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0282`.
+For more information about this error, try `rustc --explain E0284`.

tests/ui/traits/next-solver/issue-118950-root-region.rs

@@ -12,7 +12,7 @@ trait ToUnit<'a> {
 trait Overlap<T> {}
 
 type Assoc<'a, T> = <*const T as ToUnit<'a>>::Unit;
-//~^ ERROR: not well-formed
+//~^ ERROR the trait bound `*const T: ToUnit<'a>` is not satisfied
 
 impl<T> Overlap<T> for T {}
 

tests/ui/traits/next-solver/issue-118950-root-region.stderr

@@ -13,11 +13,17 @@ LL | #![feature(lazy_type_alias)]
    = note: see issue #112792 <https://github.com./rust-lang/rust/issues/112792> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-error: the type `<*const T as ToUnit<'a>>::Unit` is not well-formed
+error[E0277]: the trait bound `*const T: ToUnit<'a>` is not satisfied
   --> $DIR/issue-118950-root-region.rs:14:21
    |
 LL | type Assoc<'a, T> = <*const T as ToUnit<'a>>::Unit;
-   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `ToUnit<'a>` is not implemented for `*const T`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/issue-118950-root-region.rs:8:1
+   |
+LL | trait ToUnit<'a> {
+   | ^^^^^^^^^^^^^^^^
 
 WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: ['^0.Named(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), "'a"), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) }
 WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: ['^0.Named(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), "'a"), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) }
@@ -34,5 +40,5 @@ LL | impl<T> Overlap<for<'a> fn(Assoc<'a, T>)> for T where Missing: Overlap<T> {
 
 error: aborting due to 3 previous errors; 1 warning emitted
 
-Some errors have detailed explanations: E0119, E0412.
+Some errors have detailed explanations: E0119, E0277, E0412.
 For more information about an error, try `rustc --explain E0119`.

tests/ui/traits/next-solver/object-unsafety.rs

@@ -10,9 +10,8 @@ fn copy<U: Setup + ?Sized>(from: &U::From) -> U::From {
 
 pub fn copy_any<T>(t: &T) -> T {
     copy::<dyn Setup<From=T>>(t)
-    //~^ ERROR the type `&<dyn Setup<From = T> as Setup>::From` is not well-formed
+    //~^ ERROR the trait bound `T: Copy` is not satisfied in `dyn Setup<From = T>`
     //~| ERROR mismatched types
-    //~| ERROR the type `<dyn Setup<From = T> as Setup>::From` is not well-formed
     //~| ERROR the trait bound `T: Copy` is not satisfied
 
     // FIXME(-Znext-solver): These error messages are horrible and some of them

tests/ui/traits/next-solver/object-unsafety.stderr

@@ -15,12 +15,6 @@ help: consider restricting type parameter `T`
 LL | pub fn copy_any<T: std::marker::Copy>(t: &T) -> T {
    |                  +++++++++++++++++++
 
-error: the type `&<dyn Setup<From = T> as Setup>::From` is not well-formed
-  --> $DIR/object-unsafety.rs:12:31
-   |
-LL |     copy::<dyn Setup<From=T>>(t)
-   |                               ^
-
 error[E0308]: mismatched types
   --> $DIR/object-unsafety.rs:12:31
    |
@@ -37,13 +31,19 @@ note: function defined here
 LL | fn copy<U: Setup + ?Sized>(from: &U::From) -> U::From {
    |    ^^^^                    --------------
 
-error: the type `<dyn Setup<From = T> as Setup>::From` is not well-formed
+error[E0277]: the trait bound `T: Copy` is not satisfied in `dyn Setup<From = T>`
   --> $DIR/object-unsafety.rs:12:5
    |
 LL |     copy::<dyn Setup<From=T>>(t)
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ within `dyn Setup<From = T>`, the trait `Copy` is not implemented for `T`, which is required by `dyn Setup<From = T>: Setup`
+   |
+   = note: required because it appears within the type `dyn Setup<From = T>`
+help: consider restricting type parameter `T`
+   |
+LL | pub fn copy_any<T: std::marker::Copy>(t: &T) -> T {
+   |                  +++++++++++++++++++
 
-error: aborting due to 4 previous errors
+error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0277, E0308.
 For more information about an error, try `rustc --explain E0277`.

tests/ui/wf/wf-normalization-sized.next.stderr

@@ -1,30 +1,37 @@
-error: the type `<[[[[[[u8]]]]]] as WellUnformed>::RequestNormalize` is not well-formed
-  --> $DIR/wf-normalization-sized.rs:19:10
+error[E0277]: the size for values of type `[[[[[u8]]]]]` cannot be known at compilation time
+  --> $DIR/wf-normalization-sized.rs:19:11
    |
 LL | const _: <[[[[[[u8]]]]]] as WellUnformed>::RequestNormalize = ();
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |           ^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[[[[[u8]]]]]`
 
-error: the type `<[[[[[[u8]]]]]] as WellUnformed>::RequestNormalize` is not well-formed
-  --> $DIR/wf-normalization-sized.rs:19:10
+error[E0277]: the size for values of type `[[[[[u8]]]]]` cannot be known at compilation time
+  --> $DIR/wf-normalization-sized.rs:19:11
    |
 LL | const _: <[[[[[[u8]]]]]] as WellUnformed>::RequestNormalize = ();
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |           ^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
+   = help: the trait `Sized` is not implemented for `[[[[[u8]]]]]`
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error: the type `<Vec<str> as WellUnformed>::RequestNormalize` is not well-formed
-  --> $DIR/wf-normalization-sized.rs:22:10
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/wf-normalization-sized.rs:22:11
    |
 LL | const _: <Vec<str> as WellUnformed>::RequestNormalize = ();
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |           ^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
 
-error: the type `<Vec<str> as WellUnformed>::RequestNormalize` is not well-formed
-  --> $DIR/wf-normalization-sized.rs:22:10
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/wf-normalization-sized.rs:22:11
    |
 LL | const _: <Vec<str> as WellUnformed>::RequestNormalize = ();
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |           ^^^^^^^^ doesn't have a size known at compile-time
    |
+   = help: the trait `Sized` is not implemented for `str`
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error: aborting due to 4 previous errors
 
+For more information about this error, try `rustc --explain E0277`.

tests/ui/wf/wf-normalization-sized.rs

@@ -17,10 +17,10 @@ impl<T: ?Sized> WellUnformed for T {
 }
 
 const _: <[[[[[[u8]]]]]] as WellUnformed>::RequestNormalize = ();
-//[next]~^ the type
-//[next]~| the type
+//[next]~^ the size for values of type `[[[[[u8]]]]]` cannot be known at compilation time
+//[next]~| the size for values of type `[[[[[u8]]]]]` cannot be known at compilation time
 const _: <Vec<str> as WellUnformed>::RequestNormalize = ();
-//[next]~^ the type
-//[next]~| the type
+//[next]~^ the size for values of type `str` cannot be known at compilation time
+//[next]~| the size for values of type `str` cannot be known at compilation time
 
 fn main() {}