Strongly prefer alias and param-env bounds

compiler/rustc_trait_selection/src/solve/assembly/mod.rs

@@ -2,7 +2,6 @@
 
 use super::search_graph::OverflowHandler;
 use super::{EvalCtxt, SolverMode};
-use crate::solve::CanonicalResponseExt;
 use crate::traits::coherence;
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_hir::def_id::DefId;
@@ -744,13 +743,18 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             SolverMode::Normal => {
                 let param_env_responses = candidates
                     .iter()
-                    .filter(|c| matches!(c.source, CandidateSource::ParamEnv(_)))
+                    .filter(|c| {
+                        matches!(
+                            c.source,
+                            CandidateSource::ParamEnv(_) | CandidateSource::AliasBound
+                        )
+                    })
                     .map(|c| c.result)
                     .collect::<Vec<_>>();
                 if let Some(result) = self.try_merge_responses(&param_env_responses) {
-                    if result.has_only_region_constraints() {
+                    // We strongly prefer alias and param-env bounds here, even if they affect inference.
-                        return Ok(result);
+                    // See https://github.com/rust-lang/trait-system-refactor-initiative/issues/11.
-                    }
+                    return Ok(result);
                 }
             }
         }

tests/ui/dyn-star/param-env-infer.current.stderr

@@ -8,7 +8,7 @@ LL | #![feature(dyn_star, pointer_like_trait)]
    = note: `#[warn(incomplete_features)]` on by default
 
 error[E0282]: type annotations needed
-  --> $DIR/param-env-infer.rs:12:10
+  --> $DIR/param-env-infer.rs:13:10
    |
 LL |     t as _
    |          ^ cannot infer type

tests/ui/dyn-star/param-env-infer.next.stderr

@@ -7,12 +7,67 @@ LL | #![feature(dyn_star, pointer_like_trait)]
    = note: see issue #102425 <https://github.com/rust-lang/rust/issues/102425> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-error[E0282]: type annotations needed
+error[E0391]: cycle detected when computing type of `make_dyn_star::{opaque#0}`
-  --> $DIR/param-env-infer.rs:12:10
+  --> $DIR/param-env-infer.rs:11:60
    |
-LL |     t as _
+LL | fn make_dyn_star<'a, T: PointerLike + Debug + 'a>(t: T) -> impl PointerLike + Debug + 'a {
-   |          ^ cannot infer type
+   |                                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: ...which requires borrow-checking `make_dyn_star`...
+  --> $DIR/param-env-infer.rs:11:1
+   |
+LL | fn make_dyn_star<'a, T: PointerLike + Debug + 'a>(t: T) -> impl PointerLike + Debug + 'a {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires promoting constants in MIR for `make_dyn_star`...
+  --> $DIR/param-env-infer.rs:11:1
+   |
+LL | fn make_dyn_star<'a, T: PointerLike + Debug + 'a>(t: T) -> impl PointerLike + Debug + 'a {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires preparing `make_dyn_star` for borrow checking...
+  --> $DIR/param-env-infer.rs:11:1
+   |
+LL | fn make_dyn_star<'a, T: PointerLike + Debug + 'a>(t: T) -> impl PointerLike + Debug + 'a {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires unsafety-checking `make_dyn_star`...
+  --> $DIR/param-env-infer.rs:11:1
+   |
+LL | fn make_dyn_star<'a, T: PointerLike + Debug + 'a>(t: T) -> impl PointerLike + Debug + 'a {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires building MIR for `make_dyn_star`...
+  --> $DIR/param-env-infer.rs:11:1
+   |
+LL | fn make_dyn_star<'a, T: PointerLike + Debug + 'a>(t: T) -> impl PointerLike + Debug + 'a {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires match-checking `make_dyn_star`...
+  --> $DIR/param-env-infer.rs:11:1
+   |
+LL | fn make_dyn_star<'a, T: PointerLike + Debug + 'a>(t: T) -> impl PointerLike + Debug + 'a {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires building THIR for `make_dyn_star`...
+  --> $DIR/param-env-infer.rs:11:1
+   |
+LL | fn make_dyn_star<'a, T: PointerLike + Debug + 'a>(t: T) -> impl PointerLike + Debug + 'a {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires type-checking `make_dyn_star`...
+  --> $DIR/param-env-infer.rs:11:1
+   |
+LL | fn make_dyn_star<'a, T: PointerLike + Debug + 'a>(t: T) -> impl PointerLike + Debug + 'a {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: ...which requires computing layout of `make_dyn_star::{opaque#0}`...
+   = note: ...which requires normalizing `make_dyn_star::{opaque#0}`...
+   = note: ...which again requires computing type of `make_dyn_star::{opaque#0}`, completing the cycle
+note: cycle used when checking item types in top-level module
+  --> $DIR/param-env-infer.rs:5:1
+   |
+LL | / #![feature(dyn_star, pointer_like_trait)]
+LL | |
+LL | |
+LL | | use std::fmt::Debug;
+...  |
+LL | |
+LL | | fn main() {}
+   | |____________^
 
 error: aborting due to previous error; 1 warning emitted
 
-For more information about this error, try `rustc --explain E0282`.
+For more information about this error, try `rustc --explain E0391`.

tests/ui/dyn-star/param-env-infer.rs

@@ -9,8 +9,9 @@ use std::fmt::Debug;
 use std::marker::PointerLike;
 
 fn make_dyn_star<'a, T: PointerLike + Debug + 'a>(t: T) -> impl PointerLike + Debug + 'a {
+    //[next]~^ ERROR cycle detected when computing type of `make_dyn_star::{opaque#0}`
     t as _
-    //~^ ERROR type annotations needed
+    //[current]~^ ERROR type annotations needed
 }
 
 fn main() {}

tests/ui/traits/new-solver/alias_eq_dont_use_normalizes_to_if_substs_eq.rs

@@ -1,5 +1,8 @@
 // compile-flags: -Ztrait-solver=next
 
+// check-pass
+// (should not pass, should be turned into a coherence-only test)
+
 // check that when computing `alias-eq(<() as Foo<u16, T>>::Assoc, <() as Foo<?0, T>>::Assoc)`
 // we do not infer `?0 = u8` via the `for<STOP> (): Foo<u8, STOP>` impl or `?0 = u16` by
 // relating substs as either could be a valid solution.
@@ -36,7 +39,6 @@ where
 {
     // `<() as Foo<u16, STOP>>::Assoc == <() as Foo<_, STOP>>::Assoc`
     let _: <() as Foo<u16, T>>::Assoc = output::<_, T>();
-    //~^ error: type annotations needed
 
     // let _: <() as Foo<u16, T>>::Assoc = output::<u8, T>(); // OK
     // let _: <() as Foo<u16, T>>::Assoc = output::<u16, T>(); // OK

tests/ui/traits/new-solver/alias_eq_dont_use_normalizes_to_if_substs_eq.stderr

@@ -1,9 +0,0 @@
-error[E0282]: type annotations needed
-  --> $DIR/alias_eq_dont_use_normalizes_to_if_substs_eq.rs:38:41
-   |
-LL |     let _: <() as Foo<u16, T>>::Assoc = output::<_, T>();
-   |                                         ^^^^^^^^^^^^^^ cannot infer type of the type parameter `T` declared on the function `output`
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0282`.

tests/ui/traits/new-solver/alias_eq_substs_eq_not_intercrate.rs

@@ -1,5 +1,8 @@
 // compile-flags: -Ztrait-solver=next
 
+// check-pass
+// (should not pass, should be turned into a coherence-only test)
+
 // check that a `alias-eq(<?0 as TraitB>::Assoc, <T as TraitB>::Assoc)` goal fails.
 
 // FIXME(deferred_projection_equality): add a test that this is true during coherence
@@ -14,7 +17,6 @@ fn needs_a<T: TraitB>() -> T::Assoc {
 
 fn bar<T: TraitB>() {
     let _: <_ as TraitB>::Assoc = needs_a::<T>();
-    //~^ error: type annotations needed
 }
 
 fn main() {}

tests/ui/traits/new-solver/alias_eq_substs_eq_not_intercrate.stderr

@@ -1,9 +0,0 @@
-error[E0282]: type annotations needed
-  --> $DIR/alias_eq_substs_eq_not_intercrate.rs:16:12
-   |
-LL |     let _: <_ as TraitB>::Assoc = needs_a::<T>();
-   |            ^^^^^^^^^^^^^^^^^^^^ cannot infer type for associated type `<_ as TraitB>::Assoc`
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0282`.