Actually prevent cycle errors

Author: oli-obk

compiler/rustc_hir_analysis/src/collect/type_of.rs

@@ -4,6 +4,7 @@ use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit;
 use rustc_hir::intravisit::Visitor;
 use rustc_hir::{HirId, Node};
+use rustc_infer::infer::opaque_types::may_define_opaque_type;
 use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::print::with_forced_trimmed_paths;
 use rustc_middle::ty::subst::InternalSubsts;
@@ -589,6 +590,12 @@ fn find_opaque_ty_constraints_for_tait(tcx: TyCtxt<'_>, def_id: LocalDefId) -> T
                 debug!("no constraint: no typeck results");
                 return;
             }
+
+            if !may_define_opaque_type(self.tcx, item_def_id, self.def_id) {
+                debug!("no constraint: does not have TAIT in signature");
+                return;
+            }
+
             // Calling `mir_borrowck` can lead to cycle errors through
             // const-checking, avoid calling it if we don't have to.
             // ```rust

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -638,11 +638,19 @@ impl<'tcx> InferCtxt<'tcx> {
 /// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`.
 /// For the above example, this function returns `true` for `f1` and `false` for `f2`.
 #[instrument(skip(tcx), level = "trace", ret)]
-fn may_define_opaque_type<'tcx>(
+pub fn may_define_opaque_type<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: LocalDefId,
     opaque_def_id: LocalDefId,
 ) -> bool {
+    if tcx.is_closure(def_id.to_def_id()) {
+        return may_define_opaque_type(
+            tcx,
+            tcx.parent(def_id.to_def_id()).expect_local(),
+            opaque_def_id,
+        );
+    }
+
     let param_env = tcx.param_env(def_id);
     let opaque_hir_id = tcx.hir().local_def_id_to_hir_id(opaque_def_id);
     let mut hir_id = tcx.hir().local_def_id_to_hir_id(def_id);

tests/ui/type-alias-impl-trait/auto-trait-leakage3.rs

@@ -1,11 +1,10 @@
 #![feature(type_alias_impl_trait)]
 #![allow(dead_code)]
 
-// FIXME This should compile, but it currently doesn't
+// check-pass
 
 mod m {
     type Foo = impl std::fmt::Debug;
-    //~^ ERROR: cycle detected when computing type of `m::Foo::{opaque#0}` [E0391]
 
     pub fn foo() -> Foo {
         22_u32

tests/ui/type-alias-impl-trait/auto-trait-leakage3.stderr

@@ -1,10 +1,11 @@
 #![feature(type_alias_impl_trait)]
 
+// check-pass
+
 // Check that, even within the defining scope, we can get the
 // `Send` or `Sync` bound of a TAIT, as long as the current function
 // doesn't mention those TAITs in the signature
 type Foo = impl std::fmt::Debug;
-//~^ ERROR cycle detected
 
 fn is_send<T: Send>() {}
 

tests/ui/type-alias-impl-trait/defining-scopes-cycle-prevention.rs

@@ -7,6 +7,8 @@ type DropType<T> = ();
 //~^ ERROR type parameter `T` is unused
 
 type Foo = impl std::fmt::Debug;
+//~^ ERROR unconstrained opaque type
+
 // Check that, even though `Foo` is not part
 // of the actual type, we do allow this to be a defining scope
 fn g() -> DropType<Foo> {

tests/ui/type-alias-impl-trait/defining-scopes-cycle-prevention.stderr

@@ -4,8 +4,16 @@ error[E0091]: type parameter `T` is unused
 LL | type DropType<T> = ();
    |               ^ unused type parameter
 
+error: unconstrained opaque type
+  --> $DIR/defining-scopes-syntactic.rs:9:12
+   |
+LL | type Foo = impl std::fmt::Debug;
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same module
+
 error[E0308]: mismatched types
-  --> $DIR/defining-scopes-syntactic.rs:13:18
+  --> $DIR/defining-scopes-syntactic.rs:15:18
    |
 LL | type Foo = impl std::fmt::Debug;
    |            -------------------- the expected opaque type
@@ -18,7 +26,7 @@ LL |     let _: Foo = 42;
    = note: expected opaque type `Foo`
                      found type `{integer}`
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0091, E0308.
 For more information about an error, try `rustc --explain E0091`.

tests/ui/type-alias-impl-trait/defining-scopes-syntactic.rs

@@ -3,17 +3,20 @@
 use std::fmt::Debug;
 
 type Foo = impl Debug;
-pub trait Yay { }
-impl Yay for Foo { }
+pub trait Yay {}
+impl Yay for Foo {}
 
-fn foo() {
-    is_yay::<u32>();   //~ ERROR: the trait bound `u32: Yay` is not satisfied
+fn foo()
+where
+    Foo: Debug,
+{
+    is_yay::<u32>(); //~ ERROR: the trait bound `u32: Yay` is not satisfied
     is_debug::<u32>(); // OK
-    is_yay::<Foo>();   // OK
+    is_yay::<Foo>(); // OK
     is_debug::<Foo>(); // OK
 }
 
-fn is_yay<T: Yay>() { }
-fn is_debug<T: Debug>() { }
+fn is_yay<T: Yay>() {}
+fn is_debug<T: Debug>() {}
 
 fn main() {}

tests/ui/type-alias-impl-trait/defining-scopes-syntactic.stderr

@@ -1,14 +1,14 @@
 error[E0277]: the trait bound `u32: Yay` is not satisfied
-  --> $DIR/impl_trait_for_tait_bound.rs:10:14
+  --> $DIR/impl_trait_for_tait_bound.rs:13:14
    |
 LL |     is_yay::<u32>();
    |              ^^^ the trait `Yay` is not implemented for `u32`
    |
    = help: the trait `Yay` is implemented for `Foo`
 note: required by a bound in `is_yay`
-  --> $DIR/impl_trait_for_tait_bound.rs:16:14
+  --> $DIR/impl_trait_for_tait_bound.rs:19:14
    |
-LL | fn is_yay<T: Yay>() { }
+LL | fn is_yay<T: Yay>() {}
    |              ^^^ required by this bound in `is_yay`
 
 error: aborting due to previous error

tests/ui/type-alias-impl-trait/defining-scopes.stderr

@@ -2,15 +2,14 @@
 
 use std::fmt::Debug;
 
-type Foo = impl Debug;
-
-pub trait Yay { }
-impl Yay for u32 { }
+pub trait Yay {}
+impl Yay for u32 {}
 
 fn foo() {
+    type Foo = impl Debug;
     is_yay::<Foo>(); //~ ERROR: the trait bound `Foo: Yay` is not satisfied
 }
 
-fn is_yay<T: Yay>() { }
+fn is_yay<T: Yay>() {}
 
 fn main() {}

tests/ui/type-alias-impl-trait/impl_trait_for_tait_bound.rs

@@ -1,14 +1,14 @@
 error[E0277]: the trait bound `Foo: Yay` is not satisfied
-  --> $DIR/impl_trait_for_tait_bound2.rs:11:14
+  --> $DIR/impl_trait_for_tait_bound2.rs:10:14
    |
 LL |     is_yay::<Foo>();
    |              ^^^ the trait `Yay` is not implemented for `Foo`
    |
    = help: the trait `Yay` is implemented for `u32`
 note: required by a bound in `is_yay`
-  --> $DIR/impl_trait_for_tait_bound2.rs:14:14
+  --> $DIR/impl_trait_for_tait_bound2.rs:13:14
    |
-LL | fn is_yay<T: Yay>() { }
+LL | fn is_yay<T: Yay>() {}
    |              ^^^ required by this bound in `is_yay`
 
 error: aborting due to previous error

tests/ui/type-alias-impl-trait/impl_trait_for_tait_bound.stderr

@@ -3,7 +3,6 @@
 
 mod m {
     type Foo = impl std::fmt::Debug;
-    //~^ ERROR cycle detected
 
     // Cycle: error today, but it'd be nice if it eventually worked
 
@@ -17,6 +16,7 @@ mod m {
 
     fn baz() -> Foo {
         let f: Foo = 22_u32;
+        //~^ ERROR concrete type differs
         f
     }
 

tests/ui/type-alias-impl-trait/impl_trait_for_tait_bound2.rs

@@ -1,22 +1,14 @@
-error[E0391]: cycle detected when computing type of `m::Foo::{opaque#0}`
-  --> $DIR/inference-cycle.rs:5:16
+error: concrete type differs from previous defining opaque type use
+  --> $DIR/inference-cycle.rs:18:22
    |
-LL |     type Foo = impl std::fmt::Debug;
-   |                ^^^^^^^^^^^^^^^^^^^^
+LL |         let f: Foo = 22_u32;
+   |                      ^^^^^^ expected `()`, got `u32`
    |
-note: ...which requires type-checking `m::bar`...
-  --> $DIR/inference-cycle.rs:15:9
+note: previous use here
+  --> $DIR/inference-cycle.rs:10:9
    |
-LL |         is_send(foo()); // Today: error
-   |         ^^^^^^^
-   = note: ...which requires evaluating trait selection obligation `m::Foo: core::marker::Send`...
-   = note: ...which again requires computing type of `m::Foo::{opaque#0}`, completing the cycle
-note: cycle used when checking item types in module `m`
-  --> $DIR/inference-cycle.rs:4:1
-   |
-LL | mod m {
-   | ^^^^^
+LL |         is_send(bar())
+   |         ^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0391`.

tests/ui/type-alias-impl-trait/impl_trait_for_tait_bound2.stderr

@@ -1,6 +1,7 @@
 #![feature(type_alias_impl_trait)]
 
 type Foo = impl std::fmt::Debug;
+//~^ ERROR: unconstrained opaque type
 type Bar = impl Trait<Foo>;
 
 trait Trait<T> {}

tests/ui/type-alias-impl-trait/inference-cycle.rs

@@ -1,12 +1,20 @@
+error: unconstrained opaque type
+  --> $DIR/nested.rs:3:12
+   |
+LL | type Foo = impl std::fmt::Debug;
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same module
+
 error[E0277]: `Bar` doesn't implement `Debug`
-  --> $DIR/nested.rs:15:22
+  --> $DIR/nested.rs:16:22
    |
 LL |     println!("{:?}", bar());
    |                      ^^^^^ `Bar` cannot be formatted using `{:?}` because it doesn't implement `Debug`
    |
    = help: the trait `Debug` is not implemented for `Bar`
    = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

tests/ui/type-alias-impl-trait/inference-cycle.stderr

@@ -4,7 +4,6 @@ use std::fmt::Debug;
 
 type Foo = impl Debug;
 //~^ ERROR cycle detected
-//~| ERROR cycle detected
 
 fn is_send<T: Send>() {}
 

tests/ui/type-alias-impl-trait/nested.rs

@@ -1,36 +1,11 @@
-error[E0391]: cycle detected when computing type of `Foo::{opaque#0}`
-  --> $DIR/reveal_local.rs:5:12
-   |
-LL | type Foo = impl Debug;
-   |            ^^^^^^^^^^
-   |
-note: ...which requires type-checking `not_good`...
-  --> $DIR/reveal_local.rs:14:5
-   |
-LL |     is_send::<Foo>();
-   |     ^^^^^^^^^^^^^^
-   = note: ...which requires evaluating trait selection obligation `Foo: core::marker::Send`...
-   = note: ...which again requires computing type of `Foo::{opaque#0}`, completing the cycle
-note: cycle used when checking item types in top-level module
-  --> $DIR/reveal_local.rs:1:1
-   |
-LL | / #![feature(type_alias_impl_trait)]
-LL | |
-LL | | use std::fmt::Debug;
-LL | |
-...  |
-LL | |
-LL | | fn main() {}
-   | |____________^
-
 error[E0391]: cycle detected when computing type of `Foo::{opaque#0}`
   --> $DIR/reveal_local.rs:5:12
    |
 LL | type Foo = impl Debug;
    |            ^^^^^^^^^^
    |
 note: ...which requires type-checking `not_gooder`...
-  --> $DIR/reveal_local.rs:26:5
+  --> $DIR/reveal_local.rs:25:5
    |
 LL |     is_send::<Foo>();
    |     ^^^^^^^^^^^^^^
@@ -48,6 +23,6 @@ LL | |
 LL | | fn main() {}
    | |____________^
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0391`.