[crater only] Always make inductive cycles as ambig during typeck

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -236,7 +236,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             freshener: infcx.freshener(),
             intercrate_ambiguity_causes: None,
             query_mode: TraitQueryMode::Standard,
-            treat_inductive_cycle: TreatInductiveCycleAs::Recur,
+            treat_inductive_cycle: TreatInductiveCycleAs::Ambig,
         }
     }
 
@@ -1620,16 +1620,23 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             for bound in
                 self.tcx().item_bounds(alias_ty.def_id).instantiate(self.tcx(), alias_ty.args)
             {
-                // HACK: On subsequent recursions, we only care about bounds that don't
+                // HACK: In the initial recursion, we only care about bounds for the
+                // `self_ty`. On subsequent recursions, we care about bounds that don't
                 // share the same type as `self_ty`. This is because for truly rigid
                 // projections, we will never be able to equate, e.g. `<T as Tr>::A`
                 // with `<<T as Tr>::A as Tr>::A`.
-                if in_parent_alias_type {
-                    match bound.kind().skip_binder() {
-                        ty::ClauseKind::Trait(tr) if tr.self_ty() == self_ty => continue,
-                        ty::ClauseKind::Projection(p) if p.self_ty() == self_ty => continue,
-                        _ => {}
+                match bound.kind().skip_binder() {
+                    ty::ClauseKind::Trait(tr)
+                        if (tr.self_ty() == self_ty) ^ !in_parent_alias_type =>
+                    {
+                        continue;
+                    }
+                    ty::ClauseKind::Projection(p)
+                        if (p.self_ty() == self_ty) ^ !in_parent_alias_type =>
+                    {
+                        continue;
                     }
+                    _ => {}
                 }
 
                 for_each(self, bound, idx)?;

tests/ui/marker_trait_attr/unsound-overlap.rs

@@ -8,6 +8,7 @@ trait B {}
 impl<T: A> B for T {}
 impl<T: B> A for T {}
 impl A for &str {}
+//~^ ERROR type annotations needed: cannot satisfy `&str: A`
 impl<T: A + B> A for (T,) {}
 trait TraitWithAssoc {
     type Assoc;

tests/ui/marker_trait_attr/unsound-overlap.stderr

@@ -1,12 +1,27 @@
+error[E0283]: type annotations needed: cannot satisfy `&str: A`
+  --> $DIR/unsound-overlap.rs:10:12
+   |
+LL | impl A for &str {}
+   |            ^^^^
+   |
+note: multiple `impl`s satisfying `&str: A` found
+  --> $DIR/unsound-overlap.rs:9:1
+   |
+LL | impl<T: B> A for T {}
+   | ^^^^^^^^^^^^^^^^^^
+LL | impl A for &str {}
+   | ^^^^^^^^^^^^^^^
+
 error[E0119]: conflicting implementations of trait `TraitWithAssoc` for type `((&str,),)`
-  --> $DIR/unsound-overlap.rs:20:1
+  --> $DIR/unsound-overlap.rs:21:1
    |
 LL | impl<T: A> TraitWithAssoc for T {
    | ------------------------------- first implementation here
 ...
 LL | impl TraitWithAssoc for ((&str,),) {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `((&str,),)`
 
-error: aborting due to 1 previous error
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0119`.
+Some errors have detailed explanations: E0119, E0283.
+For more information about an error, try `rustc --explain E0119`.

tests/ui/specialization/issue-39448.rs

@@ -22,6 +22,7 @@ trait FromA<T> {
 }
 
 impl<T: A, U: A + FromA<T>> FromA<T> for U {
+    //~^ ERROR cycle detected when computing whether impls specialize one another
     default fn from(x: T) -> Self {
         ToA::to(x)
     }
@@ -42,7 +43,7 @@ where
 
 #[allow(dead_code)]
 fn foo<T: A, U: A>(x: T, y: U) -> U {
-    x.foo(y.to()).to() //~ ERROR overflow evaluating the requirement
+    x.foo(y.to()).to()
 }
 
 fn main() {

tests/ui/specialization/issue-39448.stderr

@@ -8,28 +8,21 @@ LL | #![feature(specialization)]
    = help: consider using `min_specialization` instead, which is more stable and complete
    = note: `#[warn(incomplete_features)]` on by default
 
-error[E0275]: overflow evaluating the requirement `T: FromA<U>`
-  --> $DIR/issue-39448.rs:45:13
-   |
-LL |     x.foo(y.to()).to()
-   |             ^^
-   |
-note: required for `T` to implement `FromA<U>`
-  --> $DIR/issue-39448.rs:24:29
+error[E0391]: cycle detected when computing whether impls specialize one another
+  --> $DIR/issue-39448.rs:24:1
    |
 LL | impl<T: A, U: A + FromA<T>> FromA<T> for U {
-   |                   --------  ^^^^^^^^     ^
-   |                   |
-   |                   unsatisfied trait bound introduced here
-note: required for `U` to implement `ToA<T>`
-  --> $DIR/issue-39448.rs:34:12
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: ...which requires evaluating trait selection obligation `u16: FromA<u8>`...
+   = note: ...which again requires computing whether impls specialize one another, completing the cycle
+note: cycle used when building specialization graph of trait `FromA`
+  --> $DIR/issue-39448.rs:20:1
    |
-LL | impl<T, U> ToA<U> for T
-   |            ^^^^^^     ^
-LL | where
-LL |     U: FromA<T>,
-   |        -------- unsatisfied trait bound introduced here
+LL | trait FromA<T> {
+   | ^^^^^^^^^^^^^^
+   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
 
 error: aborting due to 1 previous error; 1 warning emitted
 
-For more information about this error, try `rustc --explain E0275`.
+For more information about this error, try `rustc --explain E0391`.

tests/ui/specialization/issue-39618.rs

@@ -2,8 +2,6 @@
 // FIXME(JohnTitor): Centril pointed out this looks suspicions, we should revisit here.
 // More context: https://github.com/rust-lang/rust/pull/69192#discussion_r379846796
 
-//@ check-pass
-
 #![feature(specialization)] //~ WARN the feature `specialization` is incomplete
 
 trait Foo {
@@ -19,6 +17,7 @@ impl<T> Bar for T where T: Foo {
 }
 
 impl<T> Foo for T where T: Bar {
+    //~^ ERROR cycle detected when computing whether impls specialize one another
     fn foo(&self) {}
 }
 

tests/ui/specialization/issue-39618.stderr

@@ -1,5 +1,5 @@
 warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/issue-39618.rs:7:12
+  --> $DIR/issue-39618.rs:5:12
    |
 LL | #![feature(specialization)]
    |            ^^^^^^^^^^^^^^
@@ -8,5 +8,21 @@ LL | #![feature(specialization)]
    = help: consider using `min_specialization` instead, which is more stable and complete
    = note: `#[warn(incomplete_features)]` on by default
 
-warning: 1 warning emitted
+error[E0391]: cycle detected when computing whether impls specialize one another
+  --> $DIR/issue-39618.rs:19:1
+   |
+LL | impl<T> Foo for T where T: Bar {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: ...which requires evaluating trait selection obligation `u64: Bar`...
+   = note: ...which again requires computing whether impls specialize one another, completing the cycle
+note: cycle used when building specialization graph of trait `Foo`
+  --> $DIR/issue-39618.rs:7:1
+   |
+LL | trait Foo {
+   | ^^^^^^^^^
+   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
+
+error: aborting due to 1 previous error; 1 warning emitted
 
+For more information about this error, try `rustc --explain E0391`.