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,
         }
     }
 

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

@@ -1,5 +1,19 @@
+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
@@ -7,6 +21,7 @@ LL | impl<T: A> TraitWithAssoc for T {
 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>`
+error[E0391]: cycle detected when computing whether impls specialize one another
-  --> $DIR/issue-39448.rs:45:13
+  --> $DIR/issue-39448.rs:24:1
-   |
-LL |     x.foo(y.to()).to()
-   |             ^^
-   |
-note: required for `T` to implement `FromA<U>`
-  --> $DIR/issue-39448.rs:24:29
    |
 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
    |
-LL | impl<T, U> ToA<U> for T
+   = note: ...which requires evaluating trait selection obligation `u16: FromA<u8>`...
-   |            ^^^^^^     ^
+   = note: ...which again requires computing whether impls specialize one another, completing the cycle
-LL | where
+note: cycle used when building specialization graph of trait `FromA`
-LL |     U: FromA<T>,
+  --> $DIR/issue-39448.rs:20:1
-   |        -------- 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`.