Prevent caching projections in the case of cycles

When normalizing a projection which results in a cycle, we would
cache the result of `project_type` without the nested obligations
(because they're not needed for inference). This would result in
the nested obligations only being handled once in fulfill, which
would avoid the cycle error.

Fixes rust-lang#79714, a regresion from rust-lang#79305 caused by the removal of
`get_paranoid_cache_value_obligation`.

compiler/rustc_infer/src/traits/project.rs

@@ -90,6 +90,7 @@ impl ProjectionCacheKey<'tcx> {
 pub enum ProjectionCacheEntry<'tcx> {
     InProgress,
     Ambiguous,
+    Recur,
     Error,
     NormalizedTy(NormalizedTy<'tcx>),
 }
@@ -143,7 +144,12 @@ impl<'tcx> ProjectionCache<'_, 'tcx> {
             "ProjectionCacheEntry::insert_ty: adding cache entry: key={:?}, value={:?}",
             key, value
         );
-        let fresh_key = self.map().insert(key, ProjectionCacheEntry::NormalizedTy(value));
+        let mut map = self.map();
+        if let Some(ProjectionCacheEntry::Recur) = map.get(&key) {
+            debug!("Not overwriting Recur");
+            return;
+        }
+        let fresh_key = map.insert(key, ProjectionCacheEntry::NormalizedTy(value));
         assert!(!fresh_key, "never started projecting `{:?}`", key);
     }
 
@@ -197,6 +203,14 @@ impl<'tcx> ProjectionCache<'_, 'tcx> {
         assert!(!fresh, "never started projecting `{:?}`", key);
     }
 
+    /// Indicates that while trying to normalize `key`, `key` was required to
+    /// be normalized again. Selection or evaluation should eventually report
+    /// an error here.
+    pub fn recur(&mut self, key: ProjectionCacheKey<'tcx>) {
+        let fresh = self.map().insert(key, ProjectionCacheEntry::Recur);
+        assert!(!fresh, "never started projecting `{:?}`", key);
+    }
+
     /// Indicates that trying to normalize `key` resulted in
     /// error.
     pub fn error(&mut self, key: ProjectionCacheKey<'tcx>) {

compiler/rustc_trait_selection/src/traits/project.rs

@@ -496,12 +496,6 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
             return Ok(None);
         }
         Err(ProjectionCacheEntry::InProgress) => {
-            // If while normalized A::B, we are asked to normalize
-            // A::B, just return A::B itself. This is a conservative
-            // answer, in the sense that A::B *is* clearly equivalent
-            // to A::B, though there may be a better value we can
-            // find.
-
             // Under lazy normalization, this can arise when
             // bootstrapping.  That is, imagine an environment with a
             // where-clause like `A::B == u32`. Now, if we are asked
@@ -512,6 +506,14 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
 
             debug!("found cache entry: in-progress");
 
+            // Cache that normalizing this projection resulted in a cycle. This
+            // should ensure that, unless this happens within a snapshot that's
+            // rolled back, fulfillment or evaluation will notice the cycle.
+
+            infcx.inner.borrow_mut().projection_cache().recur(cache_key);
+            return Err(InProgress);
+        }
+        Err(ProjectionCacheEntry::Recur) => {
             return Err(InProgress);
         }
         Err(ProjectionCacheEntry::NormalizedTy(ty)) => {

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

@@ -291,6 +291,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         self.infcx.tcx
     }
 
+    pub(super) fn query_mode(&self) -> TraitQueryMode {
+        self.query_mode
+    }
+
     ///////////////////////////////////////////////////////////////////////////
     // Selection
     //

src/test/ui/associated-types/defaults-cyclic-fail-1.rs

@@ -24,13 +24,13 @@ impl Tr for u32 {
 // ...but not in an impl that redefines one of the types.
 impl Tr for bool {
     type A = Box<Self::B>;
-    //~^ ERROR type mismatch resolving `<bool as Tr>::B == _`
+    //~^ ERROR overflow evaluating the requirement `<bool as Tr>::B == _`
 }
 // (the error is shown twice for some reason)
 
 impl Tr for usize {
     type B = &'static Self::A;
-    //~^ ERROR type mismatch resolving `<usize as Tr>::A == _`
+    //~^ ERROR overflow evaluating the requirement `<usize as Tr>::A == _`
 }
 
 fn main() {

src/test/ui/associated-types/defaults-cyclic-fail-1.stderr

@@ -1,15 +1,15 @@
-error[E0271]: type mismatch resolving `<bool as Tr>::B == _`
+error[E0275]: overflow evaluating the requirement `<bool as Tr>::B == _`
   --> $DIR/defaults-cyclic-fail-1.rs:26:5
    |
 LL |     type A = Box<Self::B>;
-   |     ^^^^^^^^^^^^^^^^^^^^^^ cyclic type of infinite size
+   |     ^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0271]: type mismatch resolving `<usize as Tr>::A == _`
+error[E0275]: overflow evaluating the requirement `<usize as Tr>::A == _`
   --> $DIR/defaults-cyclic-fail-1.rs:32:5
    |
 LL |     type B = &'static Self::A;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ cyclic type of infinite size
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0271`.
+For more information about this error, try `rustc --explain E0275`.

src/test/ui/associated-types/defaults-cyclic-fail-2.rs

@@ -25,13 +25,13 @@ impl Tr for u32 {
 
 impl Tr for bool {
     type A = Box<Self::B>;
-    //~^ ERROR type mismatch resolving `<bool as Tr>::B == _`
+    //~^ ERROR overflow evaluating the requirement `<bool as Tr>::B == _`
 }
 // (the error is shown twice for some reason)
 
 impl Tr for usize {
     type B = &'static Self::A;
-    //~^ ERROR type mismatch resolving `<usize as Tr>::A == _`
+    //~^ ERROR overflow evaluating the requirement `<usize as Tr>::A == _`
 }
 
 fn main() {

src/test/ui/associated-types/defaults-cyclic-fail-2.stderr

@@ -1,15 +1,15 @@
-error[E0271]: type mismatch resolving `<bool as Tr>::B == _`
+error[E0275]: overflow evaluating the requirement `<bool as Tr>::B == _`
   --> $DIR/defaults-cyclic-fail-2.rs:27:5
    |
 LL |     type A = Box<Self::B>;
-   |     ^^^^^^^^^^^^^^^^^^^^^^ cyclic type of infinite size
+   |     ^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0271]: type mismatch resolving `<usize as Tr>::A == _`
+error[E0275]: overflow evaluating the requirement `<usize as Tr>::A == _`
   --> $DIR/defaults-cyclic-fail-2.rs:33:5
    |
 LL |     type B = &'static Self::A;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ cyclic type of infinite size
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0271`.
+For more information about this error, try `rustc --explain E0275`.

src/test/ui/associated-types/impl-wf-cycle-1.rs

@@ -0,0 +1,29 @@
+// Regression test for #79714
+
+trait Baz {}
+impl Baz for () {}
+impl<T> Baz for (T,) {}
+
+trait Fiz {}
+impl Fiz for bool {}
+
+trait Grault {
+    type A;
+    type B;
+}
+
+impl<T: Grault> Grault for (T,)
+where
+    Self::A: Baz,
+    Self::B: Fiz,
+{
+    type A = ();
+    //~^ ERROR overflow evaluating the requirement `<(T,) as Grault>::A == _`
+    type B = bool;
+    //~^ ERROR overflow evaluating the requirement `<(T,) as Grault>::A == _`
+}
+//~^^^^^^^^^^ ERROR overflow evaluating the requirement `<(T,) as Grault>::A == _`
+
+fn main() {
+    let x: <(_,) as Grault>::A = ();
+}

src/test/ui/associated-types/impl-wf-cycle-1.stderr

@@ -0,0 +1,39 @@
+error[E0275]: overflow evaluating the requirement `<(T,) as Grault>::A == _`
+  --> $DIR/impl-wf-cycle-1.rs:15:1
+   |
+LL | / impl<T: Grault> Grault for (T,)
+LL | | where
+LL | |     Self::A: Baz,
+LL | |     Self::B: Fiz,
+...  |
+LL | |
+LL | | }
+   | |_^
+   |
+   = note: required because of the requirements on the impl of `Grault` for `(T,)`
+   = note: 1 redundant requirements hidden
+   = note: required because of the requirements on the impl of `Grault` for `(T,)`
+
+error[E0275]: overflow evaluating the requirement `<(T,) as Grault>::A == _`
+  --> $DIR/impl-wf-cycle-1.rs:20:5
+   |
+LL |     type A = ();
+   |     ^^^^^^^^^^^^
+   |
+   = note: required because of the requirements on the impl of `Grault` for `(T,)`
+   = note: 1 redundant requirements hidden
+   = note: required because of the requirements on the impl of `Grault` for `(T,)`
+
+error[E0275]: overflow evaluating the requirement `<(T,) as Grault>::A == _`
+  --> $DIR/impl-wf-cycle-1.rs:22:5
+   |
+LL |     type B = bool;
+   |     ^^^^^^^^^^^^^^
+   |
+   = note: required because of the requirements on the impl of `Grault` for `(T,)`
+   = note: 1 redundant requirements hidden
+   = note: required because of the requirements on the impl of `Grault` for `(T,)`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0275`.

src/test/ui/associated-types/impl-wf-cycle-2.rs

@@ -0,0 +1,16 @@
+// Regression test for #79714
+
+trait Grault {
+    type A;
+}
+
+impl<T: Grault> Grault for (T,)
+where
+    Self::A: Copy,
+{
+    type A = ();
+    //~^ ERROR overflow evaluating the requirement `<(T,) as Grault>::A == _`
+}
+//~^^^^^^^ ERROR overflow evaluating the requirement `<(T,) as Grault>::A == _`
+
+fn main() {}

src/test/ui/associated-types/impl-wf-cycle-2.stderr

@@ -0,0 +1,25 @@
+error[E0275]: overflow evaluating the requirement `<(T,) as Grault>::A == _`
+  --> $DIR/impl-wf-cycle-2.rs:7:1
+   |
+LL | / impl<T: Grault> Grault for (T,)
+LL | | where
+LL | |     Self::A: Copy,
+LL | | {
+LL | |     type A = ();
+LL | |
+LL | | }
+   | |_^
+   |
+   = note: required because of the requirements on the impl of `Grault` for `(T,)`
+
+error[E0275]: overflow evaluating the requirement `<(T,) as Grault>::A == _`
+  --> $DIR/impl-wf-cycle-2.rs:11:5
+   |
+LL |     type A = ();
+   |     ^^^^^^^^^^^^
+   |
+   = note: required because of the requirements on the impl of `Grault` for `(T,)`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0275`.