Rollup merge of rust-lang#104269 - compiler-errors:hang-in-where-clause-sugg, r=lcnr

Fix hang in where-clause suggestion with `predicate_can_apply`

Using `predicate_may_hold` during error reporting causes an evaluation overflow, which (because we use `evaluate_obligation_no_overflow`) then causes the predicate to need to be re-evaluated locally, which results in a hang.

... but since the "add a where clause" suggestion is best-effort, just throw any overflow errors. No need for 100% accuracy.

r? `@lcnr` who has been thinking about overflows... Let me know if you want more context about this issue, and as always, feel free to reassign.

Fixes rust-lang#104225

Author: matthiaskrgr

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

@@ -2557,7 +2557,10 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             let obligation =
                 Obligation::new(self.tcx, ObligationCause::dummy(), param_env, cleaned_pred);
 
-            self.predicate_may_hold(&obligation)
+            // We don't use `InferCtxt::predicate_may_hold` because that
+            // will re-run predicates that overflow locally, which ends up
+            // taking a really long time to compute.
+            self.evaluate_obligation(&obligation).map_or(false, |eval| eval.may_apply())
         })
     }
 

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -1340,9 +1340,8 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     obligation.param_env,
                     trait_pred_and_suggested_ty,
                 );
-                let suggested_ty_would_satisfy_obligation = self
-                    .evaluate_obligation_no_overflow(&new_obligation)
-                    .must_apply_modulo_regions();
+                let suggested_ty_would_satisfy_obligation =
+                    self.predicate_must_hold_modulo_regions(&new_obligation);
                 if suggested_ty_would_satisfy_obligation {
                     let sp = self
                         .tcx

src/test/ui/traits/predicate_can_apply-hang.rs

@@ -0,0 +1,6 @@
+fn f<B>(x: Vec<[[[B; 1]; 1]; 1]>) -> impl PartialEq<B> {
+    //~^ ERROR can't compare `Vec<[[[B; 1]; 1]; 1]>` with `B`
+    x
+}
+
+fn main() {}

src/test/ui/traits/predicate_can_apply-hang.stderr

@@ -0,0 +1,21 @@
+error[E0277]: can't compare `Vec<[[[B; 1]; 1]; 1]>` with `B`
+  --> $DIR/predicate_can_apply-hang.rs:1:38
+   |
+LL | fn f<B>(x: Vec<[[[B; 1]; 1]; 1]>) -> impl PartialEq<B> {
+   |                                      ^^^^^^^^^^^^^^^^^ no implementation for `Vec<[[[B; 1]; 1]; 1]> == B`
+LL |
+LL |     x
+   |     - return type was inferred to be `Vec<[[[B; 1]; 1]; 1]>` here
+   |
+   = help: the trait `PartialEq<B>` is not implemented for `Vec<[[[B; 1]; 1]; 1]>`
+   = help: the following other types implement trait `PartialEq<Rhs>`:
+             <Vec<T, A1> as PartialEq<Vec<U, A2>>>
+             <Vec<T, A> as PartialEq<&[U; N]>>
+             <Vec<T, A> as PartialEq<&[U]>>
+             <Vec<T, A> as PartialEq<&mut [U]>>
+             <Vec<T, A> as PartialEq<[U; N]>>
+             <Vec<T, A> as PartialEq<[U]>>
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/typeck/hang-in-overflow.rs

@@ -0,0 +1,18 @@
+// normalize-stderr-test "the requirement `.*`" -> "the requirement `...`"
+// normalize-stderr-test "required for `.*` to implement `.*`" -> "required for `...` to implement `...`"
+
+// Currently this fatally aborts instead of hanging.
+// Make sure at least that this doesn't turn into a hang.
+
+fn f() {
+    foo::<_>();
+    //~^ ERROR overflow evaluating the requirement
+}
+
+fn foo<B>()
+where
+    Vec<[[[B; 1]; 1]; 1]>: PartialEq<B>,
+{
+}
+
+fn main() {}

src/test/ui/typeck/hang-in-overflow.stderr

@@ -0,0 +1,22 @@
+error[E0275]: overflow evaluating the requirement `...`
+  --> $DIR/hang-in-overflow.rs:8:5
+   |
+LL |     foo::<_>();
+   |     ^^^^^^^^
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`hang_in_overflow`)
+   = note: required for `...` to implement `...`
+   = note: 127 redundant requirements hidden
+   = note: required for `...` to implement `...`
+note: required by a bound in `foo`
+  --> $DIR/hang-in-overflow.rs:14:28
+   |
+LL | fn foo<B>()
+   |    --- required by a bound in this
+LL | where
+LL |     Vec<[[[B; 1]; 1]; 1]>: PartialEq<B>,
+   |                            ^^^^^^^^^^^^ required by this bound in `foo`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0275`.