Rollup merge of #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 #104225

4 files changed, 68 insertions, 0 deletions

diff --git a/src/test/ui/traits/predicate_can_apply-hang.rs b/src/test/ui/traits/predicate_can_apply-hang.rs
new file mode 100644
index 00000000000..5f01645da52
--- /dev/null
+++ b/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() {}
diff --git a/src/test/ui/traits/predicate_can_apply-hang.stderr b/src/test/ui/traits/predicate_can_apply-hang.stderr
new file mode 100644
index 00000000000..49fe63b412a
--- /dev/null
+++ b/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`.
diff --git a/src/test/ui/typeck/hang-in-overflow.rs b/src/test/ui/typeck/hang-in-overflow.rs
new file mode 100644
index 00000000000..a8330c9b65c
--- /dev/null
+++ b/src/test/ui/typeck/hang-in-overflow.rs
@@ -0,0 +1,19 @@
+// normalize-stderr-test "the requirement `.*`" -> "the requirement `...`"
+// normalize-stderr-test "required for `.*` to implement `.*`" -> "required for `...` to implement `...`"
+// normalize-stderr-test: ".*the full type name has been written to.*\n" -> ""
+
+// 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() {}
diff --git a/src/test/ui/typeck/hang-in-overflow.stderr b/src/test/ui/typeck/hang-in-overflow.stderr
new file mode 100644
index 00000000000..7a7b85b19b4
--- /dev/null
+++ b/src/test/ui/typeck/hang-in-overflow.stderr
@@ -0,0 +1,22 @@
+error[E0275]: overflow evaluating the requirement `...`
+  --> $DIR/hang-in-overflow.rs:9:5
+   |
+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:15:28
+   |
+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`.