Add test

tests/crashes/123461.rs

@@ -1,5 +0,0 @@
-//@ known-bug: #123461
-
-fn main() {
-    let _: [_; unsafe { std::mem::transmute(|o_b: Option<_>| {}) }];
-}

tests/ui/transmute/ambiguity-in-closure-arg.rs

@@ -0,0 +1,11 @@
+// Minimized test for <https://github.com/rust-lang/rust/issues/123461>.
+
+struct Unconstrained<T>(T);
+
+fn main() {
+    unsafe { std::mem::transmute::<_, ()>(|o_b: Unconstrained<_>| {}) };
+    //~^ ERROR type annotations needed
+    // We unfortunately don't check `Wf(Unconstrained<_>)`, so we won't
+    // hit an ambiguity error before checking the transmute. That means
+    // we still may have inference variables in our transmute src.
+}

tests/ui/transmute/ambiguity-in-closure-arg.stderr

@@ -0,0 +1,9 @@
+error[E0282]: type annotations needed
+  --> $DIR/ambiguity-in-closure-arg.rs:6:44
+   |
+LL |     unsafe { std::mem::transmute::<_, ()>(|o_b: Unconstrained<_>| {}) };
+   |                                            ^^^^^^^^^^^^^^^^^^^^^ cannot infer type
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0282`.

tests/ui/wf/closure-wf.rs

@@ -0,0 +1,11 @@
+trait Bound {}
+struct NeedsBound<T: Bound>(T);
+
+// Checks that we enforce that closure args are WF.
+
+fn constrain_inner<T, F: for<'a> FnOnce(&'a (), NeedsBound<T>)>(_: T, _: F) {}
+
+fn main() {
+    constrain_inner(1u32, |&(), _| ());
+    //~^ ERROR the trait bound `u32: Bound` is not satisfied
+}

tests/ui/wf/closure-wf.stderr

@@ -0,0 +1,20 @@
+error[E0277]: the trait bound `u32: Bound` is not satisfied
+  --> $DIR/closure-wf.rs:9:33
+   |
+LL |     constrain_inner(1u32, |&(), _| ());
+   |                                 ^ the trait `Bound` is not implemented for `u32`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/closure-wf.rs:1:1
+   |
+LL | trait Bound {}
+   | ^^^^^^^^^^^
+note: required by a bound in `NeedsBound`
+  --> $DIR/closure-wf.rs:2:22
+   |
+LL | struct NeedsBound<T: Bound>(T);
+   |                      ^^^^^ required by this bound in `NeedsBound`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.