Remove crashes, add comment

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

@@ -2224,6 +2224,8 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
     ) -> GetSafeTransmuteErrorAndReason {
         use rustc_transmute::Answer;
 
+        // We don't assemble a transmutability candidate for types that are generic
+        // and we should have ambiguity for types that still have non-region infer.
         if obligation.predicate.has_non_region_param() || obligation.has_non_region_infer() {
             return GetSafeTransmuteErrorAndReason::Default;
         }

tests/crashes/125881.rs

@@ -1,8 +0,0 @@
-//@ known-bug: rust-lang/rust#125881
-#![crate_type = "lib"]
-#![feature(transmutability)]
-#![feature(unboxed_closures,effects)]
-
-const fn test() -> impl std::mem::TransmuteFrom() {
-    || {}
-}

tests/crashes/126377.rs

@@ -1,29 +0,0 @@
-//@ known-bug: rust-lang/rust#126377
-
-#![feature(effects)]
-#![feature(generic_const_exprs)]
-
-mod assert {
-    use std::mem::{Assume, TransmuteFrom};
-
-    pub fn is_transmutable<
-        Src,
-        Dst,
-        const ASSUME_ALIGNMENT: bool,
-        const ASSUME_LIFETIMES: bool,
-        const ASSUME_SAFETY: bool,
-        const ASSUME_VALIDITY: bool,
-    >()
-    where
-        Dst: TransmuteFrom<
-            Src,
-            {  }
-        >,
-    {}
-}
-
-const fn from_options() -> Assume {
-    #[repr(C)] struct Src;
-    #[repr(C)] struct Dst;
-    assert::is_transmutable::<Src, Dst, {0u8}, false, false, false>();
-}

tests/crashes/130413.rs

@@ -1,17 +0,0 @@
-//@ known-bug: #130413
-
-#![feature(transmutability)]
-trait Aaa {
-    type Y;
-}
-
-trait Bbb {
-    type B: std::mem::TransmuteFrom<()>;
-}
-
-impl<T> Bbb for T
-where
-    T: Aaa,
-{
-    type B = T::Y;
-}