improve fixme

Author: lcnr

Diff for: compiler/rustc_mir/src/transform/validate.rs

@@ -85,9 +85,12 @@ pub fn equal_up_to_regions(
             param_env,
             ty.fold_with(&mut BottomUpFolder {
                 tcx,
-                // We just erase all late-bound lifetimes, but this is not fully correct (FIXME):
-                // lifetimes in invariant positions could matter (e.g. through associated types).
-                // We rely on the fact that layout was confirmed to be equal above.
+                // FIXME: We erase all late-bound lifetimes, but this is not fully correct.
+                // If you have a type like `<for<'a> fn(&'a u32) as SomeTrait>::Assoc`,
+                // this is not necessarily equivalent to `<fn(&'static u32) as SomeTrait>::Assoc`,
+                // since one may have an `impl SomeTrait for fn(&32)` and
+                // `impl SomeTrait for fn(&'static u32)` at the same time which
+                // specify distinct values for Assoc. (See also #56105)
                 lt_op: |_| tcx.lifetimes.re_erased,
                 // Leave consts and types unchanged.
                 ct_op: |ct| ct,