Add some FIXMEs for remaining issues that we need to fix before using more const trait things in libcore

(cherry picked from commit 16f8396)

Author: oli-obk

src/librustdoc/clean/mod.rs

@@ -2549,6 +2549,7 @@ fn clean_generic_args<'tcx>(
                     }
                     hir::GenericArg::Lifetime(_) => GenericArg::Lifetime(Lifetime::elided()),
                     hir::GenericArg::Type(ty) => GenericArg::Type(clean_ty(ty, cx)),
+                    // FIXME(effects): This will still emit `<true>` for non-const impls of const traits
                     hir::GenericArg::Const(ct)
                         if cx.tcx.has_attr(ct.value.def_id, sym::rustc_host) =>
                     {

src/librustdoc/clean/utils.rs

@@ -104,6 +104,9 @@ pub(crate) fn ty_args_to_args<'tcx>(
                     arg: index,
                 }),
             ))),
+            // FIXME(effects): this relies on the host effect being called `host`, which users could also name
+            // their const generics.
+            // FIXME(effects): this causes `host = true` and `host = false` generics to also be emitted.
             GenericArgKind::Const(ct) if let ty::ConstKind::Param(p) = ct.kind() && p.name == sym::host => None,
             GenericArgKind::Const(ct) => {
                 Some(GenericArg::Const(Box::new(clean_middle_const(kind.rebind(ct), cx))))