Tweak wording of non-const traits used as const bounds

Use verbose suggestions and add additional labels/notes.

Add more test cases for stable/nightly and feature enabled/disabled.

compiler/rustc_hir_analysis/src/collect.rs

@@ -1637,11 +1637,23 @@ fn check_impl_constness(
     }
 
     let trait_name = tcx.item_name(trait_def_id).to_string();
+    let (local_trait_span, suggestion_pre) =
+        match (trait_def_id.is_local(), tcx.sess.is_nightly_build()) {
+            (true, true) => (
+                Some(tcx.def_span(trait_def_id).shrink_to_lo()),
+                if tcx.features().const_trait_impl() {
+                    ""
+                } else {
+                    "enable `#![feature(const_trait_impl)]` in your crate and "
+                },
+            ),
+            (false, _) | (_, false) => (None, ""),
+        };
     Some(tcx.dcx().emit_err(errors::ConstImplForNonConstTrait {
         trait_ref_span: hir_trait_ref.path.span,
         trait_name,
-        local_trait_span:
-            trait_def_id.as_local().map(|_| tcx.def_span(trait_def_id).shrink_to_lo()),
+        local_trait_span,
+        suggestion_pre,
         marking: (),
         adding: (),
     }))