WIP: suggest how to fix with unconstrained type parameters

Unconstrained type parameters made no suggestion and it makes difficult
to know how to fix (#107295).

To make fixing easier, we output suggestions how to fix.

Author: kei519

compiler/rustc_hir_analysis/src/impl_wf_check.rs

@@ -12,6 +12,7 @@ use std::assert_matches::debug_assert_matches;
 
 use min_specialization::check_min_specialization;
 use rustc_data_structures::fx::FxHashSet;
+use rustc_errors::Applicability;
 use rustc_errors::codes::*;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::LocalDefId;
@@ -149,6 +150,21 @@ fn enforce_impl_params_are_constrained(
                 const_param_note2: const_param_note,
             });
             diag.code(E0207);
+
+            diag.multipart_suggestions(
+                format!(
+                    "either remove the type parameter (<{0}>) or make use of it, for example \
+                        `{1}<{0}>`",
+                    param.name, impl_self_ty,
+                ),
+                // FIXME: I'm not sure the way of getting the span of `<{param.name}>`.
+                [vec![(tcx.def_span(param.def_id), "".into())], vec![(
+                    // FIXME: I'm not sure the way of getting the span of `impl_self_ty`.
+                    tcx.def_span(param.def_id).shrink_to_hi(),
+                    format!("<{}>", param.name),
+                )]],
+                Applicability::MachineApplicable,
+            );
             res = Err(diag.emit());
         }
     }