diagnostics: use DeepRejectCtxt for check

This makes more things match, particularly applicable blankets.

compiler/rustc_trait_selection/src/error_reporting/infer/note_and_explain.rs

@@ -4,6 +4,7 @@ use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_middle::traits::{ObligationCause, ObligationCauseCode};
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
+use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
 use rustc_middle::ty::print::{FmtPrinter, Printer};
 use rustc_middle::ty::{self, suggest_constraining_type_param, Ty};
 use rustc_span::def_id::DefId;
@@ -313,9 +314,15 @@ impl<T> Trait<T> for X {
                     (ty::Dynamic(t, _, ty::DynKind::Dyn), _)
                         if let Some(def_id) = t.principal_def_id() =>
                     {
-                        let has_non_blanket_impl =
+                        let mut has_matching_impl = false;
-                            tcx.non_blanket_impls_for_ty(def_id, values.found).next().is_some();
+                        tcx.for_each_relevant_impl(def_id, values.found, |did| {
-                        if has_non_blanket_impl {
+                            if DeepRejectCtxt::new(tcx, TreatParams::ForLookup)
+                                .types_may_unify(values.found, tcx.type_of(did).skip_binder())
+                            {
+                                has_matching_impl = true;
+                            }
+                        });
+                        if has_matching_impl {
                             let trait_name = tcx.item_name(def_id);
                             diag.help(format!(
                                 "`{}` implements `{trait_name}` so you could box the found value \
@@ -328,9 +335,15 @@ impl<T> Trait<T> for X {
                     (_, ty::Dynamic(t, _, ty::DynKind::Dyn))
                         if let Some(def_id) = t.principal_def_id() =>
                     {
-                        let has_non_blanket_impl =
+                        let mut has_matching_impl = false;
-                            tcx.non_blanket_impls_for_ty(def_id, values.expected).next().is_some();
+                        tcx.for_each_relevant_impl(def_id, values.expected, |did| {
-                        if has_non_blanket_impl {
+                            if DeepRejectCtxt::new(tcx, TreatParams::ForLookup)
+                                .types_may_unify(values.expected, tcx.type_of(did).skip_binder())
+                            {
+                                has_matching_impl = true;
+                            }
+                        });
+                        if has_matching_impl {
                             let trait_name = tcx.item_name(def_id);
                             diag.help(format!(
                                 "`{}` implements `{trait_name}` so you could change the expected \
@@ -342,9 +355,15 @@ impl<T> Trait<T> for X {
                     (ty::Dynamic(t, _, ty::DynKind::DynStar), _)
                         if let Some(def_id) = t.principal_def_id() =>
                     {
-                        let has_non_blanket_impl =
+                        let mut has_matching_impl = false;
-                            tcx.non_blanket_impls_for_ty(def_id, values.found).next().is_some();
+                        tcx.for_each_relevant_impl(def_id, values.found, |did| {
-                        if has_non_blanket_impl {
+                            if DeepRejectCtxt::new(tcx, TreatParams::ForLookup)
+                                .types_may_unify(values.found, tcx.type_of(did).skip_binder())
+                            {
+                                has_matching_impl = true;
+                            }
+                        });
+                        if has_matching_impl {
                             let trait_name = tcx.item_name(def_id);
                             diag.help(format!(
                                 "`{}` implements `{trait_name}`, `#[feature(dyn_star)]` is likely \

tests/ui/cast/ptr-to-trait-obj-different-args.stderr

@@ -14,6 +14,7 @@ LL |     let y: *const dyn Trait<Y> = x as _;
    |
    = note: expected trait object `dyn Trait<X>`
               found trait object `dyn Trait<Y>`
+   = help: `dyn Trait<Y>` implements `Trait` so you could box the found value and coerce it to the trait object `Box<dyn Trait>`, you will have to change the expected type as well
 
 error[E0308]: mismatched types
   --> $DIR/ptr-to-trait-obj-different-args.rs:27:34
@@ -25,6 +26,7 @@ LL |     let _: *const dyn Trait<T> = x as _;
    |
    = note: expected trait object `dyn Trait<X>`
               found trait object `dyn Trait<T>`
+   = help: `dyn Trait<T>` implements `Trait` so you could box the found value and coerce it to the trait object `Box<dyn Trait>`, you will have to change the expected type as well
 
 error[E0308]: mismatched types
   --> $DIR/ptr-to-trait-obj-different-args.rs:28:34
@@ -37,6 +39,7 @@ LL |     let _: *const dyn Trait<X> = t as _;
    |
    = note: expected trait object `dyn Trait<T>`
               found trait object `dyn Trait<X>`
+   = help: `dyn Trait<X>` implements `Trait` so you could box the found value and coerce it to the trait object `Box<dyn Trait>`, you will have to change the expected type as well
 
 error[E0308]: mismatched types
   --> $DIR/ptr-to-trait-obj-different-args.rs:36:5

tests/ui/traits/upcast_soundness_bug.stderr

@@ -6,6 +6,7 @@ LL |     let p = p as *const dyn Trait<u8, u16>; // <- this is bad!
    |
    = note: expected trait object `dyn Trait<u8, u8>`
               found trait object `dyn Trait<u8, u16>`
+   = help: `dyn Trait<u8, u16>` implements `Trait` so you could box the found value and coerce it to the trait object `Box<dyn Trait>`, you will have to change the expected type as well
 
 error: aborting due to 1 previous error