Normalize in infcx instead of globally for Option::as_deref suggestion

The projection may contain inference variables. These inference
variables are local to the local inference context. Using
`tcx.normalize_erasing_regions` doesn't work here because this method is
global and does not have access to the inference context. It's therefore
unable to deal with the inference variables. We normalize in the local
inference context instead, which knowns about the inference variables.

Author: Noratrieb

Diff for: compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -3592,7 +3592,8 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             // Extract `<U as Deref>::Target` assoc type and check that it is `T`
             && let Some(deref_target_did) = tcx.lang_items().deref_target()
             && let projection = tcx.mk_projection(deref_target_did, tcx.mk_substs(&[ty::GenericArg::from(found_ty)]))
-            && let Ok(deref_target) = tcx.try_normalize_erasing_regions(param_env, projection)
+            && let InferOk { value: deref_target, obligations } = infcx.at(&ObligationCause::dummy(), param_env).normalize(projection)
+            && obligations.iter().all(|obligation| infcx.predicate_must_hold_modulo_regions(obligation))
             && infcx.can_eq(param_env, deref_target, target_ty)
         {
             let help = if let hir::Mutability::Mut = needs_mut

Diff for: tests/ui/mismatched_types/suggest-option-asderef-inference-var.rs

@@ -0,0 +1,9 @@
+fn deref_int(a: &i32) -> i32 {
+    *a
+}
+
+fn main() {
+    // https://github.com/rust-lang/rust/issues/112293
+    let _has_inference_vars: Option<i32> = Some(0).map(deref_int);
+    //~^ ERROR type mismatch in function arguments
+}

Diff for: tests/ui/mismatched_types/suggest-option-asderef-inference-var.stderr

@@ -0,0 +1,24 @@
+error[E0631]: type mismatch in function arguments
+  --> $DIR/suggest-option-asderef-inference-var.rs:7:56
+   |
+LL | fn deref_int(a: &i32) -> i32 {
+   | ---------------------------- found signature defined here
+...
+LL |     let _has_inference_vars: Option<i32> = Some(0).map(deref_int);
+   |                                                    --- ^^^^^^^^^ expected due to this
+   |                                                    |
+   |                                                    required by a bound introduced by this call
+   |
+   = note: expected function signature `fn({integer}) -> _`
+              found function signature `for<'a> fn(&'a i32) -> _`
+note: required by a bound in `Option::<T>::map`
+  --> $SRC_DIR/core/src/option.rs:LL:COL
+help: do not borrow the argument
+   |
+LL - fn deref_int(a: &i32) -> i32 {
+LL + fn deref_int(a: i32) -> i32 {
+   |
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0631`.