Improve fuzzy_provenance_casts lint diagnostics

Use `multipart_suggestion` instead of getting a snippet.

compiler/rustc_typeck/src/check/cast.rs

@@ -1041,16 +1041,12 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                     self.expr_ty, self.cast_ty
                 ));
                 let msg = "use `.with_addr()` to adjust a valid pointer in the same allocation, to this address";
-                if let Ok(snippet) = fcx.tcx.sess.source_map().span_to_snippet(self.expr_span) {
-                    err.span_suggestion(
-                        self.span,
-                        msg,
-                        format!("(...).with_addr({snippet})"),
-                        Applicability::HasPlaceholders,
-                    );
-                } else {
-                    err.help(msg);
-                }
+                let suggestions = vec![
+                    (self.expr_span.shrink_to_lo(), String::from("(...).with_addr(")),
+                    (self.expr_span.shrink_to_hi().to(self.cast_span), String::from(")")),
+                ];
+
+                err.multipart_suggestion(msg, suggestions, Applicability::MaybeIncorrect);
                 err.help(
                     "if you can't comply with strict provenance and don't have a pointer with \
                     the correct provenance you can use `std::ptr::from_exposed_addr()` instead"

src/test/ui/lint/lint-strict-provenance-fuzzy-casts.stderr

@@ -13,7 +13,7 @@ LL | #![deny(fuzzy_provenance_casts)]
 help: use `.with_addr()` to adjust a valid pointer in the same allocation, to this address
    |
 LL |     let dangling = (...).with_addr(16_usize);
-   |                    ~~~~~~~~~~~~~~~~~~~~~~~~~
+   |                    ++++++++++++++++        ~
 
 error: aborting due to previous error