Implement suggestion

Co-authored-by: Esteban Kuber <estebank@users.noreply.github.com>

compiler/rustc_hir_analysis/src/structured_errors/missing_cast_for_variadic_arg.rs

@@ -41,27 +41,19 @@ impl<'tcx> StructuredDiagnostic<'tcx> for MissingCastForVariadicArg<'tcx, '_> {
             err.downgrade_to_delayed_bug();
         }
 
-        if let Ok(snippet) = self.sess.source_map().span_to_snippet(self.span) {
-            if self.ty.is_fn() {
-                err.span_suggestion(
-                    self.span,
-                    "cast the value into a function pointer",
-                    format!("{} as {}", snippet, self.cast_ty),
-                    Applicability::MachineApplicable,
-                )
-                .help("a function item is zero-sized and needs to be casted into a function pointer to be used in FFI")
+            let msg = if self.ty.is_fn() {
+                err.help("a function item is zero-sized and needs to be casted into a function pointer to be used in FFI")
                 .note("for more information on function items, visit https://doc.rust-lang.org/reference/types/function-item.html");
+                "cast the value into a function pointer".to_string()
             } else {
-                err.span_suggestion(
-                    self.span,
-                    format!("cast the value to `{}`", self.cast_ty),
-                    format!("{} as {}", snippet, self.cast_ty),
-                    Applicability::MachineApplicable,
-                );
-            }
-        } else {
-            err.help(format!("cast the value into `{}`", self.cast_ty));
-        }
+                format!("cast the value to `{}`", self.cast_ty)
+            };
+            err.span_suggestion_verbose(
+                self.span.shrink_to_hi(),
+                msg,
+                format!(" as {}", snippet, self.cast_ty),
+                Applicability::MachineApplicable,
+            );
 
         err
     }