Set diagnostic applicability based on array length

Author: memoryruins

src/librustc_typeck/check/mod.rs

@@ -3344,12 +3344,24 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                                 }
                             };
                     }
-                    ty::Array(ty, _) if ty.is_numeric() => {
-                        let base = self.tcx.hir.node_to_pretty_string(base.id);
-                        let msg = format!("attempting to use tuple indexing on an array; try");
-                        let suggestion = format!("{}[{}]", base, field);
-                        err.span_suggestion(field.span, &msg, suggestion);
-                    },
+                    ty::Array(_, len) => {
+                        if let (Some(len), Ok(user_index)) = (
+                            len.assert_usize(self.tcx),
+                            field.as_str().parse::<u64>()
+                        ) {
+                            let base = self.tcx.hir.node_to_pretty_string(base.id);
+                            let help = "instead of using tuple indexing, use array indexing";
+                            let suggestion = format!("{}[{}]", base, field);
+                            let applicability = if len < user_index {
+                                Applicability::MachineApplicable
+                            } else {
+                                Applicability::MaybeIncorrect
+                            };
+                            err.span_suggestion_with_applicability(
+                                field.span, help, suggestion, applicability
+                            );
+                        }
+                    }
                     ty::RawPtr(..) => {
                         let base = self.tcx.hir.node_to_pretty_string(base.id);
                         let msg = format!("`{}` is a native pointer; try dereferencing it", base);

src/test/ui/issues/issue-53712.rs

@@ -4,6 +4,6 @@ fn main() {
     let arr = [10, 20, 30, 40, 50];
     arr.0;
     //~^ ERROR no field `0` on type `[{integer}; 5]` [E0609]
-    //~| HELP attempting to use tuple indexing on an array; try
+    //~| HELP instead of using tuple indexing, use array indexing
     //~| SUGGESTION arr[0]
 }

src/test/ui/issues/issue-53712.stderr

@@ -2,7 +2,7 @@ error[E0609]: no field `0` on type `[{integer}; 5]`
   --> $DIR/issue-53712.rs:5:9
    |
 LL |     arr.0;
-   |         ^ help: attempting to use tuple indexing on an array; try: `arr[0]`
+   |         ^ help: instead of using tuple indexing, use array indexing: `arr[0]`
 
 error: aborting due to previous error