From 7e53e27dfdb5ce4cda44912fcc6c288c34ac7d91 Mon Sep 17 00:00:00 2001
From: Krishna Sundarram <krishnasundarram@monzo.com>
Date: Thu, 16 Feb 2023 16:41:04 +0000
Subject: [PATCH] Stop bytes_nth from suggesting code that does not compile

---
 clippy_lints/src/methods/bytes_nth.rs | 42 ++++++++++++++++++---------
 tests/ui/bytes_nth.fixed              |  6 ++--
 tests/ui/bytes_nth.rs                 |  2 +-
 tests/ui/bytes_nth.stderr             | 10 +++----
 4 files changed, 38 insertions(+), 22 deletions(-)

diff --git a/clippy_lints/src/methods/bytes_nth.rs b/clippy_lints/src/methods/bytes_nth.rs
index d512cc4eeae..c5fc145b289 100644
--- a/clippy_lints/src/methods/bytes_nth.rs
+++ b/clippy_lints/src/methods/bytes_nth.rs
@@ -5,6 +5,8 @@ use rustc_errors::Applicability;
 use rustc_hir::{Expr, LangItem};
 use rustc_lint::LateContext;
 
+use crate::methods::method_call;
+
 use super::BYTES_NTH;
 
 pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>, recv: &'tcx Expr<'tcx>, n_arg: &'tcx Expr<'tcx>) {
@@ -16,18 +18,32 @@ pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>, recv: &'tcx E
     } else {
         return;
     };
+
     let mut applicability = Applicability::MachineApplicable;
-    span_lint_and_sugg(
-        cx,
-        BYTES_NTH,
-        expr.span,
-        &format!("called `.bytes().nth()` on a `{caller_type}`"),
-        "try",
-        format!(
-            "{}.as_bytes().get({})",
-            snippet_with_applicability(cx, recv.span, "..", &mut applicability),
-            snippet_with_applicability(cx, n_arg.span, "..", &mut applicability)
-        ),
-        applicability,
-    );
+    let receiver = snippet_with_applicability(cx, recv.span, "..", &mut applicability);
+    let n = snippet_with_applicability(cx, n_arg.span, "..", &mut applicability);
+
+    if let Some(parent) = clippy_utils::get_parent_expr(cx, expr)
+      && let Some((name, _, _, _, _)) = method_call(parent)
+      && name == "unwrap" {
+        span_lint_and_sugg(
+            cx,
+            BYTES_NTH,
+            parent.span,
+            &format!("called `.bytes().nth().unwrap()` on a `{caller_type}`"),
+            "try",
+            format!("{receiver}.as_bytes()[{n}]",),
+            applicability
+        );
+    } else {
+        span_lint_and_sugg(
+            cx,
+            BYTES_NTH,
+            expr.span,
+            &format!("called `.bytes().nth()` on a `{caller_type}`"),
+            "try",
+            format!("{receiver}.as_bytes().get({n}).copied()"), 
+            applicability
+        );
+    };
 }
diff --git a/tests/ui/bytes_nth.fixed b/tests/ui/bytes_nth.fixed
index b1fb2e16bd5..a35c679afb7 100644
--- a/tests/ui/bytes_nth.fixed
+++ b/tests/ui/bytes_nth.fixed
@@ -5,7 +5,7 @@
 
 fn main() {
     let s = String::from("String");
-    let _ = s.as_bytes().get(3);
-    let _ = &s.as_bytes().get(3);
-    let _ = s[..].as_bytes().get(3);
+    let _ = s.as_bytes().get(3).copied();
+    let _ = &s.as_bytes()[3];
+    let _ = s[..].as_bytes().get(3).copied();
 }
diff --git a/tests/ui/bytes_nth.rs b/tests/ui/bytes_nth.rs
index 034c54e6a42..1ecffea5303 100644
--- a/tests/ui/bytes_nth.rs
+++ b/tests/ui/bytes_nth.rs
@@ -6,6 +6,6 @@
 fn main() {
     let s = String::from("String");
     let _ = s.bytes().nth(3);
-    let _ = &s.bytes().nth(3);
+    let _ = &s.bytes().nth(3).unwrap();
     let _ = s[..].bytes().nth(3);
 }
diff --git a/tests/ui/bytes_nth.stderr b/tests/ui/bytes_nth.stderr
index 9851d4791d8..e8b15027829 100644
--- a/tests/ui/bytes_nth.stderr
+++ b/tests/ui/bytes_nth.stderr
@@ -2,21 +2,21 @@ error: called `.bytes().nth()` on a `String`
   --> $DIR/bytes_nth.rs:8:13
    |
 LL |     let _ = s.bytes().nth(3);
-   |             ^^^^^^^^^^^^^^^^ help: try: `s.as_bytes().get(3)`
+   |             ^^^^^^^^^^^^^^^^ help: try: `s.as_bytes().get(3).copied()`
    |
    = note: `-D clippy::bytes-nth` implied by `-D warnings`
 
-error: called `.bytes().nth()` on a `String`
+error: called `.bytes().nth().unwrap()` on a `String`
   --> $DIR/bytes_nth.rs:9:14
    |
-LL |     let _ = &s.bytes().nth(3);
-   |              ^^^^^^^^^^^^^^^^ help: try: `s.as_bytes().get(3)`
+LL |     let _ = &s.bytes().nth(3).unwrap();
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `s.as_bytes()[3]`
 
 error: called `.bytes().nth()` on a `str`
   --> $DIR/bytes_nth.rs:10:13
    |
 LL |     let _ = s[..].bytes().nth(3);
-   |             ^^^^^^^^^^^^^^^^^^^^ help: try: `s[..].as_bytes().get(3)`
+   |             ^^^^^^^^^^^^^^^^^^^^ help: try: `s[..].as_bytes().get(3).copied()`
 
 error: aborting due to 3 previous errors