add a new lint bytes_nth

clippy_lints/src/lib.rs

@@ -734,6 +734,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &mem_replace::MEM_REPLACE_WITH_DEFAULT,
         &mem_replace::MEM_REPLACE_WITH_UNINIT,
         &methods::BIND_INSTEAD_OF_MAP,
+        &methods::BYTES_NTH,
         &methods::CHARS_LAST_CMP,
         &methods::CHARS_NEXT_CMP,
         &methods::CLONE_DOUBLE_REF,
@@ -1531,6 +1532,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&mem_replace::MEM_REPLACE_WITH_DEFAULT),
         LintId::of(&mem_replace::MEM_REPLACE_WITH_UNINIT),
         LintId::of(&methods::BIND_INSTEAD_OF_MAP),
+        LintId::of(&methods::BYTES_NTH),
         LintId::of(&methods::CHARS_LAST_CMP),
         LintId::of(&methods::CHARS_NEXT_CMP),
         LintId::of(&methods::CLONE_DOUBLE_REF),
@@ -1749,6 +1751,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&matches::SINGLE_MATCH),
         LintId::of(&mem_replace::MEM_REPLACE_OPTION_WITH_NONE),
         LintId::of(&mem_replace::MEM_REPLACE_WITH_DEFAULT),
+        LintId::of(&methods::BYTES_NTH),
         LintId::of(&methods::CHARS_LAST_CMP),
         LintId::of(&methods::CHARS_NEXT_CMP),
         LintId::of(&methods::FROM_ITER_INSTEAD_OF_COLLECT),

clippy_lints/src/methods/bytes_nth.rs

@@ -0,0 +1,39 @@
+use crate::utils::{is_type_diagnostic_item, snippet_with_applicability, span_lint_and_sugg};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::LateContext;
+use rustc_span::sym;
+
+use super::BYTES_NTH;
+
+pub(super) fn lints<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>, iter_args: &'tcx [Expr<'tcx>]) {
+    if_chain! {
+        if let ExprKind::MethodCall(_, _, ref args, _) = expr.kind;
+        let ty = cx.typeck_results().expr_ty(&iter_args[0]).peel_refs();
+        let caller_type = if is_type_diagnostic_item(cx, ty, sym::string_type) {
+            Some("String")
+        } else if ty.is_str() {
+            Some("str")
+        } else {
+            None
+        };
+        if let Some(caller_type) = caller_type;
+        then {
+            let mut applicability = Applicability::MachineApplicable;
+            span_lint_and_sugg(
+                cx,
+                BYTES_NTH,
+                expr.span,
+                &format!("called `.byte().nth()` on a `{}`", caller_type),
+                "try calling `.as_bytes().get()`",
+                format!(
+                    "{}.as_bytes().get({})",
+                    snippet_with_applicability(cx, iter_args[0].span, "..", &mut applicability),
+                    snippet_with_applicability(cx, args[1].span, "..", &mut applicability)
+                ),
+                applicability,
+            );
+        }
+    }
+}

clippy_lints/src/methods/mod.rs

@@ -1,4 +1,5 @@
 mod bind_instead_of_map;
+mod bytes_nth;
 mod filter_map_identity;
 mod inefficient_to_string;
 mod inspect_for_each;
@@ -1490,6 +1491,28 @@ declare_clippy_lint! {
     "call to `filter_map` where `flatten` is sufficient"
 }
 
+declare_clippy_lint! {
+    /// **What it does:** Checks for the use of `.bytes().nth()`.
+    ///
+    /// **Why is this bad?** `.as_bytes().get()` is more efficient and more
+    /// readable.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// // Bad
+    /// let _ = "Hello".bytes().nth(3);;
+    ///
+    /// // Good
+    /// let _ = "Hello".as_bytes().get(3);
+    /// ```
+    pub BYTES_NTH,
+    style,
+    "replace `.bytes().nth()` with `.as_bytes().get()`"
+}
+
 pub struct Methods {
     msrv: Option<RustcVersion>,
 }
@@ -1537,6 +1560,7 @@ impl_lint_pass!(Methods => [
     ITER_NEXT_SLICE,
     ITER_NTH,
     ITER_NTH_ZERO,
+    BYTES_NTH,
     ITER_SKIP_NEXT,
     GET_UNWRAP,
     STRING_EXTEND_CHARS,
@@ -1614,6 +1638,7 @@ impl<'tcx> LateLintPass<'tcx> for Methods {
             ["extend", ..] => lint_extend(cx, expr, arg_lists[0]),
             ["nth", "iter"] => lint_iter_nth(cx, expr, &arg_lists, false),
             ["nth", "iter_mut"] => lint_iter_nth(cx, expr, &arg_lists, true),
+            ["nth", "bytes"] => bytes_nth::lints(cx, expr, &arg_lists[1]),
             ["nth", ..] => lint_iter_nth_zero(cx, expr, arg_lists[0]),
             ["step_by", ..] => lint_step_by(cx, expr, arg_lists[0]),
             ["next", "skip"] => lint_iter_skip_next(cx, expr, arg_lists[1]),