New lint

clippy_lints/src/declared_lints.rs

@@ -503,6 +503,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::methods::WAKER_CLONE_WAKE_INFO,
     crate::methods::WRONG_SELF_CONVENTION_INFO,
     crate::methods::ZST_OFFSET_INFO,
+    crate::methods::SLICE_AS_BYTES_INFO,
     crate::min_ident_chars::MIN_IDENT_CHARS_INFO,
     crate::minmax::MIN_MAX_INFO,
     crate::misc::SHORT_CIRCUIT_STATEMENT_INFO,

clippy_lints/src/methods/mod.rs

@@ -102,6 +102,7 @@ mod single_char_add_str;
 mod single_char_insert_string;
 mod single_char_push_string;
 mod skip_while_next;
+mod slice_as_bytes;
 mod stable_sort_primitive;
 mod str_split;
 mod str_splitn;
@@ -4363,6 +4364,34 @@ declare_clippy_lint! {
     "detect `repeat().take()` that can be replaced with `repeat_n()`"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for string slices immediantly followed by `as_bytes`.
+    ///
+    /// ### Why is this bad?
+    /// It involves doing an unnecessary UTF-8 alignment check which is less efficient, and can cause a panic.
+    ///
+    /// ### Known problems
+    /// In some cases, the UTF-8 validation and potential panic from string slicing may be required for
+    /// the code's correctness. If you need to ensure the slice boundaries fall on valid UTF-8 character
+    /// boundaries, the original form (`s[1..5].as_bytes()`) should be preferred.
+    ///
+    /// ### Example
+    /// ```rust
+    /// let s = "Lorem ipsum";
+    /// s[1..5].as_bytes();
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// let s = "Lorem ipsum";
+    /// &s.as_bytes()[1..5];
+    /// ```
+     #[clippy::version = "1.72.0"]
+     pub SLICE_AS_BYTES,
+     pedantic,
+     "slicing a string and immediately calling as_bytes is less efficient and can lead to panics"
+}
+
 pub struct Methods {
     avoid_breaking_exported_api: bool,
     msrv: Msrv,
@@ -4531,6 +4560,7 @@ impl_lint_pass!(Methods => [
     DOUBLE_ENDED_ITERATOR_LAST,
     USELESS_NONZERO_NEW_UNCHECKED,
     MANUAL_REPEAT_N,
+    SLICE_AS_BYTES,
 ]);
 
 /// Extracts a method call name, args, and `Span` of the method name.
@@ -4798,6 +4828,7 @@ impl Methods {
                     if let Some(("as_str", recv, [], as_str_span, _)) = method_call(recv) {
                         redundant_as_str::check(cx, expr, recv, as_str_span, span);
                     }
+                    slice_as_bytes::check(cx, expr, recv);
                 },
                 ("as_mut", []) => useless_asref::check(cx, expr, "as_mut", recv),
                 ("as_ptr", []) => manual_c_str_literals::check_as_ptr(cx, expr, recv, &self.msrv),

clippy_lints/src/methods/slice_as_bytes.rs

@@ -0,0 +1,30 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::source::snippet_with_applicability;
+use clippy_utils::ty::is_type_lang_item;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind, LangItem, is_range_literal};
+use rustc_lint::LateContext;
+
+use super::SLICE_AS_BYTES;
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, recv: &Expr<'_>) {
+    if let ExprKind::Index(indexed, index) = recv.kind
+        && is_range_literal(index)
+    {
+        let ty = cx.typeck_results().expr_ty(indexed).peel_refs();
+        if ty.is_str() || is_type_lang_item(cx, ty, LangItem::String) {
+            let mut applicability = Applicability::MaybeIncorrect;
+            let stringish = snippet_with_applicability(cx, indexed.span, "..", &mut applicability);
+            let range = snippet_with_applicability(cx, index.span, "..", &mut applicability);
+            span_lint_and_sugg(
+                cx,
+                SLICE_AS_BYTES,
+                expr.span,
+                "calling `as_bytes` after slicing a string",
+                "try",
+                format!("&{stringish}.as_bytes()[{range}]"),
+                applicability,
+            );
+        }
+    }
+}