Add slice_as_bytes lint

Detects patterns like
```rust
s[1..5].as_bytes();
```
and suggest replacing with
```rust
&s.as_bytes()[1..5];
```

Author: A-Walrus

CHANGELOG.md

@@ -5170,6 +5170,7 @@ Released 2018-09-13
 [`size_of_in_element_count`]: https://rust-lang.github.io/rust-clippy/master/index.html#size_of_in_element_count
 [`size_of_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#size_of_ref
 [`skip_while_next`]: https://rust-lang.github.io/rust-clippy/master/index.html#skip_while_next
+[`slice_as_bytes`]: https://rust-lang.github.io/rust-clippy/master/index.html#slice_as_bytes
 [`slow_vector_initialization`]: https://rust-lang.github.io/rust-clippy/master/index.html#slow_vector_initialization
 [`stable_sort_primitive`]: https://rust-lang.github.io/rust-clippy/master/index.html#stable_sort_primitive
 [`std_instead_of_alloc`]: https://rust-lang.github.io/rust-clippy/master/index.html#std_instead_of_alloc

clippy_lints/src/declared_lints.rs

@@ -575,6 +575,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::single_range_in_vec_init::SINGLE_RANGE_IN_VEC_INIT_INFO,
     crate::size_of_in_element_count::SIZE_OF_IN_ELEMENT_COUNT_INFO,
     crate::size_of_ref::SIZE_OF_REF_INFO,
+    crate::slice_as_bytes::SLICE_AS_BYTES_INFO,
     crate::slow_vector_initialization::SLOW_VECTOR_INITIALIZATION_INFO,
     crate::std_instead_of_core::ALLOC_INSTEAD_OF_CORE_INFO,
     crate::std_instead_of_core::STD_INSTEAD_OF_ALLOC_INFO,

clippy_lints/src/lib.rs

@@ -292,6 +292,7 @@ mod single_component_path_imports;
 mod single_range_in_vec_init;
 mod size_of_in_element_count;
 mod size_of_ref;
+mod slice_as_bytes;
 mod slow_vector_initialization;
 mod std_instead_of_core;
 mod strings;
@@ -1050,6 +1051,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(move |_| Box::new(large_stack_frames::LargeStackFrames::new(stack_size_threshold)));
     store.register_late_pass(|_| Box::new(single_range_in_vec_init::SingleRangeInVecInit));
     store.register_late_pass(|_| Box::new(incorrect_impls::IncorrectImpls));
+    store.register_late_pass(|_| Box::new(slice_as_bytes::SliceAsBytes));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/slice_as_bytes.rs

@@ -0,0 +1,56 @@
+use clippy_utils::{diagnostics::span_lint_and_sugg, source::snippet_with_applicability, ty::is_type_lang_item};
+use rustc_errors::Applicability::MachineApplicable;
+use rustc_hir::{Expr, ExprKind, LangItem, QPath};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for string slices immediantly followed by `as_bytes`.
+    /// ### Why is this bad?
+    /// This involves doing an unnecessary UTF-8 alignment check which is less efficient, and can cause a panic.
+    /// ### Example
+    /// ```rust
+    /// s[1..5].as_bytes();
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// &s.as_bytes()[1..5];
+    /// ```
+    #[clippy::version = "1.72.0"]
+    pub SLICE_AS_BYTES,
+    nursery,
+    "slicing a string and immediantly calling as_bytes is less efficient and can lead to panics"
+}
+declare_lint_pass!(SliceAsBytes => [SLICE_AS_BYTES]);
+
+impl<'tcx> LateLintPass<'tcx> for SliceAsBytes {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
+        if let ExprKind::MethodCall(path, recv, [], _) = expr.kind && path.ident.name == sym!(as_bytes){
+            if let ExprKind::Index(indexed, index) = recv.kind {
+                use LangItem::{Range,RangeFrom,RangeFull,RangeTo,RangeToInclusive};
+                if let ExprKind::Struct(
+                    QPath::LangItem(Range | RangeFrom | RangeTo | RangeToInclusive | RangeFull, ..),
+                    ..,
+                ) = index.kind
+                {
+                    let ty = cx.typeck_results().expr_ty(indexed).peel_refs();
+                    let is_str = ty.is_str();
+                                        let is_string = is_type_lang_item(cx, ty, LangItem::String);
+                                        if  is_str|| is_string
+                    {
+                        let mut applicability = MachineApplicable;
+                        let stringish = snippet_with_applicability(cx,indexed.span,"..",&mut applicability);
+                        let range = snippet_with_applicability(cx,index.span,"..",&mut applicability);
+                        let sugg = format!("&{stringish}.as_bytes()[{range}]");
+                        let type_name = if is_str {"str"} else {"String"};
+                        let msg = format!("Slicing a {type_name} before calling `as_bytes` results in needless UTF-8 alignment checks, and has the possiblity of panicking");
+                        let sp = expr.span;
+                        span_lint_and_sugg(cx,SLICE_AS_BYTES ,sp , &msg, "try",sugg ,applicability);
+                    }
+
+                }
+            }
+        }
+    }
+}

tests/ui/slice_as_bytes.fixed

@@ -0,0 +1,12 @@
+//@run-rustfix
+#![allow(unused)]
+#![warn(clippy::slice_as_bytes)]
+
+fn main() {
+    let s = "Lorem ipsum";
+    let string: String = "dolor sit amet".to_owned();
+
+    let bytes = &s.as_bytes()[1..5];
+    let bytes = &string.as_bytes()[1..];
+    let bytes = &"consectetur adipiscing".as_bytes()[..=5];
+}

tests/ui/slice_as_bytes.rs

@@ -0,0 +1,12 @@
+//@run-rustfix
+#![allow(unused)]
+#![warn(clippy::slice_as_bytes)]
+
+fn main() {
+    let s = "Lorem ipsum";
+    let string: String = "dolor sit amet".to_owned();
+
+    let bytes = s[1..5].as_bytes();
+    let bytes = string[1..].as_bytes();
+    let bytes = "consectetur adipiscing"[..=5].as_bytes();
+}

tests/ui/slice_as_bytes.stderr

@@ -0,0 +1,22 @@
+error: Slicing a str before calling `as_bytes` results in needless UTF-8 alignment checks, and has the possiblity of panicking
+  --> $DIR/slice_as_bytes.rs:9:17
+   |
+LL |     let bytes = s[1..5].as_bytes();
+   |                 ^^^^^^^^^^^^^^^^^^ help: try: `&s.as_bytes()[1..5]`
+   |
+   = note: `-D clippy::slice-as-bytes` implied by `-D warnings`
+
+error: Slicing a String before calling `as_bytes` results in needless UTF-8 alignment checks, and has the possiblity of panicking
+  --> $DIR/slice_as_bytes.rs:10:17
+   |
+LL |     let bytes = string[1..].as_bytes();
+   |                 ^^^^^^^^^^^^^^^^^^^^^^ help: try: `&string.as_bytes()[1..]`
+
+error: Slicing a str before calling `as_bytes` results in needless UTF-8 alignment checks, and has the possiblity of panicking
+  --> $DIR/slice_as_bytes.rs:11:17
+   |
+LL |     let bytes = "consectetur adipiscing"[..=5].as_bytes();
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&"consectetur adipiscing".as_bytes()[..=5]`
+
+error: aborting due to 3 previous errors
+