Add lint to suggest as_chunks over chunks_exact with constant

CHANGELOG.md

@@ -6252,6 +6252,7 @@ Released 2018-09-13
 [`chars_last_cmp`]: https://rust-lang.github.io/rust-clippy/master/index.html#chars_last_cmp
 [`chars_next_cmp`]: https://rust-lang.github.io/rust-clippy/master/index.html#chars_next_cmp
 [`checked_conversions`]: https://rust-lang.github.io/rust-clippy/master/index.html#checked_conversions
+[`chunks_exact_with_const_size`]: https://rust-lang.github.io/rust-clippy/master/index.html#chunks_exact_with_const_size
 [`clear_with_drain`]: https://rust-lang.github.io/rust-clippy/master/index.html#clear_with_drain
 [`clone_double_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#clone_double_ref
 [`clone_on_copy`]: https://rust-lang.github.io/rust-clippy/master/index.html#clone_on_copy

clippy_lints/src/declared_lints.rs

@@ -352,6 +352,7 @@ pub static LINTS: &[&::declare_clippy_lint::LintInfo] = &[
     crate::methods::CASE_SENSITIVE_FILE_EXTENSION_COMPARISONS_INFO,
     crate::methods::CHARS_LAST_CMP_INFO,
     crate::methods::CHARS_NEXT_CMP_INFO,
+    crate::methods::CHUNKS_EXACT_WITH_CONST_SIZE_INFO,
     crate::methods::CLEAR_WITH_DRAIN_INFO,
     crate::methods::CLONED_INSTEAD_OF_COPIED_INFO,
     crate::methods::CLONE_ON_COPY_INFO,

clippy_lints/src/methods/chunks_exact_with_const_size.rs

@@ -0,0 +1,58 @@
+use clippy_utils::consts::{ConstEvalCtxt, Constant};
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::msrvs::{self, Msrv};
+use clippy_utils::source::snippet_with_applicability;
+use clippy_utils::sym;
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_span::{Span, Symbol};
+
+use super::CHUNKS_EXACT_WITH_CONST_SIZE;
+
+pub(super) fn check(
+    cx: &LateContext<'_>,
+    recv: &Expr<'_>,
+    arg: &Expr<'_>,
+    call_span: Span,
+    method_name: Symbol,
+    msrv: Msrv,
+) {
+    // Check if receiver is slice-like
+    if !cx.typeck_results().expr_ty_adjusted(recv).peel_refs().is_slice() {
+        return;
+    }
+
+    // Check if argument is a constant
+    let constant_eval = ConstEvalCtxt::new(cx);
+    if let Some(Constant::Int(_)) = constant_eval.eval(arg) {
+        // Check for Rust version - only check after we know we would emit a lint
+        if !msrv.meets(cx, msrvs::AS_CHUNKS) {
+            return;
+        }
+
+        // Determine the suggested method name
+        let suggestion_method = if method_name == sym::chunks_exact_mut {
+            "as_chunks_mut"
+        } else {
+            "as_chunks"
+        };
+
+        // Build the suggestion with proper applicability tracking
+        let mut applicability = Applicability::MachineApplicable;
+        let arg_str = snippet_with_applicability(cx, arg.span, "_", &mut applicability);
+
+        // Suggestion replaces just "chunks_exact(N)" with "as_chunks::<N>().0.iter()"
+        let suggestion = format!("{suggestion_method}::<{arg_str}>().0.iter()");
+
+        span_lint_and_sugg(
+            cx,
+            CHUNKS_EXACT_WITH_CONST_SIZE,
+            call_span,
+            format!("using `{method_name}` with a constant chunk size"),
+            "consider using `as_chunks` instead",
+            suggestion,
+            applicability,
+        );
+    }
+}

clippy_lints/src/methods/mod.rs

@@ -9,6 +9,7 @@ mod chars_last_cmp;
 mod chars_last_cmp_with_unwrap;
 mod chars_next_cmp;
 mod chars_next_cmp_with_unwrap;
+mod chunks_exact_with_const_size;
 mod clear_with_drain;
 mod clone_on_copy;
 mod clone_on_ref_ptr;
@@ -2087,6 +2088,32 @@ declare_clippy_lint! {
     "replace `.bytes().nth()` with `.as_bytes().get()`"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for usage of `chunks_exact` or `chunks_exact_mut` with a constant chunk size.
+    ///
+    /// ### Why is this bad?
+    /// `as_chunks` provides better ergonomics and type safety by returning arrays instead of slices.
+    /// It was stabilized in Rust 1.88.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// let slice = [1, 2, 3, 4, 5, 6];
+    /// let mut it = slice.chunks_exact(2);
+    /// for chunk in it {}
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// let slice = [1, 2, 3, 4, 5, 6];
+    /// let (chunks, remainder) = slice.as_chunks::<2>();
+    /// for chunk in chunks {}
+    /// ```
+    #[clippy::version = "1.93.0"]
+    pub CHUNKS_EXACT_WITH_CONST_SIZE,
+    style,
+    "using `chunks_exact` with constant when `as_chunks` is more ergonomic"
+}
+
 declare_clippy_lint! {
     /// ### What it does
     /// Checks for the usage of `_.to_owned()`, `vec.to_vec()`, or similar when calling `_.clone()` would be clearer.
@@ -4787,6 +4814,7 @@ impl_lint_pass!(Methods => [
     ITER_NTH,
     ITER_NTH_ZERO,
     BYTES_NTH,
+    CHUNKS_EXACT_WITH_CONST_SIZE,
     ITER_SKIP_NEXT,
     GET_UNWRAP,
     GET_LAST_WITH_LEN,
@@ -5043,6 +5071,9 @@ impl Methods {
                         _ => {},
                     }
                 },
+                (name @ (sym::chunks_exact | sym::chunks_exact_mut), [arg]) => {
+                    chunks_exact_with_const_size::check(cx, recv, arg, call_span, name, self.msrv);
+                },
                 (sym::and_then, [arg]) => {
                     let biom_option_linted = bind_instead_of_map::check_and_then_some(cx, expr, recv, arg);
                     let biom_result_linted = bind_instead_of_map::check_and_then_ok(cx, expr, recv, arg);

clippy_utils/src/msrvs.rs

@@ -23,7 +23,7 @@ macro_rules! msrv_aliases {
 
 // names may refer to stabilized feature flags or library items
 msrv_aliases! {
-    1,88,0 { LET_CHAINS }
+    1,88,0 { LET_CHAINS, AS_CHUNKS }
     1,87,0 { OS_STR_DISPLAY, INT_MIDPOINT, CONST_CHAR_IS_DIGIT, UNSIGNED_IS_MULTIPLE_OF, INTEGER_SIGN_CAST }
     1,85,0 { UINT_FLOAT_MIDPOINT, CONST_SIZE_OF_VAL }
     1,84,0 { CONST_OPTION_AS_SLICE, MANUAL_DANGLING_PTR }

clippy_utils/src/sym.rs

@@ -111,6 +111,8 @@ generate! {
     checked_pow,
     checked_rem_euclid,
     checked_sub,
+    chunks_exact,
+    chunks_exact_mut,
     clamp,
     clippy_utils,
     clone_into,

tests/ui/chunks_exact_with_const_size.fixed

@@ -0,0 +1,34 @@
+#![warn(clippy::chunks_exact_with_const_size)]
+#![allow(unused)]
+#![allow(clippy::iter_cloned_collect)]
+
+fn main() {
+    let slice = [1, 2, 3, 4, 5, 6, 7, 8];
+
+    // Should trigger lint - literal constant
+    let result = slice.as_chunks::<4>().0.iter();
+    //~^ chunks_exact_with_const_size
+
+    // Should trigger lint - const value
+    const CHUNK_SIZE: usize = 4;
+    let result = slice.as_chunks::<CHUNK_SIZE>().0.iter();
+    //~^ chunks_exact_with_const_size
+
+    // Should NOT trigger - runtime value
+    let size = 4;
+    let mut it = slice.chunks_exact(size);
+    for chunk in it {}
+
+    // Should trigger lint - simple iteration
+    let result = slice.as_chunks::<3>().0.iter();
+    //~^ chunks_exact_with_const_size
+
+    // Should trigger - mutable variant
+    let mut arr = [1, 2, 3, 4, 5, 6, 7, 8];
+    let result = arr.as_chunks_mut::<4>().0.iter();
+    //~^ chunks_exact_with_const_size
+
+    // Should trigger - multiline expression
+    let result = slice.iter().copied().collect::<Vec<_>>().as_chunks::<2>().0.iter();
+    //~^ chunks_exact_with_const_size
+}

tests/ui/chunks_exact_with_const_size.rs

@@ -0,0 +1,34 @@
+#![warn(clippy::chunks_exact_with_const_size)]
+#![allow(unused)]
+#![allow(clippy::iter_cloned_collect)]
+
+fn main() {
+    let slice = [1, 2, 3, 4, 5, 6, 7, 8];
+
+    // Should trigger lint - literal constant
+    let result = slice.chunks_exact(4);
+    //~^ chunks_exact_with_const_size
+
+    // Should trigger lint - const value
+    const CHUNK_SIZE: usize = 4;
+    let result = slice.chunks_exact(CHUNK_SIZE);
+    //~^ chunks_exact_with_const_size
+
+    // Should NOT trigger - runtime value
+    let size = 4;
+    let mut it = slice.chunks_exact(size);
+    for chunk in it {}
+
+    // Should trigger lint - simple iteration
+    let result = slice.chunks_exact(3);
+    //~^ chunks_exact_with_const_size
+
+    // Should trigger - mutable variant
+    let mut arr = [1, 2, 3, 4, 5, 6, 7, 8];
+    let result = arr.chunks_exact_mut(4);
+    //~^ chunks_exact_with_const_size
+
+    // Should trigger - multiline expression
+    let result = slice.iter().copied().collect::<Vec<_>>().chunks_exact(2);
+    //~^ chunks_exact_with_const_size
+}

tests/ui/chunks_exact_with_const_size.stderr

@@ -0,0 +1,35 @@
+error: using `chunks_exact` with a constant chunk size
+  --> tests/ui/chunks_exact_with_const_size.rs:9:24
+   |
+LL |     let result = slice.chunks_exact(4);
+   |                        ^^^^^^^^^^^^^^^ help: consider using `as_chunks` instead: `as_chunks::<4>().0.iter()`
+   |
+   = note: `-D clippy::chunks-exact-with-const-size` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::chunks_exact_with_const_size)]`
+
+error: using `chunks_exact` with a constant chunk size
+  --> tests/ui/chunks_exact_with_const_size.rs:14:24
+   |
+LL |     let result = slice.chunks_exact(CHUNK_SIZE);
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `as_chunks` instead: `as_chunks::<CHUNK_SIZE>().0.iter()`
+
+error: using `chunks_exact` with a constant chunk size
+  --> tests/ui/chunks_exact_with_const_size.rs:23:24
+   |
+LL |     let result = slice.chunks_exact(3);
+   |                        ^^^^^^^^^^^^^^^ help: consider using `as_chunks` instead: `as_chunks::<3>().0.iter()`
+
+error: using `chunks_exact_mut` with a constant chunk size
+  --> tests/ui/chunks_exact_with_const_size.rs:28:22
+   |
+LL |     let result = arr.chunks_exact_mut(4);
+   |                      ^^^^^^^^^^^^^^^^^^^ help: consider using `as_chunks` instead: `as_chunks_mut::<4>().0.iter()`
+
+error: using `chunks_exact` with a constant chunk size
+  --> tests/ui/chunks_exact_with_const_size.rs:32:60
+   |
+LL |     let result = slice.iter().copied().collect::<Vec<_>>().chunks_exact(2);
+   |                                                            ^^^^^^^^^^^^^^^ help: consider using `as_chunks` instead: `as_chunks::<2>().0.iter()`
+
+error: aborting due to 5 previous errors
+