Add new lints: const_sized_chunks_exact and const_sized_windows

Suggests using (nightly) `array_chunks` and `array_windows` (#6580)

changelog: new lint: [`const_sized_chunks_exact`]
changelog: new lint: [`const_sized_chunks_exact_mut`]
changelog: new lint: [`const_sized_windows`]

Co-authored-by: Michał Chodzikiewicz <[email protected]>

Author: immersum

CHANGELOG.md

@@ -5696,6 +5696,9 @@ Released 2018-09-13
 [`comparison_to_empty`]: https://rust-lang.github.io/rust-clippy/master/index.html#comparison_to_empty
 [`confusing_method_to_numeric_cast`]: https://rust-lang.github.io/rust-clippy/master/index.html#confusing_method_to_numeric_cast
 [`const_is_empty`]: https://rust-lang.github.io/rust-clippy/master/index.html#const_is_empty
+[`const_sized_chunks_exact`]: https://rust-lang.github.io/rust-clippy/master/index.html#const_sized_chunks_exact
+[`const_sized_chunks_exact_mut`]: https://rust-lang.github.io/rust-clippy/master/index.html#const_sized_chunks_exact_mut
+[`const_sized_windows`]: https://rust-lang.github.io/rust-clippy/master/index.html#const_sized_windows
 [`const_static_lifetime`]: https://rust-lang.github.io/rust-clippy/master/index.html#const_static_lifetime
 [`copy_iterator`]: https://rust-lang.github.io/rust-clippy/master/index.html#copy_iterator
 [`crate_in_macro_def`]: https://rust-lang.github.io/rust-clippy/master/index.html#crate_in_macro_def

clippy_lints/src/declared_lints.rs

@@ -266,6 +266,9 @@ pub static LINTS: &[&::declare_clippy_lint::LintInfo] = &[
     crate::literal_representation::UNUSUAL_BYTE_GROUPINGS_INFO,
     crate::literal_string_with_formatting_args::LITERAL_STRING_WITH_FORMATTING_ARGS_INFO,
     crate::loops::CHAR_INDICES_AS_BYTE_INDICES_INFO,
+    crate::loops::CONST_SIZED_CHUNKS_EXACT_INFO,
+    crate::loops::CONST_SIZED_CHUNKS_EXACT_MUT_INFO,
+    crate::loops::CONST_SIZED_WINDOWS_INFO,
     crate::loops::EMPTY_LOOP_INFO,
     crate::loops::EXPLICIT_COUNTER_LOOP_INFO,
     crate::loops::EXPLICIT_INTO_ITER_LOOP_INFO,

clippy_lints/src/loops/const_sized_chunks.rs

@@ -0,0 +1,60 @@
+use clippy_utils::consts::{ConstEvalCtxt, Constant};
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::expr_or_init;
+use clippy_utils::source::snippet;
+use clippy_utils::ty::is_slice_like;
+use itertools::Itertools;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind, Pat, PatKind};
+use rustc_lint::LateContext;
+
+use super::{CONST_SIZED_CHUNKS_EXACT, CONST_SIZED_CHUNKS_EXACT_MUT, CONST_SIZED_WINDOWS};
+
+/// Checks for the `/CONST_SIZED(_CHUNKS_EXACT(_MUT)?|_WINDOWS)/` lint.
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>, arg: &'tcx Expr<'_>) {
+    if !arg.span.from_expansion()
+        // The `for` loop pattern should be a binding.
+        && let PatKind::Binding(..) = pat.kind
+        // The `for` loop argument expression must be a method call.
+        && let ExprKind::MethodCall(method, self_arg, [arg], span) = arg.kind
+        // The receiver of the method call must be slice-like.
+        && is_slice_like(cx, cx.typeck_results().expr_ty(self_arg).peel_refs())
+        // The parameter to the method call must be a constant.
+        && let Some(Constant::Int(n)) = ConstEvalCtxt::new(cx).eval(expr_or_init(cx, arg))
+        // The number of elements should be limited.
+        && let Ok(n) = n.try_into() && n >= 1 && n <= 1 + 'z' as usize - 'a' as usize
+    {
+        let method = method.ident.name;
+        let new_method;
+
+        let lint = match method {
+            clippy_utils::sym::chunks_exact => {
+                new_method = clippy_utils::sym::array_chunks;
+                CONST_SIZED_CHUNKS_EXACT
+            },
+            clippy_utils::sym::chunks_exact_mut => {
+                new_method = clippy_utils::sym::array_chunks_mut;
+                CONST_SIZED_CHUNKS_EXACT_MUT
+            },
+            rustc_span::sym::windows => {
+                new_method = clippy_utils::sym::array_windows;
+                CONST_SIZED_WINDOWS
+            },
+            _ => return,
+        };
+
+        let bindings = ('a'..).take(n).join(", ");
+        let self_arg = snippet(cx, self_arg.span, "..");
+        let arg = snippet(cx, arg.span, "_");
+        let msg = format!("iterating over `{method}()` with constant parameter `{arg}`");
+
+        span_lint_and_then(cx, lint, span, msg, |diag| {
+            diag.span_suggestion_verbose(
+                span.with_lo(pat.span.lo()),
+                format!("use `{new_method}::<{n}>()` instead"),
+                format!("[{bindings}] in {self_arg}.{new_method}()"),
+                Applicability::Unspecified,
+            );
+        });
+    }
+}

clippy_lints/src/loops/mod.rs

@@ -1,4 +1,5 @@
 mod char_indices_as_byte_indices;
+mod const_sized_chunks;
 mod empty_loop;
 mod explicit_counter_loop;
 mod explicit_into_iter_loop;
@@ -784,6 +785,104 @@ declare_clippy_lint! {
     "using the character position yielded by `.chars().enumerate()` in a context where a byte index is expected"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for usage of `.chunks_exact()` on slices where the `chunk_size` is a constant and suggests
+    /// replacing it with its const generic equivalent, `.array_chunks()`, in a way that the value of the
+    /// const parameter `N` can be inferred.
+    ///
+    /// Specifically, in a for-loop, consuming the iterator over the (non-overlapping) chunks, the lint
+    /// suggests destructuring the chunks to allow for `N`, the number of elements in a chunk, to be inferred.
+    ///
+    /// ### Why is this bad?
+    /// When `.chunks_exact()` is used, a bounds check is required before an element can be accessed.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// let numbers = Vec::from_iter(0..10);
+    /// for chunk in numbers.chunks_exact(2) {
+    ///     println!("{n} is an odd number", n = chunk[1]);
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// let numbers = Vec::from_iter(0..10);
+    /// for [_, n] in numbers.array_chunks() {
+    ///     println!("{n} is an odd number");
+    /// }
+    /// ```
+    #[clippy::version = "1.89.0"]
+    pub CONST_SIZED_CHUNKS_EXACT,
+    nursery,
+    "calling `.chunks_exact()` with a constant `chunk_size` where `.array_chunks()` could be used instead"
+}
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for usage of `.chunks_exact_mut()` on slices where the `chunk_size` is a constant and suggests
+    /// replacing it with its const generic equivalent, `.array_chunks_mut()`, in a way that the value of the
+    /// const parameter `N` can be inferred.
+    ///
+    /// Specifically, in a for-loop, consuming the iterator over the (non-overlapping) chunks, the lint
+    /// suggests destructuring the chunks to allow for `N`, the number of elements in a chunk, to be inferred.
+    ///
+    /// ### Why is this bad?
+    /// When `.chunks_exact_mut()` is used, a bounds check is required before an element can be accessed.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// let mut numbers = Vec::from_iter(0..10);
+    /// for chunk in numbers.chunks_exact_mut(2) {
+    ///     chunk[1] += chunk[0];
+    ///     println!("{n} is an odd number", n = chunk[1]);
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// let mut numbers = Vec::from_iter(0..10);
+    /// for [m, n] in numbers.array_chunks_mut() {
+    ///     *n += *m;
+    ///     println!("{n} is an odd number");
+    /// }
+    /// ```
+    #[clippy::version = "1.89.0"]
+    pub CONST_SIZED_CHUNKS_EXACT_MUT,
+    nursery,
+    "calling `.chunks_exact_mut()` with a constant `chunk_size` where `.array_chunks_mut()` could be used instead"
+}
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for usage of `.windows()` on slices where the `size` is a constant and suggests replacing it with
+    /// its const generic equivalent, `.array_windows()`, in a way that the value of the const parameter `N` can
+    /// be inferred.
+    ///
+    /// Specifically, in a for-loop, consuming the windowed iterator over the overlapping chunks, the lint
+    /// suggests destructuring the chunks to allow for `N`, the number of elements in a chunk, to be inferred.
+    ///
+    /// ### Why is this bad?
+    /// When `.windows()` is used, a bounds check is required before an element can be accessed.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// let numbers = Vec::from_iter(0..10);
+    /// for chunk in numbers.windows(2) {
+    ///     println!("{n} is an odd number", n = chunk[0] + chunk[1]);
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// let numbers = Vec::from_iter(0..10);
+    /// for [m, n] in numbers.array_windows() {
+    ///     println!("{n} is an odd number", n = m + n);
+    /// }
+    /// ```
+    #[clippy::version = "1.89.0"]
+    pub CONST_SIZED_WINDOWS,
+    nursery,
+    "calling `.windows()` with a constant `size` where `.array_windows()` could be used instead"
+}
+
 pub struct Loops {
     msrv: Msrv,
     enforce_iter_loop_reborrow: bool,
@@ -822,6 +921,9 @@ impl_lint_pass!(Loops => [
     INFINITE_LOOP,
     MANUAL_SLICE_FILL,
     CHAR_INDICES_AS_BYTE_INDICES,
+    CONST_SIZED_CHUNKS_EXACT,
+    CONST_SIZED_CHUNKS_EXACT_MUT,
+    CONST_SIZED_WINDOWS,
 ]);
 
 impl<'tcx> LateLintPass<'tcx> for Loops {
@@ -906,6 +1008,7 @@ impl Loops {
         manual_find::check(cx, pat, arg, body, span, expr);
         unused_enumerate_index::check(cx, pat, arg, body);
         char_indices_as_byte_indices::check(cx, pat, arg, body);
+        const_sized_chunks::check(cx, pat, arg);
     }
 
     fn check_for_loop_arg(&self, cx: &LateContext<'_>, _: &Pat<'_>, arg: &Expr<'_>) {

clippy_utils/src/sym.rs

@@ -80,6 +80,9 @@ generate! {
     ambiguous_glob_reexports,
     append,
     arg,
+    array_chunks,
+    array_chunks_mut,
+    array_windows,
     as_bytes,
     as_deref,
     as_deref_mut,
@@ -108,6 +111,8 @@ generate! {
     checked_pow,
     checked_rem_euclid,
     checked_sub,
+    chunks_exact,
+    chunks_exact_mut,
     clamp,
     clippy_utils,
     clone_into,

tests/ui/const_sized_chunks_exact.fixed

@@ -0,0 +1,89 @@
+#![warn(clippy::const_sized_chunks_exact)]
+#![feature(array_chunks)]
+#![allow(unused_variables)]
+#![allow(dead_code)]
+
+fn test_binding() {
+    let numbers = [0, 1, 2, 3, 4];
+
+    #[allow(clippy::needless_borrow)]
+    for [a, b] in numbers.array_chunks() {
+        //~^ const_sized_chunks_exact
+    }
+
+    #[allow(unused_mut)]
+    for [a, b] in numbers.array_chunks() {
+        //~^ const_sized_chunks_exact
+    }
+
+    for [a, b] in numbers.array_chunks() {
+        //~^ const_sized_chunks_exact
+    }
+}
+
+fn test_slice_like() {
+    let numbers = [2; 5];
+    for [a, b] in numbers.array_chunks() {
+        //~^ const_sized_chunks_exact
+    }
+
+    let numbers = [0, 1, 2, 3, 4];
+    for [a, b] in numbers.array_chunks() {
+        //~^ const_sized_chunks_exact
+    }
+
+    let numbers = &[0, 1, 2, 3, 4];
+    for [a, b] in numbers.array_chunks() {
+        //~^ const_sized_chunks_exact
+    }
+
+    let numbers = &&[0, 1, 2, 3, 4];
+    for [a, b] in numbers.array_chunks() {
+        //~^ const_sized_chunks_exact
+    }
+
+    let numbers = &&&[0, 1, 2, 3, 4];
+    for [a, b] in numbers.array_chunks() {
+        //~^ const_sized_chunks_exact
+    }
+
+    let numbers = Vec::from_iter(0..5);
+    for [a, b] in numbers.array_chunks() {
+        //~^ const_sized_chunks_exact
+    }
+
+    let numbers = &Vec::from_iter(0..5);
+    for [a, b] in numbers.array_chunks() {
+        //~^ const_sized_chunks_exact
+    }
+}
+
+fn test_const_eval() {
+    const N: usize = 2;
+
+    let numbers = [2; 5];
+    for [a, b] in numbers.array_chunks() {
+        //~^ const_sized_chunks_exact
+    }
+
+    for [a, b, c] in numbers.array_chunks() {
+        //~^ const_sized_chunks_exact
+    }
+}
+
+fn test_chunk_size() {
+    let numbers = Vec::from_iter(0..5);
+    for chunk in numbers.chunks_exact(0) {}
+
+    for [a] in numbers.array_chunks() {
+        //~^ const_sized_chunks_exact
+    }
+
+    for [a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z] in numbers.array_chunks() {
+        //~^ const_sized_chunks_exact
+    }
+
+    for chunk in numbers.chunks_exact(27) {}
+}
+
+fn main() {}