add a new lint for repeat().take() that can be replaced with repeat_n() (#13858)

close #13271

changelog: add new `manual_repeat_n` lint

Author: y21

CHANGELOG.md

@@ -5768,6 +5768,7 @@ Released 2018-09-13
 [`manual_range_contains`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_range_contains
 [`manual_range_patterns`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_range_patterns
 [`manual_rem_euclid`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_rem_euclid
+[`manual_repeat_n`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_repeat_n
 [`manual_retain`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_retain
 [`manual_rotate`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_rotate
 [`manual_saturating_arithmetic`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_saturating_arithmetic

clippy_lints/src/declared_lints.rs

@@ -420,6 +420,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::methods::MANUAL_IS_VARIANT_AND_INFO,
     crate::methods::MANUAL_NEXT_BACK_INFO,
     crate::methods::MANUAL_OK_OR_INFO,
+    crate::methods::MANUAL_REPEAT_N_INFO,
     crate::methods::MANUAL_SATURATING_ARITHMETIC_INFO,
     crate::methods::MANUAL_SPLIT_ONCE_INFO,
     crate::methods::MANUAL_STR_REPEAT_INFO,

clippy_lints/src/doc/lazy_continuation.rs

@@ -57,7 +57,7 @@ pub(super) fn check(
                 diag.span_suggestion_verbose(
                     span.shrink_to_hi(),
                     "indent this line",
-                    std::iter::repeat(" ").take(indent).join(""),
+                    std::iter::repeat_n(" ", indent).join(""),
                     Applicability::MaybeIncorrect,
                 );
                 diag.help("if this is supposed to be its own paragraph, add a blank line");

clippy_lints/src/methods/manual_repeat_n.rs

@@ -0,0 +1,43 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::msrvs::{self, Msrv};
+use clippy_utils::source::{snippet, snippet_with_context};
+use clippy_utils::{expr_use_ctxt, fn_def_id, is_trait_method, std_or_core};
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::LateContext;
+use rustc_span::sym;
+
+use super::MANUAL_REPEAT_N;
+
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx Expr<'tcx>,
+    repeat_expr: &Expr<'_>,
+    take_arg: &Expr<'_>,
+    msrv: &Msrv,
+) {
+    if msrv.meets(msrvs::REPEAT_N)
+        && !expr.span.from_expansion()
+        && is_trait_method(cx, expr, sym::Iterator)
+        && let ExprKind::Call(_, [repeat_arg]) = repeat_expr.kind
+        && let Some(def_id) = fn_def_id(cx, repeat_expr)
+        && cx.tcx.is_diagnostic_item(sym::iter_repeat, def_id)
+        && !expr_use_ctxt(cx, expr).is_ty_unified
+        && let Some(std_or_core) = std_or_core(cx)
+    {
+        let mut app = Applicability::MachineApplicable;
+        span_lint_and_sugg(
+            cx,
+            MANUAL_REPEAT_N,
+            expr.span,
+            "this `repeat().take()` can be written more concisely",
+            "consider using `repeat_n()` instead",
+            format!(
+                "{std_or_core}::iter::repeat_n({}, {})",
+                snippet_with_context(cx, repeat_arg.span, expr.span.ctxt(), "..", &mut app).0,
+                snippet(cx, take_arg.span, "..")
+            ),
+            app,
+        );
+    }
+}

clippy_lints/src/methods/mod.rs

@@ -58,6 +58,7 @@ mod manual_inspect;
 mod manual_is_variant_and;
 mod manual_next_back;
 mod manual_ok_or;
+mod manual_repeat_n;
 mod manual_saturating_arithmetic;
 mod manual_str_repeat;
 mod manual_try_fold;
@@ -4339,6 +4340,29 @@ declare_clippy_lint! {
     "using `NonZero::new_unchecked()` in a `const` context"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    ///
+    /// Checks for `repeat().take()` that can be replaced with `repeat_n()`.
+    ///
+    /// ### Why is this bad?
+    ///
+    /// Using `repeat_n()` is more concise and clearer. Also, `repeat_n()` is sometimes faster than `repeat().take()` when the type of the element is non-trivial to clone because the original value can be reused for the last `.next()` call rather than always cloning.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// let _ = std::iter::repeat(10).take(3);
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// let _ = std::iter::repeat_n(10, 3);
+    /// ```
+    #[clippy::version = "1.86.0"]
+    pub MANUAL_REPEAT_N,
+    style,
+    "detect `repeat().take()` that can be replaced with `repeat_n()`"
+}
+
 pub struct Methods {
     avoid_breaking_exported_api: bool,
     msrv: Msrv,
@@ -4506,6 +4530,7 @@ impl_lint_pass!(Methods => [
     UNNECESSARY_MAP_OR,
     DOUBLE_ENDED_ITERATOR_LAST,
     USELESS_NONZERO_NEW_UNCHECKED,
+    MANUAL_REPEAT_N,
 ]);
 
 /// Extracts a method call name, args, and `Span` of the method name.
@@ -5176,6 +5201,7 @@ impl Methods {
                 ("step_by", [arg]) => iterator_step_by_zero::check(cx, expr, arg),
                 ("take", [arg]) => {
                     iter_out_of_bounds::check_take(cx, expr, recv, arg);
+                    manual_repeat_n::check(cx, expr, recv, arg, &self.msrv);
                     if let Some(("cloned", recv2, [], _span2, _)) = method_call(recv) {
                         iter_overeager_cloned::check(
                             cx,

clippy_utils/src/lib.rs

@@ -88,7 +88,7 @@ use core::mem;
 use core::ops::ControlFlow;
 use std::collections::hash_map::Entry;
 use std::hash::BuildHasherDefault;
-use std::iter::{once, repeat};
+use std::iter::{once, repeat_n};
 use std::sync::{Mutex, MutexGuard, OnceLock};
 
 use itertools::Itertools;
@@ -3420,7 +3420,7 @@ fn maybe_get_relative_path(from: &DefPath, to: &DefPath, max_super: usize) -> St
             }))
             .join("::")
     } else {
-        repeat(String::from("super")).take(go_up_by).chain(path).join("::")
+        repeat_n(String::from("super"), go_up_by).chain(path).join("::")
     }
 }
 

tests/ui/from_iter_instead_of_collect.fixed

@@ -1,6 +1,6 @@
 #![warn(clippy::from_iter_instead_of_collect)]
 #![allow(unused_imports)]
-#![allow(clippy::useless_vec)]
+#![allow(clippy::useless_vec, clippy::manual_repeat_n)]
 
 use std::collections::{BTreeMap, BTreeSet, HashMap, VecDeque};
 

tests/ui/from_iter_instead_of_collect.rs

@@ -1,6 +1,6 @@
 #![warn(clippy::from_iter_instead_of_collect)]
 #![allow(unused_imports)]
-#![allow(clippy::useless_vec)]
+#![allow(clippy::useless_vec, clippy::manual_repeat_n)]
 
 use std::collections::{BTreeMap, BTreeSet, HashMap, VecDeque};
 

tests/ui/manual_repeat_n.fixed

@@ -0,0 +1,30 @@
+#![warn(clippy::manual_repeat_n)]
+
+use std::iter::repeat;
+
+fn main() {
+    let _ = std::iter::repeat_n(10, 3);
+
+    let _ = std::iter::repeat_n(String::from("foo"), 4);
+
+    for value in std::iter::repeat_n(5, 3) {}
+
+    let _: Vec<_> = std::iter::repeat_n(String::from("bar"), 10).collect();
+
+    let _ = std::iter::repeat_n(vec![1, 2], 2);
+}
+
+mod foo_lib {
+    pub fn iter() -> std::iter::Take<std::iter::Repeat<&'static [u8]>> {
+        todo!()
+    }
+}
+
+fn foo() {
+    let _ = match 1 {
+        1 => foo_lib::iter(),
+        // Shouldn't lint because `external_lib::iter` doesn't return `std::iter::RepeatN`.
+        2 => std::iter::repeat([1, 2].as_slice()).take(2),
+        _ => todo!(),
+    };
+}

tests/ui/manual_repeat_n.rs

@@ -0,0 +1,30 @@
+#![warn(clippy::manual_repeat_n)]
+
+use std::iter::repeat;
+
+fn main() {
+    let _ = repeat(10).take(3);
+
+    let _ = repeat(String::from("foo")).take(4);
+
+    for value in std::iter::repeat(5).take(3) {}
+
+    let _: Vec<_> = std::iter::repeat(String::from("bar")).take(10).collect();
+
+    let _ = repeat(vec![1, 2]).take(2);
+}
+
+mod foo_lib {
+    pub fn iter() -> std::iter::Take<std::iter::Repeat<&'static [u8]>> {
+        todo!()
+    }
+}
+
+fn foo() {
+    let _ = match 1 {
+        1 => foo_lib::iter(),
+        // Shouldn't lint because `external_lib::iter` doesn't return `std::iter::RepeatN`.
+        2 => std::iter::repeat([1, 2].as_slice()).take(2),
+        _ => todo!(),
+    };
+}