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use iter::repeat_with in suggestion and add examples

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y21 2023-12-01 17:18:37 +01:00
parent 504941591f
commit 76eb781336
3 changed files with 14 additions and 9 deletions
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17
clippy_lints/src/repeat_vec_with_capacity.rs
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@ -7,7 +7,7 @@
use rustc_errors::Applicability;
use rustc_hir::{Expr, ExprKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_session::declare_lint_pass;
use rustc_span::{sym, Span};
declare_clippy_lint! {
@ -22,20 +22,25 @@
/// expected that the yielded `Vec<_>` will have the requested capacity, otherwise one can simply write
/// `iter::repeat(Vec::new())` instead and it will have the same effect.
///
/// Similarily for `vec![x; n]`, the element `x` is cloned to fill the vec.
/// Similarly for `vec![x; n]`, the element `x` is cloned to fill the vec.
/// Unlike `iter::repeat` however, the vec repeat macro does not have to clone the value `n` times
/// but just `n - 1` times, because it can reuse the passed value for the last slot.
/// That means that the last `Vec<_>` gets the requested capacity but all other ones do not.
///
/// ### Example
/// ```rust
/// # use std::iter;
///
/// let _: Vec<Vec<u8>> = vec![Vec::with_capacity(42); 123];
/// let _: Vec<Vec<u8>> = iter::repeat(Vec::with_capacity(42)).take(123).collect();
/// ```
/// Use instead:
/// ```rust
/// let _: Vec<Vec<u8>> = (0..123).map(|_| Vec::with_capacity(42)).collect();
/// // ^^^ this closure executes 123 times
/// // and the vecs will have the expected capacity
/// # use std::iter;
///
/// let _: Vec<Vec<u8>> = iter::repeat_with(|| Vec::with_capacity(42)).take(123).collect();
/// // ^^^ this closure executes 123 times
/// // and the vecs will have the expected capacity
/// ```
#[clippy::version = "1.74.0"]
pub REPEAT_VEC_WITH_CAPACITY,
@ -96,7 +101,7 @@ fn check_repeat_fn(cx: &LateContext<'_>, expr: &Expr<'_>) {
"iter::repeat",
"none of the yielded `Vec`s will have the requested capacity",
"if you intended to create an iterator that yields `Vec`s with an initial capacity, try",
format!("std::iter::from_fn(|| Some({}))", snippet(cx, repeat_expr.span, "..")),
format!("std::iter::repeat_with(|| {})", snippet(cx, repeat_expr.span, "..")),
);
}
}

2
tests/ui/repeat_vec_with_capacity.fixed
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@ -23,7 +23,7 @@ fn main() {
}
{
std::iter::from_fn(|| Some(Vec::<()>::with_capacity(42)));
std::iter::repeat_with(|| Vec::<()>::with_capacity(42));
//~^ ERROR: repeating `Vec::with_capacity` using `iter::repeat`, which does not retain capacity
}

4
tests/ui/repeat_vec_with_capacity.stderr
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@ -33,8 +33,8 @@ LL | std::iter::repeat(Vec::<()>::with_capacity(42));
= note: none of the yielded `Vec`s will have the requested capacity
help: if you intended to create an iterator that yields `Vec`s with an initial capacity, try
|
LL | std::iter::from_fn(|| Some(Vec::<()>::with_capacity(42)));
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
LL | std::iter::repeat_with(|| Vec::<()>::with_capacity(42));
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to 3 previous errors