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| 1 | +use clippy_utils::consts::{constant, Constant}; |
| 2 | +use clippy_utils::diagnostics::span_lint_and_then; |
| 3 | +use clippy_utils::higher::VecArgs; |
| 4 | +use clippy_utils::macros::root_macro_call; |
| 5 | +use clippy_utils::source::snippet; |
| 6 | +use clippy_utils::{expr_or_init, fn_def_id, match_def_path, paths}; |
| 7 | +use rustc_errors::Applicability; |
| 8 | +use rustc_hir::{Expr, ExprKind}; |
| 9 | +use rustc_lint::{LateContext, LateLintPass}; |
| 10 | +use rustc_session::declare_lint_pass; |
| 11 | +use rustc_span::{sym, Span}; |
| 12 | + |
| 13 | +declare_clippy_lint! { |
| 14 | + /// ### What it does |
| 15 | + /// Looks for patterns such as `vec![Vec::with_capacity(x); n]` or `iter::repeat(Vec::with_capacity(x))`. |
| 16 | + /// |
| 17 | + /// ### Why is this bad? |
| 18 | + /// These constructs work by cloning the element, but cloning a `Vec<_>` does not |
| 19 | + /// respect the old vector's capacity and effectively discards it. |
| 20 | + /// |
| 21 | + /// This makes `iter::repeat(Vec::with_capacity(x))` especially suspicious because the user most certainly |
| 22 | + /// expected that the yielded `Vec<_>` will have the requested capacity, otherwise one can simply write |
| 23 | + /// `iter::repeat(Vec::new())` instead and it will have the same effect. |
| 24 | + /// |
| 25 | + /// Similarly for `vec![x; n]`, the element `x` is cloned to fill the vec. |
| 26 | + /// Unlike `iter::repeat` however, the vec repeat macro does not have to clone the value `n` times |
| 27 | + /// but just `n - 1` times, because it can reuse the passed value for the last slot. |
| 28 | + /// That means that the last `Vec<_>` gets the requested capacity but all other ones do not. |
| 29 | + /// |
| 30 | + /// ### Example |
| 31 | + /// ```rust |
| 32 | + /// # use std::iter; |
| 33 | + /// |
| 34 | + /// let _: Vec<Vec<u8>> = vec![Vec::with_capacity(42); 123]; |
| 35 | + /// let _: Vec<Vec<u8>> = iter::repeat(Vec::with_capacity(42)).take(123).collect(); |
| 36 | + /// ``` |
| 37 | + /// Use instead: |
| 38 | + /// ```rust |
| 39 | + /// # use std::iter; |
| 40 | + /// |
| 41 | + /// let _: Vec<Vec<u8>> = iter::repeat_with(|| Vec::with_capacity(42)).take(123).collect(); |
| 42 | + /// // ^^^ this closure executes 123 times |
| 43 | + /// // and the vecs will have the expected capacity |
| 44 | + /// ``` |
| 45 | + #[clippy::version = "1.74.0"] |
| 46 | + pub REPEAT_VEC_WITH_CAPACITY, |
| 47 | + suspicious, |
| 48 | + "repeating a `Vec::with_capacity` expression which does not retain capacity" |
| 49 | +} |
| 50 | + |
| 51 | +declare_lint_pass!(RepeatVecWithCapacity => [REPEAT_VEC_WITH_CAPACITY]); |
| 52 | + |
| 53 | +fn emit_lint(cx: &LateContext<'_>, span: Span, kind: &str, note: &'static str, sugg_msg: &'static str, sugg: String) { |
| 54 | + span_lint_and_then( |
| 55 | + cx, |
| 56 | + REPEAT_VEC_WITH_CAPACITY, |
| 57 | + span, |
| 58 | + &format!("repeating `Vec::with_capacity` using `{kind}`, which does not retain capacity"), |
| 59 | + |diag| { |
| 60 | + diag.note(note); |
| 61 | + diag.span_suggestion_verbose(span, sugg_msg, sugg, Applicability::MaybeIncorrect); |
| 62 | + }, |
| 63 | + ); |
| 64 | +} |
| 65 | + |
| 66 | +/// Checks `vec![Vec::with_capacity(x); n]` |
| 67 | +fn check_vec_macro(cx: &LateContext<'_>, expr: &Expr<'_>) { |
| 68 | + if let Some(mac_call) = root_macro_call(expr.span) |
| 69 | + && cx.tcx.is_diagnostic_item(sym::vec_macro, mac_call.def_id) |
| 70 | + && let Some(VecArgs::Repeat(repeat_expr, len_expr)) = VecArgs::hir(cx, expr) |
| 71 | + && fn_def_id(cx, repeat_expr).is_some_and(|did| match_def_path(cx, did, &paths::VEC_WITH_CAPACITY)) |
| 72 | + && !len_expr.span.from_expansion() |
| 73 | + && let Some(Constant::Int(2..)) = constant(cx, cx.typeck_results(), expr_or_init(cx, len_expr)) |
| 74 | + { |
| 75 | + emit_lint( |
| 76 | + cx, |
| 77 | + expr.span.source_callsite(), |
| 78 | + "vec![x; n]", |
| 79 | + "only the last `Vec` will have the capacity", |
| 80 | + "if you intended to initialize multiple `Vec`s with an initial capacity, try", |
| 81 | + format!( |
| 82 | + "(0..{}).map(|_| {}).collect::<Vec<_>>()", |
| 83 | + snippet(cx, len_expr.span, ""), |
| 84 | + snippet(cx, repeat_expr.span, "..") |
| 85 | + ), |
| 86 | + ); |
| 87 | + } |
| 88 | +} |
| 89 | + |
| 90 | +/// Checks `iter::repeat(Vec::with_capacity(x))` |
| 91 | +fn check_repeat_fn(cx: &LateContext<'_>, expr: &Expr<'_>) { |
| 92 | + if !expr.span.from_expansion() |
| 93 | + && fn_def_id(cx, expr).is_some_and(|did| cx.tcx.is_diagnostic_item(sym::iter_repeat, did)) |
| 94 | + && let ExprKind::Call(_, [repeat_expr]) = expr.kind |
| 95 | + && fn_def_id(cx, repeat_expr).is_some_and(|did| match_def_path(cx, did, &paths::VEC_WITH_CAPACITY)) |
| 96 | + && !repeat_expr.span.from_expansion() |
| 97 | + { |
| 98 | + emit_lint( |
| 99 | + cx, |
| 100 | + expr.span, |
| 101 | + "iter::repeat", |
| 102 | + "none of the yielded `Vec`s will have the requested capacity", |
| 103 | + "if you intended to create an iterator that yields `Vec`s with an initial capacity, try", |
| 104 | + format!("std::iter::repeat_with(|| {})", snippet(cx, repeat_expr.span, "..")), |
| 105 | + ); |
| 106 | + } |
| 107 | +} |
| 108 | + |
| 109 | +impl LateLintPass<'_> for RepeatVecWithCapacity { |
| 110 | + fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) { |
| 111 | + check_vec_macro(cx, expr); |
| 112 | + check_repeat_fn(cx, expr); |
| 113 | + } |
| 114 | +} |
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