copy_iterator.rs

use clippy_utils::diagnostics::span_lint_and_note;
use clippy_utils::ty::is_copy;
use rustc_hir::{Impl, Item, ItemKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::declare_lint_pass;
use rustc_span::sym;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for types that implement `Copy` as well as
    /// `Iterator`.
    ///
    /// ### Why is this bad?
    /// Implicit copies can be confusing when working with
    /// iterator combinators.
    ///
    /// ### Example
    /// ```rust,ignore
    /// #[derive(Copy, Clone)]
    /// struct Countdown(u8);
    ///
    /// impl Iterator for Countdown {
    ///     // ...
    /// }
    ///
    /// let a: Vec<_> = my_iterator.take(1).collect();
    /// let b: Vec<_> = my_iterator.collect();
    /// ```
    #[clippy::version = "1.30.0"]
    pub COPY_ITERATOR,
    pedantic,
    "implementing `Iterator` on a `Copy` type"
}

declare_lint_pass!(CopyIterator => [COPY_ITERATOR]);

impl<'tcx> LateLintPass<'tcx> for CopyIterator {
    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
        if let ItemKind::Impl(Impl {
            of_trait: Some(trait_ref),
            ..
        }) = item.kind
            && let ty = cx.tcx.type_of(item.owner_id).instantiate_identity()
            && is_copy(cx, ty)
            && let Some(trait_id) = trait_ref.trait_def_id()
            && cx.tcx.is_diagnostic_item(sym::Iterator, trait_id)
        {
            span_lint_and_note(
                cx,
                COPY_ITERATOR,
                item.span,
                "you are implementing `Iterator` on a `Copy` type",
                None,
                "consider implementing `IntoIterator` instead",
            );
        }
    }
}