src/tools/clippy/clippy_lints/src/iter_without_into_iter.rs

use clippy_utils::diagnostics::span_lint_and_then;
use clippy_utils::get_parent_as_impl;
use clippy_utils::source::snippet;
use clippy_utils::ty::{implements_trait, make_normalized_projection};
use rustc_ast::Mutability;
use rustc_errors::Applicability;
use rustc_hir::{FnRetTy, ImplItemKind, ImplicitSelfKind, ItemKind, TyKind};
use rustc_lint::{LateContext, LateLintPass, LintContext};
use rustc_middle::lint::in_external_macro;
use rustc_middle::ty::{self, Ty};
use rustc_session::declare_lint_pass;
use rustc_span::{sym, Symbol};
use std::iter;

declare_clippy_lint! {
    /// ### What it does
    /// Looks for `iter` and `iter_mut` methods without an associated `IntoIterator for (&|&mut) Type` implementation.
    ///
    /// ### Why is this bad?
    /// It's not bad, but having them is idiomatic and allows the type to be used in for loops directly
    /// (`for val in &iter {}`), without having to first call `iter()` or `iter_mut()`.
    ///
    /// ### Limitations
    /// This lint focuses on providing an idiomatic API. Therefore, it will only
    /// lint on types which are accessible outside of the crate. For internal types,
    /// the `IntoIterator` trait can be implemented on demand if it is actually needed.
    ///
    /// ### Example
    /// ```no_run
    /// struct MySlice<'a>(&'a [u8]);
    /// impl<'a> MySlice<'a> {
    ///     pub fn iter(&self) -> std::slice::Iter<'a, u8> {
    ///         self.0.iter()
    ///     }
    /// }
    /// ```
    /// Use instead:
    /// ```no_run
    /// struct MySlice<'a>(&'a [u8]);
    /// impl<'a> MySlice<'a> {
    ///     pub fn iter(&self) -> std::slice::Iter<'a, u8> {
    ///         self.0.iter()
    ///     }
    /// }
    /// impl<'a> IntoIterator for &MySlice<'a> {
    ///     type Item = &'a u8;
    ///     type IntoIter = std::slice::Iter<'a, u8>;
    ///     fn into_iter(self) -> Self::IntoIter {
    ///         self.iter()
    ///     }
    /// }
    /// ```
    #[clippy::version = "1.75.0"]
    pub ITER_WITHOUT_INTO_ITER,
    pedantic,
    "implementing `iter(_mut)` without an associated `IntoIterator for (&|&mut) Type` impl"
}

declare_clippy_lint! {
    /// ### What it does
    /// This is the opposite of the `iter_without_into_iter` lint.
    /// It looks for `IntoIterator for (&|&mut) Type` implementations without an inherent `iter` or `iter_mut` method
    /// on the type or on any of the types in its `Deref` chain.
    ///
    /// ### Why is this bad?
    /// It's not bad, but having them is idiomatic and allows the type to be used in iterator chains
    /// by just calling `.iter()`, instead of the more awkward `<&Type>::into_iter` or `(&val).into_iter()` syntax
    /// in case of ambiguity with another `IntoIterator` impl.
    ///
    /// ### Limitations
    /// This lint focuses on providing an idiomatic API. Therefore, it will only
    /// lint on types which are accessible outside of the crate. For internal types,
    /// these methods can be added on demand if they are actually needed. Otherwise,
    /// it would trigger the [`dead_code`] lint for the unused method.
    ///
    /// [`dead_code`]: https://doc.rust-lang.org/rustc/lints/listing/warn-by-default.html#dead-code
    ///
    /// ### Example
    /// ```no_run
    /// struct MySlice<'a>(&'a [u8]);
    /// impl<'a> IntoIterator for &MySlice<'a> {
    ///     type Item = &'a u8;
    ///     type IntoIter = std::slice::Iter<'a, u8>;
    ///     fn into_iter(self) -> Self::IntoIter {
    ///         self.0.iter()
    ///     }
    /// }
    /// ```
    /// Use instead:
    /// ```no_run
    /// struct MySlice<'a>(&'a [u8]);
    /// impl<'a> MySlice<'a> {
    ///     pub fn iter(&self) -> std::slice::Iter<'a, u8> {
    ///         self.into_iter()
    ///     }
    /// }
    /// impl<'a> IntoIterator for &MySlice<'a> {
    ///     type Item = &'a u8;
    ///     type IntoIter = std::slice::Iter<'a, u8>;
    ///     fn into_iter(self) -> Self::IntoIter {
    ///         self.0.iter()
    ///     }
    /// }
    /// ```
    #[clippy::version = "1.75.0"]
    pub INTO_ITER_WITHOUT_ITER,
    pedantic,
    "implementing `IntoIterator for (&|&mut) Type` without an inherent `iter(_mut)` method"
}

declare_lint_pass!(IterWithoutIntoIter => [ITER_WITHOUT_INTO_ITER, INTO_ITER_WITHOUT_ITER]);

/// Checks if a given type is nameable in a trait (impl).
/// RPIT is stable, but impl Trait in traits is not (yet), so when we have
/// a function such as `fn iter(&self) -> impl IntoIterator`, we can't
/// suggest `type IntoIter = impl IntoIterator`.
fn is_nameable_in_impl_trait(ty: &rustc_hir::Ty<'_>) -> bool {
    !matches!(ty.kind, TyKind::OpaqueDef(..))
}

fn is_ty_exported(cx: &LateContext<'_>, ty: Ty<'_>) -> bool {
    ty.ty_adt_def()
        .and_then(|adt| adt.did().as_local())
        .is_some_and(|did| cx.effective_visibilities.is_exported(did))
}

/// Returns the deref chain of a type, starting with the type itself.
fn deref_chain<'cx, 'tcx>(cx: &'cx LateContext<'tcx>, ty: Ty<'tcx>) -> impl Iterator<Item = Ty<'tcx>> + 'cx {
    iter::successors(Some(ty), |&ty| {
        if let Some(deref_did) = cx.tcx.lang_items().deref_trait()
            && implements_trait(cx, ty, deref_did, &[])
        {
            make_normalized_projection(cx.tcx, cx.param_env, deref_did, sym::Target, [ty])
        } else {
            None
        }
    })
}

fn adt_has_inherent_method(cx: &LateContext<'_>, ty: Ty<'_>, method_name: Symbol) -> bool {
    if let Some(ty_did) = ty.ty_adt_def().map(ty::AdtDef::did) {
        cx.tcx.inherent_impls(ty_did).into_iter().flatten().any(|&did| {
            cx.tcx
                .associated_items(did)
                .filter_by_name_unhygienic(method_name)
                .next()
                .is_some_and(|item| item.kind == ty::AssocKind::Fn)
        })
    } else {
        false
    }
}

impl LateLintPass<'_> for IterWithoutIntoIter {
    fn check_item(&mut self, cx: &LateContext<'_>, item: &rustc_hir::Item<'_>) {
        if !in_external_macro(cx.sess(), item.span)
            && let ItemKind::Impl(imp) = item.kind
            && let TyKind::Ref(_, self_ty_without_ref) = &imp.self_ty.kind
            && let Some(trait_ref) = imp.of_trait
            && trait_ref
                .trait_def_id()
                .is_some_and(|did| cx.tcx.is_diagnostic_item(sym::IntoIterator, did))
            && let &ty::Ref(_, ty, mtbl) = cx.tcx.type_of(item.owner_id).instantiate_identity().kind()
            && let expected_method_name = match mtbl {
                Mutability::Mut => sym::iter_mut,
                Mutability::Not => sym::iter,
            }
            && !deref_chain(cx, ty).any(|ty| {
                // We can't check inherent impls for slices, but we know that they have an `iter(_mut)` method
                ty.peel_refs().is_slice() || adt_has_inherent_method(cx, ty, expected_method_name)
            })
            && let Some(iter_assoc_span) = imp.items.iter().find_map(|item| {
                if item.ident.name == sym!(IntoIter) {
                    Some(cx.tcx.hir().impl_item(item.id).expect_type().span)
                } else {
                    None
                }
            })
            && is_ty_exported(cx, ty)
        {
            span_lint_and_then(
                cx,
                INTO_ITER_WITHOUT_ITER,
                item.span,
                format!("`IntoIterator` implemented for a reference type without an `{expected_method_name}` method"),
                |diag| {
                    // The suggestion forwards to the `IntoIterator` impl and uses a form of UFCS
                    // to avoid name ambiguities, as there might be an inherent into_iter method
                    // that we don't want to call.
                    let sugg = format!(
                        "
impl {self_ty_without_ref} {{
    fn {expected_method_name}({ref_self}self) -> {iter_ty} {{
        <{ref_self}Self as IntoIterator>::into_iter(self)
    }}
}}
",
                        self_ty_without_ref = snippet(cx, self_ty_without_ref.ty.span, ".."),
                        ref_self = mtbl.ref_prefix_str(),
                        iter_ty = snippet(cx, iter_assoc_span, ".."),
                    );

                    diag.span_suggestion_verbose(
                        item.span.shrink_to_lo(),
                        format!("consider implementing `{expected_method_name}`"),
                        sugg,
                        // Just like iter_without_into_iter, this suggestion is on a best effort basis
                        // and requires potentially adding lifetimes or moving them around.
                        Applicability::Unspecified,
                    );
                },
            );
        }
    }

    fn check_impl_item(&mut self, cx: &LateContext<'_>, item: &rustc_hir::ImplItem<'_>) {
        let item_did = item.owner_id.to_def_id();
        let (borrow_prefix, expected_implicit_self) = match item.ident.name {
            sym::iter => ("&", ImplicitSelfKind::RefImm),
            sym::iter_mut => ("&mut ", ImplicitSelfKind::RefMut),
            _ => return,
        };

        if !in_external_macro(cx.sess(), item.span)
            && let ImplItemKind::Fn(sig, _) = item.kind
            && let FnRetTy::Return(ret) = sig.decl.output
            && is_nameable_in_impl_trait(ret)
            && cx.tcx.generics_of(item_did).own_params.is_empty()
            && sig.decl.implicit_self == expected_implicit_self
            && sig.decl.inputs.len() == 1
            && let Some(imp) = get_parent_as_impl(cx.tcx, item.hir_id())
            && imp.of_trait.is_none()
            && let sig = cx.tcx.liberate_late_bound_regions(
                item_did,
                cx.tcx.fn_sig(item_did).instantiate_identity()
            )
            && let ref_ty = sig.inputs()[0]
            && let Some(into_iter_did) = cx.tcx.get_diagnostic_item(sym::IntoIterator)
            && let Some(iterator_did) = cx.tcx.get_diagnostic_item(sym::Iterator)
            && let ret_ty = sig.output()
            // Order is important here, we need to check that the `fn iter` return type actually implements `IntoIterator`
            // *before* normalizing `<_ as IntoIterator>::Item` (otherwise make_normalized_projection ICEs)
            && implements_trait(cx, ret_ty, iterator_did, &[])
            && let Some(iter_ty) = make_normalized_projection(
                cx.tcx,
                cx.param_env,
                iterator_did,
                sym::Item,
                [ret_ty],
            )
            // Only lint if the `IntoIterator` impl doesn't actually exist
            && !implements_trait(cx, ref_ty, into_iter_did, &[])
            && is_ty_exported(cx, ref_ty.peel_refs())
        {
            let self_ty_snippet = format!("{borrow_prefix}{}", snippet(cx, imp.self_ty.span, ".."));

            span_lint_and_then(
                cx,
                ITER_WITHOUT_INTO_ITER,
                item.span,
                format!(
                    "`{}` method without an `IntoIterator` impl for `{self_ty_snippet}`",
                    item.ident
                ),
                |diag| {
                    // Get the lower span of the `impl` block, and insert the suggestion right before it:
                    // impl X {
                    // ^   fn iter(&self) -> impl IntoIterator { ... }
                    // }
                    let span_behind_impl = cx
                        .tcx
                        .def_span(cx.tcx.parent_hir_id(item.hir_id()).owner.def_id)
                        .shrink_to_lo();

                    let sugg = format!(
                        "
impl IntoIterator for {self_ty_snippet} {{
    type IntoIter = {ret_ty};
    type Item = {iter_ty};
    fn into_iter(self) -> Self::IntoIter {{
        self.iter()
    }}
}}
"
                    );
                    diag.span_suggestion_verbose(
                        span_behind_impl,
                        format!("consider implementing `IntoIterator` for `{self_ty_snippet}`"),
                        sugg,
                        // Suggestion is on a best effort basis, might need some adjustments by the user
                        // such as adding some lifetimes in the associated types, or importing types.
                        Applicability::Unspecified,
                    );
                },
            );
        }
    }
}