array_into_iter.rs

use crate::{LateContext, LateLintPass, LintContext};
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_middle::ty;
use rustc_middle::ty::adjustment::{Adjust, Adjustment};
use rustc_session::lint::FutureBreakage;
use rustc_span::symbol::sym;

declare_lint! {
    /// The `array_into_iter` lint detects calling `into_iter` on arrays.
    ///
    /// ### Example
    ///
    /// ```rust
    /// # #![allow(unused)]
    /// [1, 2, 3].into_iter().for_each(|n| { *n; });
    /// ```
    ///
    /// {{produces}}
    ///
    /// ### Explanation
    ///
    /// In the future, it is planned to add an `IntoIter` implementation for
    /// arrays such that it will iterate over *values* of the array instead of
    /// references. Due to how method resolution works, this will change
    /// existing code that uses `into_iter` on arrays. The solution to avoid
    /// this warning is to use `iter()` instead of `into_iter()`.
    ///
    /// This is a [future-incompatible] lint to transition this to a hard error
    /// in the future. See [issue #66145] for more details and a more thorough
    /// description of the lint.
    ///
    /// [issue #66145]: https://github.com/rust-lang/rust/issues/66145
    /// [future-incompatible]: ../index.md#future-incompatible-lints
    pub ARRAY_INTO_ITER,
    Warn,
    "detects calling `into_iter` on arrays",
    @future_incompatible = FutureIncompatibleInfo {
        reference: "issue #66145 <https://github.com/rust-lang/rust/issues/66145>",
        edition: None,
        future_breakage: Some(FutureBreakage {
            date: None
        })
    };
}

declare_lint_pass!(
    /// Checks for instances of calling `into_iter` on arrays.
    ArrayIntoIter => [ARRAY_INTO_ITER]
);

impl<'tcx> LateLintPass<'tcx> for ArrayIntoIter {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
        // We only care about method call expressions.
        if let hir::ExprKind::MethodCall(call, span, args, _) = &expr.kind {
            if call.ident.name != sym::into_iter {
                return;
            }

            // Check if the method call actually calls the libcore
            // `IntoIterator::into_iter`.
            let def_id = cx.typeck_results().type_dependent_def_id(expr.hir_id).unwrap();
            match cx.tcx.trait_of_item(def_id) {
                Some(trait_id) if cx.tcx.is_diagnostic_item(sym::IntoIterator, trait_id) => {}
                _ => return,
            };

            // As this is a method call expression, we have at least one
            // argument.
            let receiver_arg = &args[0];

            // Peel all `Box<_>` layers. We have to special case `Box` here as
            // `Box` is the only thing that values can be moved out of via
            // method call. `Box::new([1]).into_iter()` should trigger this
            // lint.
            let mut recv_ty = cx.typeck_results().expr_ty(receiver_arg);
            let mut num_box_derefs = 0;
            while recv_ty.is_box() {
                num_box_derefs += 1;
                recv_ty = recv_ty.boxed_ty();
            }

            // Make sure we found an array after peeling the boxes.
            if !matches!(recv_ty.kind(), ty::Array(..)) {
                return;
            }

            // Make sure that there is an autoref coercion at the expected
            // position. The first `num_box_derefs` adjustments are the derefs
            // of the box.
            match cx.typeck_results().expr_adjustments(receiver_arg).get(num_box_derefs) {
                Some(Adjustment { kind: Adjust::Borrow(_), .. }) => {}
                _ => return,
            }

            // Emit lint diagnostic.
            let target = match *cx.typeck_results().expr_ty_adjusted(receiver_arg).kind() {
                ty::Ref(_, inner_ty, _) if inner_ty.is_array() => "[T; N]",
                ty::Ref(_, inner_ty, _) if matches!(inner_ty.kind(), ty::Slice(..)) => "[T]",

                // We know the original first argument type is an array type,
                // we know that the first adjustment was an autoref coercion
                // and we know that `IntoIterator` is the trait involved. The
                // array cannot be coerced to something other than a reference
                // to an array or to a slice.
                _ => bug!("array type coerced to something other than array or slice"),
            };
            cx.struct_span_lint(ARRAY_INTO_ITER, *span, |lint| {
                lint.build(&format!(
                "this method call currently resolves to `<&{} as IntoIterator>::into_iter` (due \
                    to autoref coercions), but that might change in the future when \
                    `IntoIterator` impls for arrays are added.",
                target,
                ))
                .span_suggestion(
                    call.ident.span,
                    "use `.iter()` instead of `.into_iter()` to avoid ambiguity",
                    "iter".into(),
                    Applicability::MachineApplicable,
                )
                .emit();
            })
        }
    }
}