clippy_lints/src/unused_peekable.rs

use clippy_utils::diagnostics::span_lint_hir_and_then;
use clippy_utils::ty::{is_type_diagnostic_item, peel_mid_ty_refs_is_mutable};
use clippy_utils::{fn_def_id, is_trait_method, path_to_local_id, peel_ref_operators, sym};
use rustc_ast::Mutability;
use rustc_hir::intravisit::{Visitor, walk_expr};
use rustc_hir::{Block, Expr, ExprKind, HirId, LetStmt, Node, PatKind, PathSegment, StmtKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::hir::nested_filter::OnlyBodies;
use rustc_session::declare_lint_pass;
use std::ops::ControlFlow;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for the creation of a `peekable` iterator that is never `.peek()`ed
    ///
    /// ### Why is this bad?
    /// Creating a peekable iterator without using any of its methods is likely a mistake,
    /// or just a leftover after a refactor.
    ///
    /// ### Example
    /// ```no_run
    /// let collection = vec![1, 2, 3];
    /// let iter = collection.iter().peekable();
    ///
    /// for item in iter {
    ///     // ...
    /// }
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// let collection = vec![1, 2, 3];
    /// let iter = collection.iter();
    ///
    /// for item in iter {
    ///     // ...
    /// }
    /// ```
    #[clippy::version = "1.65.0"]
    pub UNUSED_PEEKABLE,
    nursery,
    "creating a peekable iterator without using any of its methods"
}

declare_lint_pass!(UnusedPeekable => [UNUSED_PEEKABLE]);

impl<'tcx> LateLintPass<'tcx> for UnusedPeekable {
    fn check_block(&mut self, cx: &LateContext<'tcx>, block: &Block<'tcx>) {
        // Don't lint `Peekable`s returned from a block
        if let Some(expr) = block.expr
            && let Some(ty) = cx.typeck_results().expr_ty_opt(peel_ref_operators(cx, expr))
            && is_type_diagnostic_item(cx, ty, sym::IterPeekable)
        {
            return;
        }

        for (idx, stmt) in block.stmts.iter().enumerate() {
            if !stmt.span.from_expansion()
                && let StmtKind::Let(local) = stmt.kind
                && let PatKind::Binding(_, binding, ident, _) = local.pat.kind
                && let Some(init) = local.init
                && !init.span.from_expansion()
                && let Some(ty) = cx.typeck_results().expr_ty_opt(init)
                && let (ty, _, Mutability::Mut) = peel_mid_ty_refs_is_mutable(ty)
                && is_type_diagnostic_item(cx, ty, sym::IterPeekable)
            {
                let mut vis = PeekableVisitor::new(cx, binding);

                if idx + 1 == block.stmts.len() && block.expr.is_none() {
                    return;
                }

                let mut found_peek_call = block.stmts[idx..].iter().any(|stmt| vis.visit_stmt(stmt).is_break());

                if !found_peek_call
                    && let Some(expr) = block.expr
                    && vis.visit_expr(expr).is_break()
                {
                    found_peek_call = true;
                }

                if !found_peek_call {
                    span_lint_hir_and_then(
                        cx,
                        UNUSED_PEEKABLE,
                        local.hir_id,
                        ident.span,
                        "`peek` never called on `Peekable` iterator",
                        |diag| {
                            diag.help("consider removing the call to `peekable`");
                        },
                    );
                }
            }
        }
    }
}

struct PeekableVisitor<'a, 'tcx> {
    cx: &'a LateContext<'tcx>,
    expected_hir_id: HirId,
}

impl<'a, 'tcx> PeekableVisitor<'a, 'tcx> {
    fn new(cx: &'a LateContext<'tcx>, expected_hir_id: HirId) -> Self {
        Self { cx, expected_hir_id }
    }
}

impl<'tcx> Visitor<'tcx> for PeekableVisitor<'_, 'tcx> {
    type NestedFilter = OnlyBodies;
    type Result = ControlFlow<()>;

    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
        self.cx.tcx
    }

    fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) -> ControlFlow<()> {
        if path_to_local_id(ex, self.expected_hir_id) {
            for (_, node) in self.cx.tcx.hir_parent_iter(ex.hir_id) {
                match node {
                    Node::Expr(expr) => {
                        match expr.kind {
                            // some_function(peekable)
                            //
                            // If the Peekable is passed to a function, stop
                            ExprKind::Call(_, args) => {
                                if let Some(func_did) = fn_def_id(self.cx, expr)
                                    && let Some(into_iter_did) = self.cx.tcx.lang_items().into_iter_fn()
                                    && func_did == into_iter_did
                                {
                                    // Probably a for loop desugar, stop searching
                                    return ControlFlow::Continue(());
                                }

                                if args.iter().any(|arg| arg_is_mut_peekable(self.cx, arg)) {
                                    return ControlFlow::Break(());
                                }

                                return ControlFlow::Continue(());
                            },
                            // Catch anything taking a Peekable mutably
                            ExprKind::MethodCall(
                                PathSegment {
                                    ident: method_name_ident,
                                    ..
                                },
                                self_arg,
                                remaining_args,
                                _,
                            ) => {
                                let method_name = method_name_ident.name;

                                // `Peekable` methods
                                if matches!(method_name, sym::peek | sym::peek_mut | sym::next_if | sym::next_if_eq)
                                    && arg_is_mut_peekable(self.cx, self_arg)
                                {
                                    return ControlFlow::Break(());
                                }

                                // foo.some_method() excluding Iterator methods
                                if remaining_args.iter().any(|arg| arg_is_mut_peekable(self.cx, arg))
                                    && !is_trait_method(self.cx, expr, sym::Iterator)
                                {
                                    return ControlFlow::Break(());
                                }

                                // foo.by_ref(), keep checking for `peek`
                                if method_name == sym::by_ref {
                                    continue;
                                }

                                return ControlFlow::Continue(());
                            },
                            ExprKind::AddrOf(_, Mutability::Mut, _) | ExprKind::Unary(..) | ExprKind::DropTemps(_) => {
                            },
                            ExprKind::AddrOf(_, Mutability::Not, _) => return ControlFlow::Continue(()),
                            _ => {
                                return ControlFlow::Break(());
                            },
                        }
                    },
                    Node::LetStmt(LetStmt { init: Some(init), .. }) => {
                        if arg_is_mut_peekable(self.cx, init) {
                            return ControlFlow::Break(());
                        }

                        return ControlFlow::Continue(());
                    },
                    Node::Stmt(stmt) => {
                        match stmt.kind {
                            StmtKind::Let(_) | StmtKind::Item(_) => {
                                return ControlFlow::Break(());
                            },
                            StmtKind::Expr(_) | StmtKind::Semi(_) => {},
                        }

                        return ControlFlow::Continue(());
                    },
                    Node::Block(_) | Node::ExprField(_) => {},
                    _ => {
                        return ControlFlow::Continue(());
                    },
                }
            }
        }

        walk_expr(self, ex)
    }
}

fn arg_is_mut_peekable(cx: &LateContext<'_>, arg: &Expr<'_>) -> bool {
    if let Some(ty) = cx.typeck_results().expr_ty_opt(arg)
        && let (ty, _, Mutability::Mut) = peel_mid_ty_refs_is_mutable(ty)
        && is_type_diagnostic_item(cx, ty, sym::IterPeekable)
    {
        true
    } else {
        false
    }
}