rust/clippy_lints/src/collection_is_never_read.rs

use clippy_utils::diagnostics::span_lint;
use clippy_utils::ty::{is_type_diagnostic_item, is_type_lang_item};
use clippy_utils::visitors::for_each_expr_with_closures;
use clippy_utils::{get_enclosing_block, path_to_local_id};
use core::ops::ControlFlow;
use rustc_hir::{Block, ExprKind, HirId, LangItem, Local, Node, PatKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::declare_lint_pass;
use rustc_span::symbol::sym;
use rustc_span::Symbol;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for collections that are never queried.
    ///
    /// ### Why is this bad?
    /// Putting effort into constructing a collection but then never querying it might indicate that
    /// the author forgot to do whatever they intended to do with the collection. Example: Clone
    /// a vector, sort it for iteration, but then mistakenly iterate the original vector
    /// instead.
    ///
    /// ### Example
    /// ```no_run
    /// # let samples = vec![3, 1, 2];
    /// let mut sorted_samples = samples.clone();
    /// sorted_samples.sort();
    /// for sample in &samples { // Oops, meant to use `sorted_samples`.
    ///     println!("{sample}");
    /// }
    /// ```
    /// Use instead:
    /// ```no_run
    /// # let samples = vec![3, 1, 2];
    /// let mut sorted_samples = samples.clone();
    /// sorted_samples.sort();
    /// for sample in &sorted_samples {
    ///     println!("{sample}");
    /// }
    /// ```
    #[clippy::version = "1.70.0"]
    pub COLLECTION_IS_NEVER_READ,
    nursery,
    "a collection is never queried"
}
declare_lint_pass!(CollectionIsNeverRead => [COLLECTION_IS_NEVER_READ]);

// Add `String` here when it is added to diagnostic items
static COLLECTIONS: [Symbol; 9] = [
    sym::BTreeMap,
    sym::BTreeSet,
    sym::BinaryHeap,
    sym::HashMap,
    sym::HashSet,
    sym::LinkedList,
    sym::Option,
    sym::Vec,
    sym::VecDeque,
];

impl<'tcx> LateLintPass<'tcx> for CollectionIsNeverRead {
    fn check_local(&mut self, cx: &LateContext<'tcx>, local: &'tcx Local<'tcx>) {
        // Look for local variables whose type is a container. Search surrounding bock for read access.
        if match_acceptable_type(cx, local, &COLLECTIONS)
            && let PatKind::Binding(_, local_id, _, _) = local.pat.kind
            && let Some(enclosing_block) = get_enclosing_block(cx, local.hir_id)
            && has_no_read_access(cx, local_id, enclosing_block)
        {
            span_lint(cx, COLLECTION_IS_NEVER_READ, local.span, "collection is never read");
        }
    }
}

fn match_acceptable_type(cx: &LateContext<'_>, local: &Local<'_>, collections: &[rustc_span::Symbol]) -> bool {
    let ty = cx.typeck_results().pat_ty(local.pat);
    collections.iter().any(|&sym| is_type_diagnostic_item(cx, ty, sym))
    // String type is a lang item but not a diagnostic item for now so we need a separate check
        || is_type_lang_item(cx, ty, LangItem::String)
}

fn has_no_read_access<'tcx>(cx: &LateContext<'tcx>, id: HirId, block: &'tcx Block<'tcx>) -> bool {
    let mut has_access = false;
    let mut has_read_access = false;

    // Inspect all expressions and sub-expressions in the block.
    for_each_expr_with_closures(cx, block, |expr| {
        // Ignore expressions that are not simply `id`.
        if !path_to_local_id(expr, id) {
            return ControlFlow::Continue(());
        }

        // `id` is being accessed. Investigate if it's a read access.
        has_access = true;

        // `id` appearing in the left-hand side of an assignment is not a read access:
        //
        // id = ...; // Not reading `id`.
        if let Node::Expr(parent) = cx.tcx.parent_hir_node(expr.hir_id)
            && let ExprKind::Assign(lhs, ..) = parent.kind
            && path_to_local_id(lhs, id)
        {
            return ControlFlow::Continue(());
        }

        // Look for method call with receiver `id`. It might be a non-read access:
        //
        // id.foo(args)
        //
        // Only assuming this for "official" methods defined on the type. For methods defined in extension
        // traits (identified as local, based on the orphan rule), pessimistically assume that they might
        // have side effects, so consider them a read.
        if let Node::Expr(parent) = cx.tcx.parent_hir_node(expr.hir_id)
            && let ExprKind::MethodCall(_, receiver, _, _) = parent.kind
            && path_to_local_id(receiver, id)
            && let Some(method_def_id) = cx.typeck_results().type_dependent_def_id(parent.hir_id)
            && !method_def_id.is_local()
        {
            // The method call is a statement, so the return value is not used. That's not a read access:
            //
            // id.foo(args);
            if let Node::Stmt(..) = cx.tcx.parent_hir_node(parent.hir_id) {
                return ControlFlow::Continue(());
            }

            // The method call is not a statement, so its return value is used somehow but its type is the
            // unit type, so this is not a real read access. Examples:
            //
            // let y = x.clear();
            // println!("{:?}", x.clear());
            if cx.typeck_results().expr_ty(parent).is_unit() {
                return ControlFlow::Continue(());
            }
        }

        // Any other access to `id` is a read access. Stop searching.
        has_read_access = true;
        ControlFlow::Break(())
    });

    // Ignore collections that have no access at all. Other lints should catch them.
    has_access && !has_read_access
}