clippy_lints/src/zero_sized_map_values.rs

use clippy_utils::diagnostics::span_lint_and_help;
use clippy_utils::ty::{is_type_diagnostic_item, ty_from_hir_ty};
use rustc_hir::{self as hir, AmbigArg, HirId, ItemKind, Node};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty::layout::LayoutOf as _;
use rustc_middle::ty::{self, TypeVisitableExt};
use rustc_session::declare_lint_pass;
use rustc_span::sym;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for maps with zero-sized value types anywhere in the code.
    ///
    /// ### Why is this bad?
    /// Since there is only a single value for a zero-sized type, a map
    /// containing zero sized values is effectively a set. Using a set in that case improves
    /// readability and communicates intent more clearly.
    ///
    /// ### Known problems
    /// * A zero-sized type cannot be recovered later if it contains private fields.
    /// * This lints the signature of public items
    ///
    /// ### Example
    /// ```no_run
    /// # use std::collections::HashMap;
    /// fn unique_words(text: &str) -> HashMap<&str, ()> {
    ///     todo!();
    /// }
    /// ```
    /// Use instead:
    /// ```no_run
    /// # use std::collections::HashSet;
    /// fn unique_words(text: &str) -> HashSet<&str> {
    ///     todo!();
    /// }
    /// ```
    #[clippy::version = "1.50.0"]
    pub ZERO_SIZED_MAP_VALUES,
    pedantic,
    "usage of map with zero-sized value type"
}

declare_lint_pass!(ZeroSizedMapValues => [ZERO_SIZED_MAP_VALUES]);

impl LateLintPass<'_> for ZeroSizedMapValues {
    fn check_ty<'tcx>(&mut self, cx: &LateContext<'tcx>, hir_ty: &hir::Ty<'tcx, AmbigArg>) {
        if !hir_ty.span.from_expansion()
            && !in_trait_impl(cx, hir_ty.hir_id)
            // We don't care about infer vars
            && let ty = ty_from_hir_ty(cx, hir_ty.as_unambig_ty())
            && (is_type_diagnostic_item(cx, ty, sym::HashMap) || is_type_diagnostic_item(cx, ty, sym::BTreeMap))
            && let ty::Adt(_, args) = ty.kind()
            && let ty = args.type_at(1)
            // Ensure that no type information is missing, to avoid a delayed bug in the compiler if this is not the case.
            // This might happen when computing a reference/pointer metadata on a type for which we
            // cannot check if it is `Sized` or not, such as an incomplete associated type in a
            // type alias. See an example in `issue14822()` of `tests/ui/zero_sized_hashmap_values.rs`.
            && !ty.has_non_region_param()
            && let Ok(layout) = cx.layout_of(ty)
            && layout.is_zst()
        {
            span_lint_and_help(
                cx,
                ZERO_SIZED_MAP_VALUES,
                hir_ty.span,
                "map with zero-sized value type",
                None,
                "consider using a set instead",
            );
        }
    }
}

fn in_trait_impl(cx: &LateContext<'_>, hir_id: HirId) -> bool {
    let parent_id = cx.tcx.hir_get_parent_item(hir_id);
    let second_parent_id = cx.tcx.hir_get_parent_item(parent_id.into()).def_id;
    if let Node::Item(item) = cx.tcx.hir_node_by_def_id(second_parent_id)
        && let ItemKind::Impl(hir::Impl { of_trait: Some(_), .. }) = item.kind
    {
        return true;
    }
    false
}