rust-clippy/clippy_lints/src/drop_bounds.rs

use crate::utils::{match_def_path, paths, span_lint};
use if_chain::if_chain;
use rustc::hir::*;
use rustc::lint::{LateLintPass, LintArray, LintPass};
use rustc::{declare_tool_lint, lint_array};

/// **What it does:** Checks for generics with `std::ops::Drop` as bounds.
///
/// **Why is this bad?** `Drop` bounds do not really accomplish anything.
/// A type may have compiler-generated drop glue without implementing the
/// `Drop` trait itself. The `Drop` trait also only has one method,
/// `Drop::drop`, and that function is by fiat not callable in user code.
/// So there is really no use case for using `Drop` in trait bounds.
///
/// The most likely use case of a drop bound is to distinguish between types
/// that have destructors and types that don't. Combined with specialization,
/// a naive coder would write an implementation that assumed a type could be
/// trivially dropped, then write a specialization for `T: Drop` that actually
/// calls the destructor. Except that doing so is not correct; String, for
/// example, doesn't actually implement Drop, but because String contains a
/// Vec, assuming it can be trivially dropped will leak memory.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// fn foo<T: Drop>() {}
/// ```
declare_clippy_lint! {
    pub DROP_BOUNDS,
    correctness,
    "Bounds of the form `T: Drop` are useless"
}

const DROP_BOUNDS_SUMMARY: &str = "Bounds of the form `T: Drop` are useless. \
                                   Use `std::mem::needs_drop` to detect if a type has drop glue.";

pub struct Pass;

impl LintPass for Pass {
    fn get_lints(&self) -> LintArray {
        lint_array!(DROP_BOUNDS)
    }

    fn name(&self) -> &'static str {
        "DropBounds"
    }
}

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
    fn check_generic_param(&mut self, cx: &rustc::lint::LateContext<'a, 'tcx>, p: &'tcx GenericParam) {
        for bound in &p.bounds {
            lint_bound(cx, bound);
        }
    }
    fn check_where_predicate(&mut self, cx: &rustc::lint::LateContext<'a, 'tcx>, p: &'tcx WherePredicate) {
        if let WherePredicate::BoundPredicate(WhereBoundPredicate { bounds, .. }) = p {
            for bound in bounds {
                lint_bound(cx, bound);
            }
        }
    }
}

fn lint_bound<'a, 'tcx>(cx: &rustc::lint::LateContext<'a, 'tcx>, bound: &'tcx GenericBound) {
    if_chain! {
        if let GenericBound::Trait(t, _) = bound;
        if let Some(def_id) = t.trait_ref.path.def.opt_def_id();
        if match_def_path(cx.tcx, def_id, &paths::DROP_TRAIT);
        then {
            span_lint(
                cx,
                DROP_BOUNDS,
                t.span,
                DROP_BOUNDS_SUMMARY
            );
        }
    }
}