src/tools/clippy/clippy_lints/src/unit_return_expecting_ord.rs

use clippy_utils::diagnostics::{span_lint, span_lint_and_help};
use if_chain::if_chain;
use rustc_hir::def_id::DefId;
use rustc_hir::{Closure, Expr, ExprKind, StmtKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty;
use rustc_middle::ty::{ClauseKind, GenericPredicates, ProjectionPredicate, TraitPredicate};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::{sym, BytePos, Span};

declare_clippy_lint! {
    /// ### What it does
    /// Checks for functions that expect closures of type
    /// Fn(...) -> Ord where the implemented closure returns the unit type.
    /// The lint also suggests to remove the semi-colon at the end of the statement if present.
    ///
    /// ### Why is this bad?
    /// Likely, returning the unit type is unintentional, and
    /// could simply be caused by an extra semi-colon. Since () implements Ord
    /// it doesn't cause a compilation error.
    /// This is the same reasoning behind the unit_cmp lint.
    ///
    /// ### Known problems
    /// If returning unit is intentional, then there is no
    /// way of specifying this without triggering needless_return lint
    ///
    /// ### Example
    /// ```rust
    /// let mut twins = vec!((1, 1), (2, 2));
    /// twins.sort_by_key(|x| { x.1; });
    /// ```
    #[clippy::version = "1.47.0"]
    pub UNIT_RETURN_EXPECTING_ORD,
    correctness,
    "fn arguments of type Fn(...) -> Ord returning the unit type ()."
}

declare_lint_pass!(UnitReturnExpectingOrd => [UNIT_RETURN_EXPECTING_ORD]);

fn get_trait_predicates_for_trait_id<'tcx>(
    cx: &LateContext<'tcx>,
    generics: GenericPredicates<'tcx>,
    trait_id: Option<DefId>,
) -> Vec<TraitPredicate<'tcx>> {
    let mut preds = Vec::new();
    for (pred, _) in generics.predicates {
        if_chain! {
            if let ClauseKind::Trait(poly_trait_pred) = pred.kind().skip_binder();
            let trait_pred = cx.tcx.erase_late_bound_regions(pred.kind().rebind(poly_trait_pred));
            if let Some(trait_def_id) = trait_id;
            if trait_def_id == trait_pred.trait_ref.def_id;
            then {
                preds.push(trait_pred);
            }
        }
    }
    preds
}

fn get_projection_pred<'tcx>(
    cx: &LateContext<'tcx>,
    generics: GenericPredicates<'tcx>,
    trait_pred: TraitPredicate<'tcx>,
) -> Option<ProjectionPredicate<'tcx>> {
    generics.predicates.iter().find_map(|(proj_pred, _)| {
        if let ClauseKind::Projection(pred) = proj_pred.kind().skip_binder() {
            let projection_pred = cx.tcx.erase_late_bound_regions(proj_pred.kind().rebind(pred));
            if projection_pred.projection_ty.args == trait_pred.trait_ref.args {
                return Some(projection_pred);
            }
        }
        None
    })
}

fn get_args_to_check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> Vec<(usize, String)> {
    let mut args_to_check = Vec::new();
    if let Some(def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id) {
        let fn_sig = cx.tcx.fn_sig(def_id).instantiate_identity();
        let generics = cx.tcx.predicates_of(def_id);
        let fn_mut_preds = get_trait_predicates_for_trait_id(cx, generics, cx.tcx.lang_items().fn_mut_trait());
        let ord_preds = get_trait_predicates_for_trait_id(cx, generics, cx.tcx.get_diagnostic_item(sym::Ord));
        let partial_ord_preds =
            get_trait_predicates_for_trait_id(cx, generics, cx.tcx.lang_items().partial_ord_trait());
        // Trying to call erase_late_bound_regions on fn_sig.inputs() gives the following error
        // The trait `rustc::ty::TypeFoldable<'_>` is not implemented for
        // `&[rustc_middle::ty::Ty<'_>]`
        let inputs_output = cx.tcx.erase_late_bound_regions(fn_sig.inputs_and_output());
        inputs_output
            .iter()
            .rev()
            .skip(1)
            .rev()
            .enumerate()
            .for_each(|(i, inp)| {
                for trait_pred in &fn_mut_preds {
                    if_chain! {
                        if trait_pred.self_ty() == inp;
                        if let Some(return_ty_pred) = get_projection_pred(cx, generics, *trait_pred);
                        then {
                            if ord_preds
                                .iter()
                                .any(|ord| Some(ord.self_ty()) == return_ty_pred.term.ty())
                            {
                                args_to_check.push((i, "Ord".to_string()));
                            } else if partial_ord_preds
                                .iter()
                                .any(|pord| pord.self_ty() == return_ty_pred.term.ty().unwrap())
                            {
                                args_to_check.push((i, "PartialOrd".to_string()));
                            }
                        }
                    }
                }
            });
    }
    args_to_check
}

fn check_arg<'tcx>(cx: &LateContext<'tcx>, arg: &'tcx Expr<'tcx>) -> Option<(Span, Option<Span>)> {
    if_chain! {
        if let ExprKind::Closure(&Closure { body, fn_decl_span, .. }) = arg.kind;
        if let ty::Closure(_def_id, args) = &cx.typeck_results().node_type(arg.hir_id).kind();
        let ret_ty = args.as_closure().sig().output();
        let ty = cx.tcx.erase_late_bound_regions(ret_ty);
        if ty.is_unit();
        then {
            let body = cx.tcx.hir().body(body);
            if_chain! {
                if let ExprKind::Block(block, _) = body.value.kind;
                if block.expr.is_none();
                if let Some(stmt) = block.stmts.last();
                if let StmtKind::Semi(_) = stmt.kind;
                then {
                    let data = stmt.span.data();
                    // Make a span out of the semicolon for the help message
                    Some((fn_decl_span, Some(data.with_lo(data.hi-BytePos(1)))))
                } else {
                    Some((fn_decl_span, None))
                }
            }
        } else {
            None
        }
    }
}

impl<'tcx> LateLintPass<'tcx> for UnitReturnExpectingOrd {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
        if let ExprKind::MethodCall(_, receiver, args, _) = expr.kind {
            let arg_indices = get_args_to_check(cx, expr);
            let args = std::iter::once(receiver).chain(args.iter()).collect::<Vec<_>>();
            for (i, trait_name) in arg_indices {
                if i < args.len() {
                    match check_arg(cx, args[i]) {
                        Some((span, None)) => {
                            span_lint(
                                cx,
                                UNIT_RETURN_EXPECTING_ORD,
                                span,
                                &format!(
                                    "this closure returns \
                                   the unit type which also implements {trait_name}"
                                ),
                            );
                        },
                        Some((span, Some(last_semi))) => {
                            span_lint_and_help(
                                cx,
                                UNIT_RETURN_EXPECTING_ORD,
                                span,
                                &format!(
                                    "this closure returns \
                                   the unit type which also implements {trait_name}"
                                ),
                                Some(last_semi),
                                "probably caused by this trailing semicolon",
                            );
                        },
                        None => {},
                    }
                }
            }
        }
    }
}