clippy_lints/src/functions/result.rs

use clippy_utils::msrvs::{self, Msrv};
use clippy_utils::res::MaybeDef;
use rustc_errors::Diag;
use rustc_hir as hir;
use rustc_lint::{LateContext, LintContext};
use rustc_middle::ty::{self, Ty};
use rustc_span::def_id::DefIdSet;
use rustc_span::{Span, sym};

use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_then};
use clippy_utils::ty::{AdtVariantInfo, approx_ty_size};
use clippy_utils::{is_no_std_crate, trait_ref_of_method};

use super::{RESULT_LARGE_ERR, RESULT_UNIT_ERR};

/// The type of the `Err`-variant in a `std::result::Result` returned by the
/// given `FnDecl`
fn result_err_ty<'tcx>(
    cx: &LateContext<'tcx>,
    decl: &hir::FnDecl<'tcx>,
    id: hir::def_id::LocalDefId,
    item_span: Span,
) -> Option<(&'tcx hir::Ty<'tcx>, Ty<'tcx>)> {
    if !item_span.in_external_macro(cx.sess().source_map())
        && let hir::FnRetTy::Return(hir_ty) = decl.output
        && let ty = cx
            .tcx
            .instantiate_bound_regions_with_erased(cx.tcx.fn_sig(id).instantiate_identity().output())
        && ty.is_diag_item(cx, sym::Result)
        && let ty::Adt(_, args) = ty.kind()
    {
        let err_ty = args.type_at(1);
        Some((hir_ty, err_ty))
    } else {
        None
    }
}

pub(super) fn check_item<'tcx>(
    cx: &LateContext<'tcx>,
    item: &hir::Item<'tcx>,
    large_err_threshold: u64,
    large_err_ignored: &DefIdSet,
    msrv: Msrv,
) {
    if let hir::ItemKind::Fn { ref sig, .. } = item.kind
        && let Some((hir_ty, err_ty)) = result_err_ty(cx, sig.decl, item.owner_id.def_id, item.span)
    {
        if cx.effective_visibilities.is_exported(item.owner_id.def_id) {
            let fn_header_span = item.span.with_hi(sig.decl.output.span().hi());
            check_result_unit_err(cx, err_ty, fn_header_span, msrv);
        }
        check_result_large_err(cx, err_ty, hir_ty.span, large_err_threshold, large_err_ignored);
    }
}

pub(super) fn check_impl_item<'tcx>(
    cx: &LateContext<'tcx>,
    item: &hir::ImplItem<'tcx>,
    large_err_threshold: u64,
    large_err_ignored: &DefIdSet,
    msrv: Msrv,
) {
    // Don't lint if method is a trait's implementation, we can't do anything about those
    if let hir::ImplItemKind::Fn(ref sig, _) = item.kind
        && let Some((hir_ty, err_ty)) = result_err_ty(cx, sig.decl, item.owner_id.def_id, item.span)
        && trait_ref_of_method(cx, item.owner_id).is_none()
    {
        if cx.effective_visibilities.is_exported(item.owner_id.def_id) {
            let fn_header_span = item.span.with_hi(sig.decl.output.span().hi());
            check_result_unit_err(cx, err_ty, fn_header_span, msrv);
        }
        check_result_large_err(cx, err_ty, hir_ty.span, large_err_threshold, large_err_ignored);
    }
}

pub(super) fn check_trait_item<'tcx>(
    cx: &LateContext<'tcx>,
    item: &hir::TraitItem<'tcx>,
    large_err_threshold: u64,
    large_err_ignored: &DefIdSet,
    msrv: Msrv,
) {
    if let hir::TraitItemKind::Fn(ref sig, _) = item.kind {
        let fn_header_span = item.span.with_hi(sig.decl.output.span().hi());
        if let Some((hir_ty, err_ty)) = result_err_ty(cx, sig.decl, item.owner_id.def_id, item.span) {
            if cx.effective_visibilities.is_exported(item.owner_id.def_id) {
                check_result_unit_err(cx, err_ty, fn_header_span, msrv);
            }
            check_result_large_err(cx, err_ty, hir_ty.span, large_err_threshold, large_err_ignored);
        }
    }
}

fn check_result_unit_err(cx: &LateContext<'_>, err_ty: Ty<'_>, fn_header_span: Span, msrv: Msrv) {
    if err_ty.is_unit() && (!is_no_std_crate(cx) || msrv.meets(cx, msrvs::ERROR_IN_CORE)) {
        span_lint_and_help(
            cx,
            RESULT_UNIT_ERR,
            fn_header_span,
            "this returns a `Result<_, ()>`",
            None,
            "use a custom `Error` type instead",
        );
    }
}

fn check_result_large_err<'tcx>(
    cx: &LateContext<'tcx>,
    err_ty: Ty<'tcx>,
    hir_ty_span: Span,
    large_err_threshold: u64,
    large_err_ignored: &DefIdSet,
) {
    if let ty::Adt(adt, _) = err_ty.kind()
        && large_err_ignored.contains(&adt.did())
    {
        return;
    }
    if let ty::Adt(adt, subst) = err_ty.kind()
        && let Some(local_def_id) = adt.did().as_local()
        && let hir::Node::Item(item) = cx.tcx.hir_node_by_def_id(local_def_id)
        && let hir::ItemKind::Enum(_, _, ref def) = item.kind
    {
        let variants_size = AdtVariantInfo::new(cx, *adt, subst);
        if let Some((first_variant, variants)) = variants_size.split_first()
            && first_variant.size >= large_err_threshold
        {
            span_lint_and_then(
                cx,
                RESULT_LARGE_ERR,
                hir_ty_span,
                "the `Err`-variant returned from this function is very large",
                |diag| {
                    diag.span_label(
                        def.variants[first_variant.ind].span,
                        format!("the largest variant contains at least {} bytes", variants_size[0].size),
                    );

                    for variant in variants {
                        if variant.size >= large_err_threshold {
                            let variant_def = &def.variants[variant.ind];
                            diag.span_label(
                                variant_def.span,
                                format!(
                                    "the variant `{}` contains at least {} bytes",
                                    variant_def.ident, variant.size
                                ),
                            );
                        }
                    }

                    diag.help(format!("try reducing the size of `{err_ty}`, for example by boxing large elements or replacing it with `Box<{err_ty}>`"));
                },
            );
        }
    } else {
        let ty_size = approx_ty_size(cx, err_ty);
        if ty_size >= large_err_threshold {
            span_lint_and_then(
                cx,
                RESULT_LARGE_ERR,
                hir_ty_span,
                "the `Err`-variant returned from this function is very large",
                |diag: &mut Diag<'_, ()>| {
                    diag.span_label(hir_ty_span, format!("the `Err`-variant is at least {ty_size} bytes"));
                    diag.help(format!("try reducing the size of `{err_ty}`, for example by boxing large elements or replacing it with `Box<{err_ty}>`"));
                },
            );
        }
    }
}