compiler/rustc_infer/src/infer/error_reporting/nice_region_error/trait_impl_difference.rs

//! Error Reporting for `impl` items that do not match the obligations from their `trait`.

use crate::errors::{ConsiderBorrowingParamHelp, RelationshipHelp, TraitImplDiff};
use crate::infer::error_reporting::nice_region_error::NiceRegionError;
use crate::infer::lexical_region_resolve::RegionResolutionError;
use crate::infer::{Subtype, ValuePairs};
use rustc_errors::ErrorGuaranteed;
use rustc_hir as hir;
use rustc_hir::def::Res;
use rustc_hir::def_id::DefId;
use rustc_hir::intravisit::Visitor;
use rustc_middle::hir::nested_filter;
use rustc_middle::traits::ObligationCauseCode;
use rustc_middle::ty::error::ExpectedFound;
use rustc_middle::ty::print::RegionHighlightMode;
use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitor};
use rustc_span::Span;

impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
    /// Print the error message for lifetime errors when the `impl` doesn't conform to the `trait`.
    pub(super) fn try_report_impl_not_conforming_to_trait(&self) -> Option<ErrorGuaranteed> {
        let error = self.error.as_ref()?;
        debug!("try_report_impl_not_conforming_to_trait {:?}", error);
        if let RegionResolutionError::SubSupConflict(
            _,
            var_origin,
            sub_origin,
            _sub,
            sup_origin,
            _sup,
            _,
        ) = error.clone()
            && let (Subtype(sup_trace), Subtype(sub_trace)) = (&sup_origin, &sub_origin)
            && let ObligationCauseCode::CompareImplItem { trait_item_def_id, .. } =
                sub_trace.cause.code()
            && sub_trace.values == sup_trace.values
            && let ValuePairs::PolySigs(ExpectedFound { expected, found }) = sub_trace.values
        {
            // FIXME(compiler-errors): Don't like that this needs `Ty`s, but
            // all of the region highlighting machinery only deals with those.
            let guar = self.emit_err(
                var_origin.span(),
                Ty::new_fn_ptr(self.cx.tcx, expected),
                Ty::new_fn_ptr(self.cx.tcx, found),
                *trait_item_def_id,
            );
            return Some(guar);
        }
        None
    }

    fn emit_err(
        &self,
        sp: Span,
        expected: Ty<'tcx>,
        found: Ty<'tcx>,
        trait_def_id: DefId,
    ) -> ErrorGuaranteed {
        let trait_sp = self.tcx().def_span(trait_def_id);

        // Mark all unnamed regions in the type with a number.
        // This diagnostic is called in response to lifetime errors, so be informative.
        struct HighlightBuilder<'tcx> {
            highlight: RegionHighlightMode<'tcx>,
            counter: usize,
        }

        impl<'tcx> HighlightBuilder<'tcx> {
            fn build(ty: Ty<'tcx>) -> RegionHighlightMode<'tcx> {
                let mut builder =
                    HighlightBuilder { highlight: RegionHighlightMode::default(), counter: 1 };
                builder.visit_ty(ty);
                builder.highlight
            }
        }

        impl<'tcx> ty::visit::TypeVisitor<TyCtxt<'tcx>> for HighlightBuilder<'tcx> {
            fn visit_region(&mut self, r: ty::Region<'tcx>) {
                if !r.has_name() && self.counter <= 3 {
                    self.highlight.highlighting_region(r, self.counter);
                    self.counter += 1;
                }
            }
        }

        let expected_highlight = HighlightBuilder::build(expected);
        let expected = self
            .cx
            .extract_inference_diagnostics_data(expected.into(), Some(expected_highlight))
            .name;
        let found_highlight = HighlightBuilder::build(found);
        let found =
            self.cx.extract_inference_diagnostics_data(found.into(), Some(found_highlight)).name;

        // Get the span of all the used type parameters in the method.
        let assoc_item = self.tcx().associated_item(trait_def_id);
        let mut visitor = TypeParamSpanVisitor { tcx: self.tcx(), types: vec![] };
        match assoc_item.kind {
            ty::AssocKind::Fn => {
                let hir = self.tcx().hir();
                if let Some(hir_id) =
                    assoc_item.def_id.as_local().map(|id| self.tcx().local_def_id_to_hir_id(id))
                {
                    if let Some(decl) = hir.fn_decl_by_hir_id(hir_id) {
                        visitor.visit_fn_decl(decl);
                    }
                }
            }
            _ => {}
        }

        let diag = TraitImplDiff {
            sp,
            trait_sp,
            note: (),
            param_help: ConsiderBorrowingParamHelp { spans: visitor.types.to_vec() },
            rel_help: visitor.types.is_empty().then_some(RelationshipHelp),
            expected,
            found,
        };

        self.tcx().dcx().emit_err(diag)
    }
}

struct TypeParamSpanVisitor<'tcx> {
    tcx: TyCtxt<'tcx>,
    types: Vec<Span>,
}

impl<'tcx> Visitor<'tcx> for TypeParamSpanVisitor<'tcx> {
    type NestedFilter = nested_filter::OnlyBodies;

    fn nested_visit_map(&mut self) -> Self::Map {
        self.tcx.hir()
    }

    fn visit_ty(&mut self, arg: &'tcx hir::Ty<'tcx>) {
        match arg.kind {
            hir::TyKind::Ref(_, ref mut_ty) => {
                // We don't want to suggest looking into borrowing `&T` or `&Self`.
                hir::intravisit::walk_ty(self, mut_ty.ty);
                return;
            }
            hir::TyKind::Path(hir::QPath::Resolved(None, path)) => match &path.segments {
                [segment]
                    if matches!(
                        segment.res,
                        Res::SelfTyParam { .. }
                            | Res::SelfTyAlias { .. }
                            | Res::Def(hir::def::DefKind::TyParam, _)
                    ) =>
                {
                    self.types.push(path.span);
                }
                _ => {}
            },
            _ => {}
        }
        hir::intravisit::walk_ty(self, arg);
    }
}