compiler/rustc_infer/src/traits/error_reporting/mod.rs

use super::ObjectSafetyViolation;

use crate::infer::InferCtxt;
use rustc_data_structures::fx::FxIndexSet;
use rustc_errors::{struct_span_err, DiagnosticBuilder, ErrorGuaranteed, MultiSpan};
use rustc_hir as hir;
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_middle::ty::print::with_no_trimmed_paths;
use rustc_middle::ty::{self, TyCtxt};
use rustc_span::Span;
use std::fmt;
use std::iter;

impl<'tcx> InferCtxt<'tcx> {
    pub fn report_extra_impl_obligation(
        &self,
        error_span: Span,
        impl_item_def_id: LocalDefId,
        trait_item_def_id: DefId,
        requirement: &dyn fmt::Display,
    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
        let mut err = struct_span_err!(
            self.tcx.sess,
            error_span,
            E0276,
            "impl has stricter requirements than trait"
        );

        if !self.tcx.is_impl_trait_in_trait(trait_item_def_id) {
            if let Some(span) = self.tcx.hir().span_if_local(trait_item_def_id) {
                let item_name = self.tcx.item_name(impl_item_def_id.to_def_id());
                err.span_label(span, format!("definition of `{item_name}` from trait"));
            }
        }

        err.span_label(error_span, format!("impl has extra requirement {requirement}"));

        err
    }
}

pub fn report_object_safety_error<'tcx>(
    tcx: TyCtxt<'tcx>,
    span: Span,
    trait_def_id: DefId,
    violations: &[ObjectSafetyViolation],
) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
    let trait_str = tcx.def_path_str(trait_def_id);
    let trait_span = tcx.hir().get_if_local(trait_def_id).and_then(|node| match node {
        hir::Node::Item(item) => Some(item.ident.span),
        _ => None,
    });
    let mut err = struct_span_err!(
        tcx.sess,
        span,
        E0038,
        "the trait `{}` cannot be made into an object",
        trait_str
    );
    err.span_label(span, format!("`{trait_str}` cannot be made into an object"));

    let mut reported_violations = FxIndexSet::default();
    let mut multi_span = vec![];
    let mut messages = vec![];
    for violation in violations {
        if let ObjectSafetyViolation::SizedSelf(sp) = &violation
            && !sp.is_empty()
        {
            // Do not report `SizedSelf` without spans pointing at `SizedSelf` obligations
            // with a `Span`.
            reported_violations.insert(ObjectSafetyViolation::SizedSelf(vec![].into()));
        }
        if reported_violations.insert(violation.clone()) {
            let spans = violation.spans();
            let msg = if trait_span.is_none() || spans.is_empty() {
                format!("the trait cannot be made into an object because {}", violation.error_msg())
            } else {
                format!("...because {}", violation.error_msg())
            };
            if spans.is_empty() {
                err.note(msg);
            } else {
                for span in spans {
                    multi_span.push(span);
                    messages.push(msg.clone());
                }
            }
        }
    }
    let has_multi_span = !multi_span.is_empty();
    let mut note_span = MultiSpan::from_spans(multi_span.clone());
    if let (Some(trait_span), true) = (trait_span, has_multi_span) {
        note_span.push_span_label(trait_span, "this trait cannot be made into an object...");
    }
    for (span, msg) in iter::zip(multi_span, messages) {
        note_span.push_span_label(span, msg);
    }
    err.span_note(
        note_span,
        "for a trait to be \"object safe\" it needs to allow building a vtable to allow the call \
         to be resolvable dynamically; for more information visit \
         <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
    );
    if trait_span.is_some() {
        let mut reported_violations: Vec<_> = reported_violations.into_iter().collect();
        reported_violations.sort();
        for violation in reported_violations {
            // Only provide the help if its a local trait, otherwise it's not actionable.
            violation.solution(&mut err);
        }
    }

    let impls_of = tcx.trait_impls_of(trait_def_id);
    let impls = if impls_of.blanket_impls().is_empty() {
        impls_of
            .non_blanket_impls()
            .values()
            .flatten()
            .filter(|def_id| {
                !matches!(tcx.type_of(*def_id).instantiate_identity().kind(), ty::Dynamic(..))
            })
            .collect::<Vec<_>>()
    } else {
        vec![]
    };
    let externally_visible = if !impls.is_empty()
        && let Some(def_id) = trait_def_id.as_local()
        && tcx.effective_visibilities(()).is_exported(def_id)
    {
        true
    } else {
        false
    };
    match &impls[..] {
        [] => {}
        _ if impls.len() > 9 => {}
        [only] if externally_visible => {
            err.help(with_no_trimmed_paths!(format!(
                "only type `{}` is seen to implement the trait in this crate, consider using it \
                 directly instead",
                tcx.type_of(*only).instantiate_identity(),
            )));
        }
        [only] => {
            err.help(with_no_trimmed_paths!(format!(
                "only type `{}` implements the trait, consider using it directly instead",
                tcx.type_of(*only).instantiate_identity(),
            )));
        }
        impls => {
            let types = impls
                .iter()
                .map(|t| {
                    with_no_trimmed_paths!(format!("  {}", tcx.type_of(*t).instantiate_identity(),))
                })
                .collect::<Vec<_>>();
            err.help(format!(
                "the following types implement the trait, consider defining an enum where each \
                 variant holds one of these types, implementing `{}` for this new enum and using \
                 it instead:\n{}",
                trait_str,
                types.join("\n"),
            ));
        }
    }
    if externally_visible {
        err.note(format!(
            "`{trait_str}` can be implemented in other crates; if you want to support your users \
             passing their own types here, you can't refer to a specific type",
        ));
    }

    err
}