rustc_trait_selection/error_reporting/infer/nice_region_error/
different_lifetimes.rs

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//! Error Reporting for Anonymous Region Lifetime Errors
//! where both the regions are anonymous.

use rustc_errors::{Diag, ErrorGuaranteed, Subdiagnostic};
use rustc_hir::Ty;
use rustc_hir::def_id::LocalDefId;
use rustc_middle::ty::{Region, TyCtxt};
use tracing::debug;

use crate::error_reporting::infer::nice_region_error::NiceRegionError;
use crate::error_reporting::infer::nice_region_error::find_anon_type::find_anon_type;
use crate::error_reporting::infer::nice_region_error::util::AnonymousParamInfo;
use crate::errors::{AddLifetimeParamsSuggestion, LifetimeMismatch, LifetimeMismatchLabels};
use crate::infer::{RegionResolutionError, SubregionOrigin};

impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
    /// Print the error message for lifetime errors when both the concerned regions are anonymous.
    ///
    /// Consider a case where we have
    ///
    /// ```compile_fail
    /// fn foo(x: &mut Vec<&u8>, y: &u8) {
    ///     x.push(y);
    /// }
    /// ```
    ///
    /// The example gives
    ///
    /// ```text
    /// fn foo(x: &mut Vec<&u8>, y: &u8) {
    ///                    ---      --- these references are declared with different lifetimes...
    ///     x.push(y);
    ///     ^ ...but data from `y` flows into `x` here
    /// ```
    ///
    /// It has been extended for the case of structs too.
    ///
    /// Consider the example
    ///
    /// ```no_run
    /// struct Ref<'a> { x: &'a u32 }
    /// ```
    ///
    /// ```text
    /// fn foo(mut x: Vec<Ref>, y: Ref) {
    ///                   ---      --- these structs are declared with different lifetimes...
    ///     x.push(y);
    ///     ^ ...but data from `y` flows into `x` here
    /// }
    /// ```
    ///
    /// It will later be extended to trait objects.
    pub(super) fn try_report_anon_anon_conflict(&self) -> Option<ErrorGuaranteed> {
        let (span, sub, sup) = self.regions()?;

        if let Some(RegionResolutionError::ConcreteFailure(
            SubregionOrigin::ReferenceOutlivesReferent(..),
            ..,
        )) = self.error
        {
            // This error doesn't make much sense in this case.
            return None;
        }

        // Determine whether the sub and sup consist of both anonymous (elided) regions.
        let sup_info = self.tcx().is_suitable_region(self.generic_param_scope, sup)?;

        let sub_info = self.tcx().is_suitable_region(self.generic_param_scope, sub)?;

        let ty_sup = find_anon_type(self.tcx(), self.generic_param_scope, sup)?;

        let ty_sub = find_anon_type(self.tcx(), self.generic_param_scope, sub)?;

        debug!("try_report_anon_anon_conflict: found_param1={:?} sup={:?}", ty_sub, sup);
        debug!("try_report_anon_anon_conflict: found_param2={:?} sub={:?}", ty_sup, sub);

        let (ty_sup, ty_fndecl_sup) = ty_sup;
        let (ty_sub, ty_fndecl_sub) = ty_sub;

        let AnonymousParamInfo { param: anon_param_sup, .. } =
            self.find_param_with_region(sup, sup)?;
        let AnonymousParamInfo { param: anon_param_sub, .. } =
            self.find_param_with_region(sub, sub)?;

        let sup_is_ret_type =
            self.is_return_type_anon(sup_info.scope, sup_info.region_def_id, ty_fndecl_sup);
        let sub_is_ret_type =
            self.is_return_type_anon(sub_info.scope, sub_info.region_def_id, ty_fndecl_sub);

        debug!(
            "try_report_anon_anon_conflict: sub_is_ret_type={:?} sup_is_ret_type={:?}",
            sub_is_ret_type, sup_is_ret_type
        );

        let labels = match (sup_is_ret_type, sub_is_ret_type) {
            (ret_capture @ Some(ret_span), _) | (_, ret_capture @ Some(ret_span)) => {
                let param_span =
                    if sup_is_ret_type == ret_capture { ty_sub.span } else { ty_sup.span };
                LifetimeMismatchLabels::InRet {
                    param_span,
                    ret_span,
                    span,
                    label_var1: anon_param_sup.pat.simple_ident(),
                }
            }

            (None, None) => LifetimeMismatchLabels::Normal {
                hir_equal: ty_sup.hir_id == ty_sub.hir_id,
                ty_sup: ty_sup.span,
                ty_sub: ty_sub.span,
                span,
                sup: anon_param_sup.pat.simple_ident(),
                sub: anon_param_sub.pat.simple_ident(),
            },
        };

        let suggestion = AddLifetimeParamsSuggestion {
            tcx: self.tcx(),
            sub,
            ty_sup,
            ty_sub,
            add_note: true,
            generic_param_scope: self.generic_param_scope,
        };
        let err = LifetimeMismatch { span, labels, suggestion };
        let reported = self.tcx().dcx().emit_err(err);
        Some(reported)
    }
}

/// Currently only used in rustc_borrowck, probably should be
/// removed in favour of public_errors::AddLifetimeParamsSuggestion
pub fn suggest_adding_lifetime_params<'tcx>(
    tcx: TyCtxt<'tcx>,
    err: &mut Diag<'_>,
    generic_param_scope: LocalDefId,
    sub: Region<'tcx>,
    ty_sup: &'tcx Ty<'_>,
    ty_sub: &'tcx Ty<'_>,
) {
    let suggestion = AddLifetimeParamsSuggestion {
        tcx,
        sub,
        ty_sup,
        ty_sub,
        add_note: false,
        generic_param_scope,
    };
    suggestion.add_to_diag(err);
}