src/librustc/middle/free_region.rs

//! This module handles the relationships between "free regions", i.e., lifetime parameters.
//! Ordinarily, free regions are unrelated to one another, but they can be related via implied
//! or explicit bounds. In that case, we track the bounds using the `TransitiveRelation` type,
//! and use that to decide when one free region outlives another, and so forth.

use crate::middle::region;
use crate::ty::free_region_map::{FreeRegionMap, FreeRegionRelations};
use crate::ty::{self, Region, TyCtxt};
use rustc_hir::def_id::DefId;

/// Combines a `region::ScopeTree` (which governs relationships between
/// scopes) and a `FreeRegionMap` (which governs relationships between
/// free regions) to yield a complete relation between concrete
/// regions.
///
/// This stuff is a bit convoluted and should be refactored, but as we
/// transition to NLL, it'll all go away anyhow.
pub struct RegionRelations<'a, 'tcx> {
    pub tcx: TyCtxt<'tcx>,

    /// The context used to fetch the region maps.
    pub context: DefId,

    /// The region maps for the given context.
    pub region_scope_tree: &'a region::ScopeTree,

    /// Free-region relationships.
    pub free_regions: &'a FreeRegionMap<'tcx>,
}

impl<'a, 'tcx> RegionRelations<'a, 'tcx> {
    pub fn new(
        tcx: TyCtxt<'tcx>,
        context: DefId,
        region_scope_tree: &'a region::ScopeTree,
        free_regions: &'a FreeRegionMap<'tcx>,
    ) -> Self {
        Self { tcx, context, region_scope_tree, free_regions }
    }

    /// Determines whether one region is a subregion of another. This is intended to run *after
    /// inference* and sadly the logic is somewhat duplicated with the code in infer.rs.
    pub fn is_subregion_of(
        &self,
        sub_region: ty::Region<'tcx>,
        super_region: ty::Region<'tcx>,
    ) -> bool {
        let result = sub_region == super_region || {
            match (sub_region, super_region) {
                (ty::ReEmpty, _) | (_, ty::ReStatic) => true,

                (ty::ReScope(sub_scope), ty::ReScope(super_scope)) => {
                    self.region_scope_tree.is_subscope_of(*sub_scope, *super_scope)
                }

                (ty::ReScope(sub_scope), ty::ReEarlyBound(ref br)) => {
                    let fr_scope = self.region_scope_tree.early_free_scope(self.tcx, br);
                    self.region_scope_tree.is_subscope_of(*sub_scope, fr_scope)
                }

                (ty::ReScope(sub_scope), ty::ReFree(fr)) => {
                    let fr_scope = self.region_scope_tree.free_scope(self.tcx, fr);
                    self.region_scope_tree.is_subscope_of(*sub_scope, fr_scope)
                }

                (ty::ReEarlyBound(_), ty::ReEarlyBound(_))
                | (ty::ReFree(_), ty::ReEarlyBound(_))
                | (ty::ReEarlyBound(_), ty::ReFree(_))
                | (ty::ReFree(_), ty::ReFree(_)) => {
                    self.free_regions.sub_free_regions(sub_region, super_region)
                }

                _ => false,
            }
        };
        let result = result || self.is_static(super_region);
        debug!(
            "is_subregion_of(sub_region={:?}, super_region={:?}) = {:?}",
            sub_region, super_region, result
        );
        result
    }

    /// Determines whether this free region is required to be `'static`.
    fn is_static(&self, super_region: ty::Region<'tcx>) -> bool {
        debug!("is_static(super_region={:?})", super_region);
        match *super_region {
            ty::ReStatic => true,
            ty::ReEarlyBound(_) | ty::ReFree(_) => {
                let re_static = self.tcx.mk_region(ty::ReStatic);
                self.free_regions.sub_free_regions(&re_static, &super_region)
            }
            _ => false,
        }
    }

    pub fn lub_free_regions(&self, r_a: Region<'tcx>, r_b: Region<'tcx>) -> Region<'tcx> {
        self.free_regions.lub_free_regions(self.tcx, r_a, r_b)
    }
}