rework opaque types region inference

See the ui tests for user-visible changes.

Author: aliemjay

compiler/rustc_borrowck/src/region_infer/opaque_types.rs

@@ -1,10 +1,10 @@
-use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
+use rustc_data_structures::fx::FxIndexMap;
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::LocalDefId;
 use rustc_hir::OpaqueTyOrigin;
-use rustc_infer::infer::InferCtxt;
 use rustc_infer::infer::TyCtxtInferExt as _;
+use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin};
 use rustc_infer::traits::{Obligation, ObligationCause};
 use rustc_middle::traits::DefiningAnchor;
 use rustc_middle::ty::visit::TypeVisitableExt;
@@ -66,85 +66,48 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     ) -> FxIndexMap<LocalDefId, OpaqueHiddenType<'tcx>> {
         let mut result: FxIndexMap<LocalDefId, OpaqueHiddenType<'tcx>> = FxIndexMap::default();
 
-        let member_constraints: FxIndexMap<_, _> = self
-            .member_constraints
-            .all_indices()
-            .map(|ci| (self.member_constraints[ci].key, ci))
-            .collect();
-        debug!(?member_constraints);
-
         for (opaque_type_key, concrete_type) in opaque_ty_decls {
-            let args = opaque_type_key.args;
-            debug!(?concrete_type, ?args);
+            debug!(?opaque_type_key, ?concrete_type);
 
-            let mut subst_regions = vec![self.universal_regions.fr_static];
+            let mut arg_regions: Vec<(ty::RegionVid, ty::Region<'_>)> =
+                vec![(self.universal_regions.fr_static, infcx.tcx.lifetimes.re_static)];
 
-            let to_universal_region = |vid, subst_regions: &mut Vec<_>| {
-                trace!(?vid);
-                let scc = self.constraint_sccs.scc(vid);
-                trace!(?scc);
-                match self.scc_values.universal_regions_outlived_by(scc).find_map(|lb| {
-                    self.eval_equal(vid, lb).then_some(self.definitions[lb].external_name?)
-                }) {
-                    Some(region) => {
-                        let vid = self.universal_regions.to_region_vid(region);
-                        subst_regions.push(vid);
-                        region
+            let opaque_type_key =
+                opaque_type_key.fold_captured_lifetime_args(infcx.tcx, |region| {
+                    let scc = self.constraint_sccs.scc(self.to_region_vid(region));
+                    let vid = self.scc_representatives[scc];
+                    let named = match self.definitions[vid].origin {
+                        NllRegionVariableOrigin::FreeRegion => self.definitions[vid].external_name,
+                        NllRegionVariableOrigin::Placeholder(placeholder) => {
+                            Some(ty::Region::new_placeholder(infcx.tcx, placeholder))
+                        }
+                        NllRegionVariableOrigin::Existential { .. } => None,
                     }
-                    None => {
-                        subst_regions.push(vid);
+                    .unwrap_or_else(|| {
                         ty::Region::new_error_with_message(
                             infcx.tcx,
                             concrete_type.span,
                             "opaque type with non-universal region args",
                         )
-                    }
-                }
-            };
+                    });
 
-            // Start by inserting universal regions from the member_constraint choice regions.
-            // This will ensure they get precedence when folding the regions in the concrete type.
-            if let Some(&ci) = member_constraints.get(&opaque_type_key) {
-                for &vid in self.member_constraints.choice_regions(ci) {
-                    to_universal_region(vid, &mut subst_regions);
-                }
-            }
-            debug!(?subst_regions);
-
-            // Next, insert universal regions from args, so we can translate regions that appear
-            // in them but are not subject to member constraints, for instance closure args.
-            let universal_args = infcx.tcx.fold_regions(args, |region, _| {
-                if let ty::RePlaceholder(..) = region.kind() {
-                    // Higher kinded regions don't need remapping, they don't refer to anything outside of this the args.
-                    return region;
-                }
+                    arg_regions.push((vid, named));
+                    named
+                });
+            debug!(?opaque_type_key, ?arg_regions);
+
+            let concrete_type = infcx.tcx.fold_regions(concrete_type, |region, _| {
                 let vid = self.to_region_vid(region);
-                to_universal_region(vid, &mut subst_regions)
+                arg_regions
+                    .iter()
+                    .find(|&&(ur_vid, _)| self.eval_equal(vid, ur_vid))
+                    .map(|&(_, ur_name)| ur_name)
+                    .unwrap_or(infcx.tcx.lifetimes.re_erased)
             });
-            debug!(?universal_args);
-            debug!(?subst_regions);
-
-            // Deduplicate the set of regions while keeping the chosen order.
-            let subst_regions = subst_regions.into_iter().collect::<FxIndexSet<_>>();
-            debug!(?subst_regions);
-
-            let universal_concrete_type =
-                infcx.tcx.fold_regions(concrete_type, |region, _| match *region {
-                    ty::ReVar(vid) => subst_regions
-                        .iter()
-                        .find(|ur_vid| self.eval_equal(vid, **ur_vid))
-                        .and_then(|ur_vid| self.definitions[*ur_vid].external_name)
-                        .unwrap_or(infcx.tcx.lifetimes.re_erased),
-                    _ => region,
-                });
-            debug!(?universal_concrete_type);
+            debug!(?concrete_type);
 
-            let opaque_type_key =
-                OpaqueTypeKey { def_id: opaque_type_key.def_id, args: universal_args };
-            let ty = infcx.infer_opaque_definition_from_instantiation(
-                opaque_type_key,
-                universal_concrete_type,
-            );
+            let ty =
+                infcx.infer_opaque_definition_from_instantiation(opaque_type_key, concrete_type);
             // Sometimes two opaque types are the same only after we remap the generic parameters
             // back to the opaque type definition. E.g. we may have `OpaqueType<X, Y>` mapped to `(X, Y)`
             // and `OpaqueType<Y, X>` mapped to `(Y, X)`, and those are the same, but we only know that

compiler/rustc_middle/src/ty/mod.rs

@@ -1504,6 +1504,25 @@ pub struct OpaqueTypeKey<'tcx> {
     pub args: GenericArgsRef<'tcx>,
 }
 
+impl<'tcx> OpaqueTypeKey<'tcx> {
+    pub fn fold_captured_lifetime_args(
+        self,
+        tcx: TyCtxt<'tcx>,
+        mut f: impl FnMut(Region<'tcx>) -> Region<'tcx>,
+    ) -> Self {
+        let Self { def_id, args } = self;
+        let args = std::iter::zip(args, tcx.variances_of(def_id)).map(|(arg, v)| {
+            match (arg.unpack(), v) {
+                (ty::GenericArgKind::Lifetime(_), ty::Bivariant) => arg,
+                (ty::GenericArgKind::Lifetime(lt), _) => f(lt).into(),
+                _ => arg,
+            }
+        });
+        let args = tcx.mk_args_from_iter(args);
+        Self { def_id, args }
+    }
+}
+
 #[derive(Copy, Clone, Debug, TypeFoldable, TypeVisitable, HashStable, TyEncodable, TyDecodable)]
 pub struct OpaqueHiddenType<'tcx> {
     /// The span of this particular definition of the opaque type. So

tests/ui/impl-trait/defining-use-uncaptured-non-universal-region-2.rs

@@ -0,0 +1,74 @@
+// issue: #110623
+// check-pass
+
+use std::{collections::BTreeMap, num::ParseIntError, str::FromStr};
+
+enum FileSystem {
+    File(usize),
+    Directory(BTreeMap<String, FileSystem>),
+}
+
+impl FromStr for FileSystem {
+    type Err = ParseIntError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        if s.starts_with("dir") {
+            Ok(Self::new_dir())
+        } else {
+            Ok(Self::File(s.split_whitespace().next().unwrap().parse()?))
+        }
+    }
+}
+
+impl FileSystem {
+    fn new_dir() -> FileSystem {
+        FileSystem::Directory(BTreeMap::new())
+    }
+
+    fn insert(&mut self, name: String, other: FileSystem) -> Option<FileSystem> {
+        match self {
+            FileSystem::File(_) => panic!("can only insert into directory!"),
+            FileSystem::Directory(tree) => tree.insert(name, other),
+        }
+    }
+
+    // Recursively build a tree from commands. This uses (abuses?)
+    // the fact that `cd /` only appears at the start and that
+    // subsequent `cd`s can only move ONE level to use the recursion
+    // stack as the filesystem stack
+    fn build<'a>(
+        &mut self,
+        mut commands: impl Iterator<Item = &'a str> + 'a,
+    ) -> Option<impl Iterator<Item = &'a str> + 'a> {
+        let cmd = commands.next()?;
+        let mut elements = cmd.lines();
+        match elements.next().map(str::trim) {
+            Some("cd /") | None => self.build(commands),
+            Some("cd ..") => {
+                // return to higher scope
+                Some(commands)
+            }
+            Some("ls") => {
+                for item in elements {
+                    let name = item.split_whitespace().last().unwrap();
+                    let prior = self.insert(name.to_string(), item.parse().unwrap());
+                    debug_assert!(prior.is_none());
+                }
+                // continue on
+                self.build(commands)
+            }
+            Some(other_cd) => {
+                let name = other_cd
+                    .trim()
+                    .strip_prefix("cd ")
+                    .expect("expected a cd command");
+                let mut directory = FileSystem::new_dir();
+                let further_commands = directory.build(commands);
+                self.insert(name.to_string(), directory);
+                self.build(further_commands?) // THIS LINE FAILS TO COMPILE
+            }
+        }
+    }
+}
+
+fn main() {}

tests/ui/impl-trait/defining-use-uncaptured-non-universal-region-3.rs

@@ -0,0 +1,13 @@
+// check-pass
+
+#![feature(adt_const_params)]
+#![allow(incomplete_features)]
+
+trait Bar<const FOO: &'static str> {}
+impl Bar<"asdf"> for () {}
+
+fn foo<const FOO: &'static str>() -> impl Bar<"asdf"> {
+    ()
+}
+
+fn main() {}

tests/ui/impl-trait/defining-use-uncaptured-non-universal-region.rs

@@ -0,0 +1,16 @@
+// issue: #111906
+// check-pass
+
+#![allow(unconditional_recursion)]
+
+fn foo<'a: 'a>() -> impl Sized {
+    let _: () = foo::<'a>();
+    loop {}
+}
+
+fn bar<'a: 'a>() -> impl Sized + 'a {
+    let _: *mut &'a () = bar::<'a>();
+    loop {}
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/defined-by-user-annotation.rs

@@ -1,5 +1,6 @@
 // User type annotation in fn bodies is a a valid defining site for opaque types.
 // check-pass
+
 #![feature(type_alias_impl_trait)]
 
 trait Equate { type Proj; }
@@ -8,12 +9,24 @@ impl<T> Equate for T { type Proj = T; }
 trait Indirect { type Ty; }
 impl<A, B: Equate<Proj = A>> Indirect for (A, B) { type Ty = (); }
 
-type Opq = impl Sized;
-fn define_1(_: Opq) {
-    let _ = None::<<(Opq, u8) as Indirect>::Ty>;
+mod basic {
+    use super::*;
+    type Opq = impl Sized;
+    fn define_1(_: Opq) {
+        let _ = None::<<(Opq, u8) as Indirect>::Ty>;
+    }
+    fn define_2() -> Opq {
+        0u8
+    }
 }
-fn define_2() -> Opq {
-    0u8
+
+// `Opq<'a> == &'b u8` shouldn't be an error because `'a == 'b`.
+mod lifetime {
+    use super::*;
+    type Opq<'a> = impl Sized + 'a;
+    fn define<'a: 'b, 'b: 'a>(_: Opq<'a>) {
+        let _ = None::<<(Opq<'a>, &'b u8) as Indirect>::Ty>;
+    }
 }
 
 fn main() {}

tests/ui/type-alias-impl-trait/generic-not-strictly-equal.basic.stderr

@@ -0,0 +1,12 @@
+error[E0792]: expected generic lifetime parameter, found `'_`
+  --> $DIR/generic-not-strictly-equal.rs:33:5
+   |
+LL | type Opaque<'a> = impl Copy + Captures<'a>;
+   |             -- this generic parameter must be used with a generic lifetime parameter
+...
+LL |     relate(opaque, hidden); // defining use: Opaque<'?1> := u8
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0792`.

tests/ui/type-alias-impl-trait/generic-not-strictly-equal.member_constraints.stderr

@@ -0,0 +1,15 @@
+error[E0700]: hidden type for `Opaque<'x>` captures lifetime that does not appear in bounds
+  --> $DIR/generic-not-strictly-equal.rs:33:5
+   |
+LL | type Opaque<'a> = impl Copy + Captures<'a>;
+   |                   ------------------------ opaque type defined here
+LL |
+LL | fn test<'x>(_: Opaque<'x>) {
+   |         -- hidden type `&'x u8` captures the lifetime `'x` as defined here
+...
+LL |     relate(opaque, hidden); // defining use: Opaque<'?1> := u8
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0700`.

tests/ui/type-alias-impl-trait/generic-not-strictly-equal.rs

@@ -0,0 +1,38 @@
+// `Opaque<'?1> := u8` is not a valid defining use here.
+//
+// Due to fundamental limitations of the member constraints algorithm,
+// we require '?1 to be *equal* to some universal region.
+//
+// While '?1 is eventually inferred to be equal to 'x because of the constraint '?1: 'x,
+// we don't consider them equal in the strict sense because they lack the bidirectional outlives
+// constraints ['?1: 'x, 'x: '?1]. In NLL terms, they are not part of the same SCC.
+//
+// See #113971 for details.
+
+// revisions: basic member_constraints
+#![feature(type_alias_impl_trait)]
+
+trait Captures<'a> {}
+impl<T> Captures<'_> for T {}
+
+fn ensure_outlives<'a, X: 'a>(_: X) {}
+fn relate<X>(_: X, _: X) {}
+
+type Opaque<'a> = impl Copy + Captures<'a>;
+
+fn test<'x>(_: Opaque<'x>) {
+    let opaque = None::<Opaque<'_>>; // let's call this lifetime '?1
+
+    #[cfg(basic)]
+    let hidden = None::<u8>;
+
+    #[cfg(member_constraints)]
+    let hidden = None::<&'x u8>;
+
+    ensure_outlives::<'x>(opaque); // outlives constraint: '?1: 'x
+    relate(opaque, hidden); // defining use: Opaque<'?1> := u8
+    //[basic]~^ ERROR expected generic lifetime parameter, found `'_`
+    //[member_constraints]~^^ ERROR captures lifetime that does not appear in bounds
+}
+
+fn main() {}