Auto merge of #116891 - aliemjay:opaque-region-infer-rework-2, r=<try>

rework opaque type region inference

fixes #113971 Pass -> Error

fixes #111906 ICE -> Pass
fixes #110623 ==
fixes #109059 ==

fixes #112841 Pass -> Error

fixes #110726 ICE->Error

fixes #111935 Pass -> Error
fixes #113916 ==

r? `@ghost`

Author: bors

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -97,9 +97,11 @@ pub struct RegionInferenceContext<'tcx> {
     /// visible from this index.
     scc_universes: IndexVec<ConstraintSccIndex, ty::UniverseIndex>,
 
-    /// Contains a "representative" from each SCC. This will be the
-    /// minimal RegionVid belonging to that universe. It is used as a
-    /// kind of hacky way to manage checking outlives relationships,
+    /// Contains the "representative" region of each SCC.
+    /// It is defined as the one with the minimal RegionVid, favoring
+    /// free regions, then placeholders, then existential regions.
+    ///
+    /// It is a hacky way to manage checking regions for equality,
     /// since we can 'canonicalize' each region to the representative
     /// of its SCC and be sure that -- if they have the same repr --
     /// they *must* be equal (though not having the same repr does not
@@ -487,22 +489,27 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         scc_universes
     }
 
-    /// For each SCC, we compute a unique `RegionVid` (in fact, the
-    /// minimal one that belongs to the SCC). See
+    /// For each SCC, we compute a unique `RegionVid`. See
     /// `scc_representatives` field of `RegionInferenceContext` for
     /// more details.
     fn compute_scc_representatives(
         constraints_scc: &Sccs<RegionVid, ConstraintSccIndex>,
         definitions: &IndexSlice<RegionVid, RegionDefinition<'tcx>>,
     ) -> IndexVec<ConstraintSccIndex, ty::RegionVid> {
         let num_sccs = constraints_scc.num_sccs();
-        let next_region_vid = definitions.next_index();
-        let mut scc_representatives = IndexVec::from_elem_n(next_region_vid, num_sccs);
-
-        for region_vid in definitions.indices() {
-            let scc = constraints_scc.scc(region_vid);
-            let prev_min = scc_representatives[scc];
-            scc_representatives[scc] = region_vid.min(prev_min);
+        let mut scc_representatives = IndexVec::from_elem_n(ty::RegionVid::MAX, num_sccs);
+
+        for (vid, def) in definitions.iter_enumerated() {
+            use NllRegionVariableOrigin as VarOrigin;
+            let scc = constraints_scc.scc(vid);
+            let repr = &mut scc_representatives[scc];
+            if *repr == ty::RegionVid::MAX {
+                *repr = vid;
+            } else if matches!(def.origin, VarOrigin::Placeholder(_))
+                && matches!(definitions[*repr].origin, VarOrigin::Existential { .. })
+            {
+                *repr = vid;
+            }
         }
 
         scc_representatives

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 +66,60 @@ 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
+                            .universal_regions
+                            .universal_regions()
+                            .filter(|&ur| {
+                                use crate::universal_regions::RegionClassification as Class;
+                                matches!(
+                                    self.universal_regions.region_classification(ur),
+                                    Some(Class::Global | Class::Local)
+                                )
+                            })
+                            .filter(|&ur| ur != self.universal_regions.fr_fn_body)
+                            .find(|&ur| self.universal_region_relations.equal(vid, ur))
+                            .map(|ur| self.definitions[ur].external_name.unwrap()),
+                        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
@@ -365,38 +340,33 @@ fn check_opaque_type_well_formed<'tcx>(
     }
 }
 
-fn check_opaque_type_parameter_valid(
-    tcx: TyCtxt<'_>,
-    opaque_type_key: OpaqueTypeKey<'_>,
+fn check_opaque_type_parameter_valid<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    opaque_type_key: OpaqueTypeKey<'tcx>,
     span: Span,
 ) -> Result<(), ErrorGuaranteed> {
-    let opaque_ty_hir = tcx.hir().expect_item(opaque_type_key.def_id);
-    let is_ty_alias = match opaque_ty_hir.expect_opaque_ty().origin {
-        OpaqueTyOrigin::TyAlias { .. } => true,
-        OpaqueTyOrigin::AsyncFn(..) | OpaqueTyOrigin::FnReturn(..) => false,
-    };
-
+    let opaque_env = LazyOpaqueTyEnv::new(tcx, opaque_type_key.def_id);
     let opaque_generics = tcx.generics_of(opaque_type_key.def_id);
     let mut seen_params: FxIndexMap<_, Vec<_>> = FxIndexMap::default();
-    for (i, arg) in opaque_type_key.args.iter().enumerate() {
+    for (i, arg) in opaque_type_key.iter_captured_args(tcx) {
         if let Err(guar) = arg.error_reported() {
             return Err(guar);
         }
 
         let arg_is_param = match arg.unpack() {
             GenericArgKind::Type(ty) => matches!(ty.kind(), ty::Param(_)),
-            GenericArgKind::Lifetime(lt) if is_ty_alias => {
+            GenericArgKind::Lifetime(lt) => {
                 matches!(*lt, ty::ReEarlyBound(_) | ty::ReFree(_))
+                    || (lt.is_static() && opaque_env.param_equal_static(i))
             }
-            // FIXME(#113916): we can't currently check for unique lifetime params,
-            // see that issue for more. We will also have to ignore unused lifetime
-            // params for RPIT, but that's comparatively trivial ✨
-            GenericArgKind::Lifetime(_) => continue,
             GenericArgKind::Const(ct) => matches!(ct.kind(), ty::ConstKind::Param(_)),
         };
 
         if arg_is_param {
-            seen_params.entry(arg).or_default().push(i);
+            let seen_where = seen_params.entry(arg).or_default();
+            if !seen_where.first().is_some_and(|&prev_i| opaque_env.params_equal(i, prev_i)) {
+                seen_where.push(i);
+            }
         } else {
             // Prevent `fn foo() -> Foo<u32>` from being defining.
             let opaque_param = opaque_generics.param_at(i, tcx);
@@ -428,3 +398,81 @@ fn check_opaque_type_parameter_valid(
 
     Ok(())
 }
+
+struct LazyOpaqueTyEnv<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    def_id: LocalDefId,
+    canonical_args: std::cell::Cell<Option<ty::GenericArgsRef<'tcx>>>,
+}
+
+impl<'tcx> LazyOpaqueTyEnv<'tcx> {
+    pub fn new(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Self {
+        Self { tcx, def_id, canonical_args: std::cell::Cell::new(None) }
+    }
+
+    pub fn param_equal_static(&self, param_index: usize) -> bool {
+        self.get_canonical_args()[param_index].expect_region().is_static()
+    }
+
+    pub fn params_equal(&self, param1: usize, param2: usize) -> bool {
+        let canonical_args = self.get_canonical_args();
+        canonical_args[param1] == canonical_args[param2]
+    }
+
+    fn get_canonical_args(&self) -> ty::GenericArgsRef<'tcx> {
+        use rustc_hir as hir;
+        use rustc_infer::infer::outlives::env::OutlivesEnvironment;
+        use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _;
+
+        if let Some(canonical_args) = self.canonical_args.get() {
+            return canonical_args;
+        }
+
+        let &Self { tcx, def_id, .. } = self;
+        let origin = tcx.opaque_type_origin(def_id);
+        let defining_use_anchor = match origin {
+            hir::OpaqueTyOrigin::FnReturn(did) | hir::OpaqueTyOrigin::AsyncFn(did) => did,
+            hir::OpaqueTyOrigin::TyAlias { .. } => tcx.impl_trait_parent(def_id),
+        };
+        let param_env = tcx.param_env(defining_use_anchor);
+
+        let infcx = tcx.infer_ctxt().build();
+        let ocx = ObligationCtxt::new(&infcx);
+
+        let args = match origin {
+            hir::OpaqueTyOrigin::FnReturn(parent) | hir::OpaqueTyOrigin::AsyncFn(parent) => {
+                GenericArgs::identity_for_item(tcx, parent).extend_to(
+                    tcx,
+                    def_id.to_def_id(),
+                    |param, _| {
+                        tcx.map_rpit_lifetime_to_fn_lifetime(param.def_id.expect_local()).into()
+                    },
+                )
+            }
+            hir::OpaqueTyOrigin::TyAlias { .. } => GenericArgs::identity_for_item(tcx, def_id),
+        };
+
+        let wf_tys = ocx.assumed_wf_types(param_env, defining_use_anchor).unwrap_or_else(|_| {
+            tcx.sess.delay_span_bug(tcx.def_span(def_id), "error getting implied bounds");
+            Default::default()
+        });
+        let implied_bounds = infcx.implied_bounds_tys(param_env, defining_use_anchor, wf_tys);
+        let outlives_env = OutlivesEnvironment::with_bounds(param_env, implied_bounds);
+
+        let mut seen = vec![tcx.lifetimes.re_static];
+        let canonical_args = tcx.fold_regions(args, |r1, _| {
+            if let Some(&r2) = seen.iter().find(|&&r2| {
+                let free_regions = outlives_env.free_region_map();
+                free_regions.sub_free_regions(tcx, r1, r2)
+                    && free_regions.sub_free_regions(tcx, r2, r1)
+            }) {
+                r2
+            } else {
+                seen.push(r1);
+                r1
+            }
+        });
+        self.canonical_args.set(Some(canonical_args));
+        canonical_args
+    }
+}

compiler/rustc_borrowck/src/type_check/free_region_relations.rs

@@ -159,6 +159,13 @@ impl UniversalRegionRelations<'_> {
         self.outlives.contains(fr1, fr2)
     }
 
+    /// Returns `true` if fr1 is known to equal fr2.
+    ///
+    /// This will only ever be true for universally quantified regions.
+    pub(crate) fn equal(&self, fr1: RegionVid, fr2: RegionVid) -> bool {
+        self.outlives.contains(fr1, fr2) && self.outlives.contains(fr2, fr1)
+    }
+
     /// Returns a vector of free regions `x` such that `fr1: x` is
     /// known to hold.
     pub(crate) fn regions_outlived_by(&self, fr1: RegionVid) -> Vec<RegionVid> {

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -243,6 +243,22 @@ pub(crate) fn type_check<'mir, 'tcx>(
                 hidden_type.ty = Ty::new_error(infcx.tcx, reported);
             }
 
+            // Convert all regions to nll vars.
+            let (opaque_type_key, hidden_type) =
+                infcx.tcx.fold_regions((opaque_type_key, hidden_type), |region, _| {
+                    match region.kind() {
+                        ty::ReVar(_) => region,
+                        ty::RePlaceholder(placeholder) => checker
+                            .borrowck_context
+                            .constraints
+                            .placeholder_region(infcx, placeholder),
+                        _ => ty::Region::new_var(
+                            infcx.tcx,
+                            checker.borrowck_context.universal_regions.to_region_vid(region),
+                        ),
+                    }
+                });
+
             (opaque_type_key, hidden_type)
         })
         .collect();

compiler/rustc_middle/src/ty/mod.rs

@@ -1504,6 +1504,38 @@ 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 }
+    }
+
+    pub fn iter_captured_args(
+        self,
+        tcx: TyCtxt<'tcx>,
+    ) -> impl Iterator<Item = (usize, GenericArg<'tcx>)> {
+        std::iter::zip(self.args, tcx.variances_of(self.def_id)).enumerate().filter_map(
+            |(i, (arg, v))| match (arg.unpack(), v) {
+                (_, ty::Invariant) => Some((i, arg)),
+                (ty::GenericArgKind::Lifetime(_), ty::Bivariant) => None,
+                _ => bug!("unexpected opaque type arg variance"),
+            },
+        )
+    }
+}
+
 #[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-captured-non-universal-region.infer.stderr

@@ -0,0 +1,12 @@
+error[E0792]: expected generic lifetime parameter, found `'_`
+  --> $DIR/defining-use-captured-non-universal-region.rs:14:18
+   |
+LL | fn foo<'a>() -> impl Sized + 'a {
+   |                              -- this generic parameter must be used with a generic lifetime parameter
+...
+LL |     let i: i32 = foo::<'_>();
+   |                  ^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0792`.

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

@@ -0,0 +1,23 @@
+// This was an ICE. See #110726.
+
+// revisions: statik infer fixed
+//[fixed] check-pass
+//[statik] check-fail
+#![allow(unconditional_recursion)]
+
+fn foo<'a>() -> impl Sized + 'a {
+    #[cfg(statik)]
+    let i: i32 = foo::<'static>();
+    //[statik]~^ ERROR expected generic lifetime parameter, found `'static`
+
+    #[cfg(infer)]
+    let i: i32 = foo::<'_>();
+    //[infer]~^ ERROR expected generic lifetime parameter, found `'_`
+
+    #[cfg(fixed)]
+    let i: i32 = foo::<'a>();
+
+    i
+}
+
+fn main() {}

tests/ui/impl-trait/defining-use-captured-non-universal-region.statik.stderr

@@ -0,0 +1,12 @@
+error[E0792]: expected generic lifetime parameter, found `'static`
+  --> $DIR/defining-use-captured-non-universal-region.rs:10:18
+   |
+LL | fn foo<'a>() -> impl Sized + 'a {
+   |                              -- cannot use static lifetime; use a bound lifetime instead or remove the lifetime parameter from the opaque type
+LL |     #[cfg(statik)]
+LL |     let i: i32 = foo::<'static>();
+   |                  ^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0792`.

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/impl-trait/impl-fn-predefined-lifetimes.rs

@@ -2,10 +2,8 @@
 use std::fmt::Debug;
 
 fn a<'a>() -> impl Fn(&'a u8) -> (impl Debug + '_) {
-    //~^ ERROR cannot resolve opaque type
-
     |x| x
-    //~^ ERROR concrete type differs from previous defining opaque type use
+    //~^ ERROR expected generic lifetime parameter, found `'_`
 }
 
 fn _b<'a>() -> impl Fn(&'a u8) -> (impl Debug + 'a) {

tests/ui/impl-trait/impl-fn-predefined-lifetimes.stderr

@@ -1,21 +1,11 @@
-error: concrete type differs from previous defining opaque type use
-  --> $DIR/impl-fn-predefined-lifetimes.rs:7:9
-   |
-LL |     |x| x
-   |         ^ expected `impl Debug + '_`, got `&u8`
-   |
-note: previous use here
-  --> $DIR/impl-fn-predefined-lifetimes.rs:7:5
-   |
-LL |     |x| x
-   |     ^^^^^
-
-error[E0720]: cannot resolve opaque type
-  --> $DIR/impl-fn-predefined-lifetimes.rs:4:35
+error[E0792]: expected generic lifetime parameter, found `'_`
+  --> $DIR/impl-fn-predefined-lifetimes.rs:5:9
    |
 LL | fn a<'a>() -> impl Fn(&'a u8) -> (impl Debug + '_) {
-   |                                   ^^^^^^^^^^^^^^^ cannot resolve opaque type
+   |                                                -- this generic parameter must be used with a generic lifetime parameter
+LL |     |x| x
+   |         ^
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0720`.
+For more information about this error, try `rustc --explain E0792`.

tests/ui/impl-trait/in-trait/bad-item-bound-within-rpitit-2.rs

@@ -5,6 +5,7 @@ trait Foo {
     fn bar<'other: 'a>() -> impl Sized + 'a {}
     //~^ ERROR use of undeclared lifetime name `'a`
     //~| ERROR use of undeclared lifetime name `'a`
+    //~| ERROR expected generic lifetime parameter, found `'static`
 }
 
 fn main() {}

tests/ui/impl-trait/in-trait/bad-item-bound-within-rpitit-2.stderr

@@ -28,6 +28,15 @@ help: consider introducing lifetime `'a` here
 LL | trait Foo<'a> {
    |          ++++
 
-error: aborting due to 2 previous errors
+error[E0792]: expected generic lifetime parameter, found `'static`
+  --> $DIR/bad-item-bound-within-rpitit-2.rs:5:45
+   |
+LL |     fn bar<'other: 'a>() -> impl Sized + 'a {}
+   |            ------                           ^^
+   |            |
+   |            cannot use static lifetime; use a bound lifetime instead or remove the lifetime parameter from the opaque type
+
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0261`.
+Some errors have detailed explanations: E0261, E0792.
+For more information about an error, try `rustc --explain E0261`.

tests/ui/impl-trait/rpit/non-defining-use-lifetimes.rs

@@ -0,0 +1,37 @@
+// issue: #111935
+
+#![allow(unconditional_recursion)]
+
+// Lt indirection is necessary to make the lifetime of the function late-bound,
+// in order to bypass some other bugs.
+type Lt<'lt> = Option<*mut &'lt ()>;
+
+mod statik {
+    use super::*;
+    // invalid defining use: Opaque<'static> := ()
+    fn foo<'a>(_: Lt<'a>) -> impl Sized + 'a {
+        let _: () = foo(Lt::<'static>::None);
+        //~^ ERROR expected generic lifetime parameter, found `'static`
+    }
+}
+
+mod infer {
+    use super::*;
+    // invalid defining use: Opaque<'_> := ()
+    fn foo<'a>(_: Lt<'a>) -> impl Sized + 'a {
+        let _: () = foo(Lt::<'_>::None);
+        //~^ ERROR expected generic lifetime parameter, found `'_`
+    }
+}
+
+mod equal {
+    use super::*;
+    // invalid defining use: Opaque<'a, 'a> := ()
+    // because of the use of equal lifetimes in args
+    fn foo<'a, 'b>(_: Lt<'a>, _: Lt<'b>) -> impl Sized + 'a + 'b {
+        let _: () = foo(Lt::<'a>::None, Lt::<'a>::None);
+        //~^ ERROR non-defining opaque type use in defining scope
+    }
+}
+
+fn main() {}

tests/ui/impl-trait/rpit/non-defining-use-lifetimes.stderr

@@ -0,0 +1,31 @@
+error[E0792]: expected generic lifetime parameter, found `'static`
+  --> $DIR/non-defining-use-lifetimes.rs:13:16
+   |
+LL |     fn foo<'a>(_: Lt<'a>) -> impl Sized + 'a {
+   |                                           -- cannot use static lifetime; use a bound lifetime instead or remove the lifetime parameter from the opaque type
+LL |         let _: () = foo(Lt::<'static>::None);
+   |                ^^
+
+error[E0792]: expected generic lifetime parameter, found `'_`
+  --> $DIR/non-defining-use-lifetimes.rs:22:16
+   |
+LL |     fn foo<'a>(_: Lt<'a>) -> impl Sized + 'a {
+   |                                           -- this generic parameter must be used with a generic lifetime parameter
+LL |         let _: () = foo(Lt::<'_>::None);
+   |                ^^
+
+error: non-defining opaque type use in defining scope
+  --> $DIR/non-defining-use-lifetimes.rs:32:16
+   |
+LL |         let _: () = foo(Lt::<'a>::None, Lt::<'a>::None);
+   |                ^^
+   |
+note: lifetime used multiple times
+  --> $DIR/non-defining-use-lifetimes.rs:31:58
+   |
+LL |     fn foo<'a, 'b>(_: Lt<'a>, _: Lt<'b>) -> impl Sized + 'a + 'b {
+   |                                                          ^^   ^^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0792`.

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/defined-in-closure-external-lifetime.rs

@@ -0,0 +1,19 @@
+#![feature(type_alias_impl_trait)]
+
+mod case1 {
+    type Opaque<'x> = impl Sized + 'x;
+    fn foo<'s>() -> Opaque<'s> {
+        let _ = || { let _: Opaque<'s> = (); };
+        //~^ ERROR expected generic lifetime parameter, found `'_`
+    }
+}
+
+mod case2 {
+    type Opaque<'x> = impl Sized + 'x;
+    fn foo<'s>() -> Opaque<'s> {
+        let _ = || -> Opaque<'s> {};
+        //~^ ERROR expected generic lifetime parameter, found `'_`
+    }
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/defined-in-closure-external-lifetime.stderr

@@ -0,0 +1,21 @@
+error[E0792]: expected generic lifetime parameter, found `'_`
+  --> $DIR/defined-in-closure-external-lifetime.rs:6:29
+   |
+LL |     type Opaque<'x> = impl Sized + 'x;
+   |                 -- this generic parameter must be used with a generic lifetime parameter
+LL |     fn foo<'s>() -> Opaque<'s> {
+LL |         let _ = || { let _: Opaque<'s> = (); };
+   |                             ^^^^^^^^^^
+
+error[E0792]: expected generic lifetime parameter, found `'_`
+  --> $DIR/defined-in-closure-external-lifetime.rs:14:34
+   |
+LL |     type Opaque<'x> = impl Sized + 'x;
+   |                 -- this generic parameter must be used with a generic lifetime parameter
+LL |     fn foo<'s>() -> Opaque<'s> {
+LL |         let _ = || -> Opaque<'s> {};
+   |                                  ^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0792`.

tests/ui/type-alias-impl-trait/equal-lifetime-params-not-ok.rs

@@ -0,0 +1,37 @@
+// issue: #112841
+
+#![feature(type_alias_impl_trait)]
+
+trait Trait<'a, 'b> {}
+impl<T> Trait<'_, '_> for T {}
+
+mod mod1 {
+    type Opaque<'a, 'b> = impl super::Trait<'a, 'b>;
+    fn test<'a>() -> Opaque<'a, 'a> {}
+    //~^ ERROR non-defining opaque type use in defining scope
+    //~| ERROR non-defining opaque type use in defining scope
+}
+
+mod mod2 {
+    type Opaque<'a, 'b> = impl super::Trait<'a, 'b>;
+    fn test<'a: 'b, 'b: 'a>() -> Opaque<'a, 'b> {}
+    //~^ ERROR non-defining opaque type use in defining scope
+}
+
+mod mod3 {
+    type Opaque<'a, 'b> = impl super::Trait<'a, 'b>;
+    fn test<'a: 'b, 'b: 'a>(a: &'a str) -> Opaque<'a, 'b> { a }
+    //~^ ERROR non-defining opaque type use in defining scope
+}
+
+// This is similar to the previous cases in that 'a is equal to 'static,
+// which is is some sense an implicit parameter to `Opaque`.
+// For example, given a defining use `Opaque<'a> := &'a ()`,
+// it is ambiguous whether `Opaque<'a> := &'a ()` or `Opaque<'a> := &'static ()`
+mod mod4 {
+    type Opaque<'a> = impl super::Trait<'a, 'a>;
+    fn test<'a: 'static>() -> Opaque<'a> {}
+    //~^ ERROR expected generic lifetime parameter, found `'static`
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/equal-lifetime-params-not-ok.stderr

@@ -0,0 +1,59 @@
+error: non-defining opaque type use in defining scope
+  --> $DIR/equal-lifetime-params-not-ok.rs:10:22
+   |
+LL |     fn test<'a>() -> Opaque<'a, 'a> {}
+   |                      ^^^^^^^^^^^^^^ generic argument `'a` used twice
+   |
+note: for this opaque type
+  --> $DIR/equal-lifetime-params-not-ok.rs:9:27
+   |
+LL |     type Opaque<'a, 'b> = impl super::Trait<'a, 'b>;
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: non-defining opaque type use in defining scope
+  --> $DIR/equal-lifetime-params-not-ok.rs:10:37
+   |
+LL |     fn test<'a>() -> Opaque<'a, 'a> {}
+   |                                     ^^
+   |
+note: lifetime used multiple times
+  --> $DIR/equal-lifetime-params-not-ok.rs:9:17
+   |
+LL |     type Opaque<'a, 'b> = impl super::Trait<'a, 'b>;
+   |                 ^^  ^^
+
+error: non-defining opaque type use in defining scope
+  --> $DIR/equal-lifetime-params-not-ok.rs:17:49
+   |
+LL |     fn test<'a: 'b, 'b: 'a>() -> Opaque<'a, 'b> {}
+   |                                                 ^^
+   |
+note: lifetime used multiple times
+  --> $DIR/equal-lifetime-params-not-ok.rs:16:17
+   |
+LL |     type Opaque<'a, 'b> = impl super::Trait<'a, 'b>;
+   |                 ^^  ^^
+
+error: non-defining opaque type use in defining scope
+  --> $DIR/equal-lifetime-params-not-ok.rs:23:61
+   |
+LL |     fn test<'a: 'b, 'b: 'a>(a: &'a str) -> Opaque<'a, 'b> { a }
+   |                                                             ^
+   |
+note: lifetime used multiple times
+  --> $DIR/equal-lifetime-params-not-ok.rs:22:17
+   |
+LL |     type Opaque<'a, 'b> = impl super::Trait<'a, 'b>;
+   |                 ^^  ^^
+
+error[E0792]: expected generic lifetime parameter, found `'static`
+  --> $DIR/equal-lifetime-params-not-ok.rs:33:42
+   |
+LL |     type Opaque<'a> = impl super::Trait<'a, 'a>;
+   |                 -- cannot use static lifetime; use a bound lifetime instead or remove the lifetime parameter from the opaque type
+LL |     fn test<'a: 'static>() -> Opaque<'a> {}
+   |                                          ^^
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0792`.

tests/ui/type-alias-impl-trait/equal-lifetime-params-ok.rs

@@ -0,0 +1,50 @@
+// FIXME: description
+// issue: #113916
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
+
+trait Trait<'a, 'b> {}
+impl<T> Trait<'_, '_> for T {}
+
+mod equal_params {
+    type Opaque<'a: 'b, 'b: 'a> = impl super::Trait<'a, 'b>;
+    fn test<'a: 'b, 'b: 'a>() -> Opaque<'a, 'b> {
+        let _ = None::<&'a &'b &'a ()>;
+        0u8
+    }
+}
+
+mod equal_static {
+    type Opaque<'a: 'static> = impl Sized + 'a;
+    fn test<'a: 'static>() -> Opaque<'a> {
+        let _ = None::<&'static &'a ()>;
+        0u8
+    }
+}
+
+mod implied_bounds {
+    trait Traitor {
+        type Assoc;
+        fn define(self) -> Self::Assoc;
+    }
+
+    impl<'a> Traitor for &'static &'a () {
+        type Assoc = impl Sized + 'a;
+        fn define(self) -> Self::Assoc {
+            let _ = None::<&'static &'a ()>;
+            0u8
+        }
+    }
+
+    impl<'a, 'b> Traitor for (&'a &'b (), &'b &'a ()) {
+        type Assoc = impl Sized + 'a + 'b;
+        fn define(self) -> Self::Assoc {
+            let _ = None::<(&'a &'b (), &'b &'a ())>;
+            0u8
+        }
+    }
+}
+
+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() {}