Propagate the object lifetime defaults of GAT's own params

Notably, this excludes the self ty.

This automatically fixes object lifetime defaulting for trait refs, too.
These used to be broken because the index calculation for going from
middle generic args back to HIR ones didn't take into account the implicit
self ty param present of traits.

Author: fmease

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -777,7 +777,7 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
                         // use the object lifetime defaulting
                         // rules. So e.g., `Box<dyn Debug>` becomes
                         // `Box<dyn Debug + 'static>`.
-                        self.resolve_object_lifetime_default(&*lifetime)
+                        self.resolve_object_lifetime_default(&*lifetime);
                     }
                     LifetimeKind::Infer => {
                         // If the user writes `'_`, we use the *ordinary* elision
@@ -796,7 +796,7 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
             hir::TyKind::Ref(lifetime_ref, ref mt) => {
                 self.visit_lifetime(lifetime_ref);
                 let scope = Scope::ObjectLifetimeDefault {
-                    lifetime: self.rbv.defs.get(&lifetime_ref.hir_id.local_id).cloned(),
+                    lifetime: self.rbv.defs.get(&lifetime_ref.hir_id.local_id).copied(),
                     s: self.scope,
                 };
                 self.with(scope, |this| this.visit_ty_unambig(mt.ty));
@@ -884,11 +884,35 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
         }
     }
 
+    fn visit_qpath(&mut self, qpath: &'tcx hir::QPath<'tcx>, id: HirId, _: Span) {
+        match qpath {
+            hir::QPath::Resolved(maybe_qself, path) => {
+                if let Some(qself) = maybe_qself {
+                    // FIXME: Actually determine the ambient object lifetime defaults for the self ty!
+                    let scope = Scope::ObjectLifetimeDefault { lifetime: None, s: self.scope };
+                    self.with(scope, |this| this.visit_ty_unambig(qself));
+                }
+                self.visit_path(path, id);
+            }
+            hir::QPath::TypeRelative(qself, segment) => {
+                // Resolving object lifetime defaults for type-relative paths requires full
+                // type-dependent resolution as performed by HIR ty lowering whose results
+                // we don't have access to (and we'd results for both FnCtxts and ItemCtxts).
+                // FIXME: Figure out if there's a feasible way to obtain the map of
+                //        type-dependent definitions.
+                let scope = Scope::ObjectLifetimeDefault { lifetime: None, s: self.scope };
+                self.with(scope, |this| {
+                    this.visit_ty_unambig(qself);
+                    this.visit_path_segment(segment)
+                });
+            }
+        }
+    }
+
     fn visit_path(&mut self, path: &hir::Path<'tcx>, hir_id: HirId) {
-        for (i, segment) in path.segments.iter().enumerate() {
-            let depth = path.segments.len() - i - 1;
+        for (index, segment) in path.segments.iter().enumerate() {
             if let Some(args) = segment.args {
-                self.visit_segment_args(path.res, depth, args);
+                self.visit_segment_args(path, index, args);
             }
         }
         if let Res::Def(DefKind::TyParam | DefKind::ConstParam, param_def_id) = path.res {
@@ -1617,55 +1641,72 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
     #[instrument(level = "debug", skip(self))]
     fn visit_segment_args(
         &mut self,
-        res: Res,
-        depth: usize,
+        path: &hir::Path<'tcx>,
+        index: usize,
         generic_args: &'tcx hir::GenericArgs<'tcx>,
     ) {
         if let Some((inputs, output)) = generic_args.paren_sugar_inputs_output() {
             self.visit_fn_like_elision(inputs, Some(output), false);
             return;
         }
 
+        // Let's first resolve all lifetime arguments because we need their resolution
+        // for computing the ambient object lifetime defaults.
         for arg in generic_args.args {
             if let hir::GenericArg::Lifetime(lt) = arg {
                 self.visit_lifetime(lt);
             }
         }
 
-        // Figure out if this is a type/trait segment,
-        // which requires object lifetime defaults.
-        let type_def_id = match res {
-            Res::Def(DefKind::AssocTy, def_id) if depth == 1 => Some(self.tcx.parent(def_id)),
-            Res::Def(DefKind::Variant, def_id) if depth == 0 => Some(self.tcx.parent(def_id)),
-            Res::Def(
-                DefKind::Struct
-                | DefKind::Union
-                | DefKind::Enum
-                | DefKind::TyAlias
-                | DefKind::Trait,
-                def_id,
-            ) if depth == 0 => Some(def_id),
+        // Figure out if this is an "eligible generic container" that brings along ambient object
+        // lifetime defaults for trait object types contained in any of the type arguments passed to
+        // it (any inner generic containers will of course end up shadowing that the default).
+        let depth = path.segments.len() - index - 1;
+        let container = match (path.res, depth) {
+            (Res::Def(DefKind::AssocTy, def_id), 1) => {
+                Some((self.tcx.parent(def_id), &path.segments[..=index]))
+            }
+            (Res::Def(DefKind::Variant, def_id), 0) => {
+                Some((self.tcx.parent(def_id), path.segments))
+            }
+            // FIXME(trait_alias): Arguably, trait aliases are eligible generic containers.
+            (
+                Res::Def(
+                    DefKind::Struct
+                    | DefKind::Union
+                    | DefKind::Enum
+                    | DefKind::TyAlias
+                    | DefKind::Trait
+                    | DefKind::AssocTy,
+                    def_id,
+                ),
+                0,
+            ) => Some((def_id, path.segments)),
+            // Note: We don't need to care about definition kinds that may have generics if they
+            // can only ever appear in positions where we can perform type inference (i.e., bodies).
+            // FIXME(mgca): @fmease thinks that under (m)GCA we now also need to care about e.g.,
+            //              type-level Consts (GCI) and AssocConsts (maybe also Fns, AssocFns) here
+            //              since they appear outside of bodies (once the feature is more complete).
             _ => None,
         };
 
-        debug!(?type_def_id);
+        debug!(?container);
 
-        // Compute a vector of defaults, one for each type parameter,
-        // per the rules given in RFCs 599 and 1156. Example:
+        // Compute a vector of ambient object lifetime defaults, one for each type parameter,
+        // per the rules initially given in RFCs 599 and 1156. Example:
         //
         // ```rust
-        // struct Foo<'a, T: 'a, U> { }
+        // struct Foo<'a, T: 'a + ?Sized, U: ?Sized>(&'a T, &'a U);
         // ```
         //
         // If you have `Foo<'x, dyn Bar, dyn Baz>`, we want to default
         // `dyn Bar` to `dyn Bar + 'x` (because of the `T: 'a` bound)
         // and `dyn Baz` to `dyn Baz + 'static` (because there is no
         // such bound).
         //
-        // Therefore, we would compute `object_lifetime_defaults` to a
-        // vector like `['x, 'static]`. Note that the vector only
-        // includes type parameters.
-        let object_lifetime_defaults = type_def_id.map_or_else(Vec::new, |def_id| {
+        // Therefore, we would compute a vector like `['x, 'static]`.
+        // Note that the vector only includes type parameters.
+        let object_lifetime_defaults = container.map_or_else(Vec::new, |(def_id, segments)| {
             let in_body = {
                 let mut scope = self.scope;
                 loop {
@@ -1689,9 +1730,6 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
             let rbv = &self.rbv;
             let generics = self.tcx.generics_of(def_id);
 
-            // `type_def_id` points to an item, so there is nothing to inherit generics from.
-            debug_assert_eq!(generics.parent_count, 0);
-
             let set_to_region = |set: ObjectLifetimeDefault| match set {
                 ObjectLifetimeDefault::Empty => {
                     if in_body {
@@ -1702,12 +1740,31 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                 }
                 ObjectLifetimeDefault::Static => Some(ResolvedArg::StaticLifetime),
                 ObjectLifetimeDefault::Param(param_def_id) => {
-                    // This index can be used with `generic_args` since `parent_count == 0`.
-                    let index = generics.param_def_id_to_index[&param_def_id] as usize;
-                    generic_args.args.get(index).and_then(|arg| match arg {
-                        GenericArg::Lifetime(lt) => rbv.defs.get(&lt.hir_id.local_id).copied(),
-                        _ => None,
-                    })
+                    fn param_to_depth_and_index(
+                        generics: &ty::Generics,
+                        tcx: TyCtxt<'_>,
+                        def_id: DefId,
+                    ) -> (usize, usize) {
+                        if let Some(&index) = generics.param_def_id_to_index.get(&def_id) {
+                            let has_self = generics.parent.is_none() && generics.has_self;
+                            (0, index as usize - generics.parent_count - has_self as usize)
+                        } else if let Some(parent) = generics.parent {
+                            let parent = tcx.generics_of(parent);
+                            let (depth, index) = param_to_depth_and_index(parent, tcx, def_id);
+                            (depth + 1, index)
+                        } else {
+                            unreachable!()
+                        }
+                    }
+
+                    let (depth, index) = param_to_depth_and_index(generics, self.tcx, param_def_id);
+                    segments[segments.len() - depth - 1]
+                        .args
+                        .and_then(|args| args.args.get(index))
+                        .and_then(|arg| match arg {
+                            GenericArg::Lifetime(lt) => rbv.defs.get(&lt.hir_id.local_id).copied(),
+                            _ => None,
+                        })
                 }
                 ObjectLifetimeDefault::Ambiguous => None,
             };
@@ -1737,6 +1794,7 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
         let mut i = 0;
         for arg in generic_args.args {
             match arg {
+                // We've already visited all lifetime arguments at the start.
                 GenericArg::Lifetime(_) => {}
                 GenericArg::Type(ty) => {
                     if let Some(&lt) = object_lifetime_defaults.get(i) {
@@ -1811,11 +1869,11 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
             //    `for<'a> for<'r> <T as Trait<'a>>::x::<'r, T>::{opaque#0}: for<'b> Other<'b>`.
             if constraint.gen_args.parenthesized == hir::GenericArgsParentheses::ReturnTypeNotation
             {
-                let bound_vars = if let Some(type_def_id) = type_def_id
-                    && self.tcx.def_kind(type_def_id) == DefKind::Trait
+                let bound_vars = if let Some((container_def_id, _)) = container
+                    && self.tcx.def_kind(container_def_id) == DefKind::Trait
                     && let Some((mut bound_vars, assoc_fn)) = BoundVarContext::supertrait_hrtb_vars(
                         self.tcx,
-                        type_def_id,
+                        container_def_id,
                         constraint.ident,
                         ty::AssocTag::Fn,
                     ) {
@@ -1844,10 +1902,10 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                         this.visit_assoc_item_constraint(constraint)
                     });
                 });
-            } else if let Some(type_def_id) = type_def_id {
+            } else if let Some((container_def_id, _)) = container {
                 let bound_vars = BoundVarContext::supertrait_hrtb_vars(
                     self.tcx,
-                    type_def_id,
+                    container_def_id,
                     constraint.ident,
                     ty::AssocTag::Type,
                 )

src/librustdoc/clean/mod.rs

@@ -1968,6 +1968,9 @@ impl<'tcx> ContainerTy<'_, 'tcx> {
         match self {
             Self::Ref(region) => ObjectLifetimeDefault::Arg(region),
             Self::Regular { ty: container, args, arg: index } => {
+                // FIXME(fmease): rustc now also computes ambient object lifetime defaults for
+                //                `AssocTy`s. Re-elide these, too!
+
                 let (DefKind::Struct
                 | DefKind::Union
                 | DefKind::Enum

tests/ui/deriving/issue-89188-gat-hrtb.rs

@@ -1,4 +1,5 @@
-//@ check-pass
+// FIXME(fmease): I've regressed this one since we now reject TypeRelative paths as too complex.
+//@ known-bug: unknown
 
 trait CallWithShim: Sized {
     type Shim<'s>

tests/ui/deriving/issue-89188-gat-hrtb.stderr

@@ -0,0 +1,25 @@
+error[E0228]: the lifetime bound for this object type cannot be deduced from context; please supply an explicit bound
+  --> $DIR/issue-89188-gat-hrtb.rs:27:56
+   |
+LL |         &'s mut T::Shim<dyn for<'t> Fn(&'s mut T::Shim<dyn for<'u> Trait<'s, 't, 'u>>)>,
+   |                                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0228]: the lifetime bound for this object type cannot be deduced from context; please supply an explicit bound
+  --> $DIR/issue-89188-gat-hrtb.rs:27:56
+   |
+LL |         &'s mut T::Shim<dyn for<'t> Fn(&'s mut T::Shim<dyn for<'u> Trait<'s, 't, 'u>>)>,
+   |                                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error[E0228]: the lifetime bound for this object type cannot be deduced from context; please supply an explicit bound
+  --> $DIR/issue-89188-gat-hrtb.rs:27:56
+   |
+LL |         &'s mut T::Shim<dyn for<'t> Fn(&'s mut T::Shim<dyn for<'u> Trait<'s, 't, 'u>>)>,
+   |                                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0228`.

tests/ui/did_you_mean/bad-assoc-ty.edition2015.stderr

@@ -193,20 +193,11 @@ help: if this is a dyn-compatible trait, use `dyn`
 LL | type H = <dyn Fn(u8) -> (u8)>::Output;
    |          ++++               +
 
-error[E0223]: ambiguous associated type
+error[E0228]: the lifetime bound for this object type cannot be deduced from context; please supply an explicit bound
   --> $DIR/bad-assoc-ty.rs:37:10
    |
 LL | type H = Fn(u8) -> (u8)::Output;
-   |          ^^^^^^^^^^^^^^^^^^^^^^
-   |
-help: use fully-qualified syntax
-   |
-LL - type H = Fn(u8) -> (u8)::Output;
-LL + type H = <(dyn Fn(u8) -> u8 + 'static) as BitOr>::Output;
-   |
-LL - type H = Fn(u8) -> (u8)::Output;
-LL + type H = <(dyn Fn(u8) -> u8 + 'static) as IntoFuture>::Output;
-   |
+   |          ^^^^^^^^^^^^^^
 
 error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:44:19
@@ -310,5 +301,5 @@ LL |     fn foo<F>(_: F) where F: Fn() -> _ {}
 
 error: aborting due to 30 previous errors; 1 warning emitted
 
-Some errors have detailed explanations: E0121, E0223, E0740.
+Some errors have detailed explanations: E0121, E0223, E0228, E0740.
 For more information about an error, try `rustc --explain E0121`.

tests/ui/did_you_mean/bad-assoc-ty.rs

@@ -35,10 +35,10 @@ type G = dyn 'static + (Send)::AssocTy;
 // This is actually a legal path with fn-like generic arguments in the middle!
 // Recovery should not apply in this context.
 type H = Fn(u8) -> (u8)::Output;
-//[edition2015]~^ ERROR ambiguous associated type
+//[edition2021]~^ ERROR expected a type, found a trait
+//[edition2015]~^^ ERROR the lifetime bound for this object type cannot be deduced from context; please supply an explicit bound
 //[edition2015]~| WARN trait objects without an explicit `dyn` are deprecated
 //[edition2015]~| WARN this is accepted in the current edition
-//[edition2021]~^^^^ ERROR expected a type, found a trait
 
 macro_rules! ty {
     ($ty: ty) => ($ty::AssocTy);

tests/ui/object-lifetime/object-lifetime-default-assoc-ty-self-ty-static.rs

@@ -0,0 +1,13 @@
+// FIXME: Explainer.
+//@ known-bug: unknown
+
+trait Outer { type Ty; }
+trait Inner {}
+
+impl<'a> Outer for dyn Inner + 'a { type Ty = &'a (); }
+
+// FIXME: Deduce `dyn Inner + 'static` from absence of any bounds on self ty param of trait `Outer`.
+fn f<'r>(x: &'r <dyn Inner as Outer>::Ty) { g(x) }
+fn g<'r>(x: &'r <dyn Inner + 'static as Outer>::Ty) {}
+
+fn main() {}

tests/ui/object-lifetime/object-lifetime-default-assoc-ty-self-ty-static.stderr

@@ -0,0 +1,9 @@
+error[E0228]: the lifetime bound for this object type cannot be deduced from context; please supply an explicit bound
+  --> $DIR/object-lifetime-default-assoc-ty-self-ty-static.rs:10:18
+   |
+LL | fn f<'r>(x: &'r <dyn Inner as Outer>::Ty) { g(x) }
+   |                  ^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0228`.

tests/ui/object-lifetime/object-lifetime-default-assoc-ty-self-ty.rs

@@ -0,0 +1,13 @@
+// FIXME: Explainer.
+//@ known-bug: unknown
+
+trait Outer<'a>: 'a { type Ty; }
+trait Inner {}
+
+impl<'a> Outer<'a> for dyn Inner + 'a { type Ty = &'a (); }
+
+fn f<'r>(x: <dyn Inner + 'r as Outer<'r>>::Ty) { g(x) }
+// FIXME: Deduce `dyn Inner + 'r` from bound `'a` on self ty param of trait `Outer`.
+fn g<'r>(x: <dyn Inner as Outer<'r>>::Ty) {}
+
+fn main() {}

tests/ui/object-lifetime/object-lifetime-default-assoc-ty-self-ty.stderr

@@ -0,0 +1,9 @@
+error[E0228]: the lifetime bound for this object type cannot be deduced from context; please supply an explicit bound
+  --> $DIR/object-lifetime-default-assoc-ty-self-ty.rs:11:14
+   |
+LL | fn g<'r>(x: <dyn Inner as Outer<'r>>::Ty) {}
+   |              ^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0228`.