Rollup merge of #132832 - compiler-errors:late-ty, r=cjgillot

Deny capturing late-bound ty/const params in nested opaques

First, this reverts a7f609504c. I can't exactly remember why I approved this specific bit of https://github.com/rust-lang/rust/pull/132466; specifically, I don't know that the purpose of that commit is, and afaict we will never have an opaque that captures late-bound params through a const because opaques can't be used inside of anon consts. Am I missing something `@cjgillot?` Since I can't see a case where this matters, and no tests seem to fail.

The second commit adds a `deny_late_regions: bool` to distinguish `Scope::LateBoundary` which should deny *any* late-bound params or just ty/consts. Then, when resolving opaques we wrap ourselves in a `Scope::LateBoundary { deny_late_regions: false }` so that we deny late-bound ty/const, which fixes a bunch of ICEs that all vaguely look like `impl for<T> Trait<Assoc = impl OtherTrait<T>>`.

I guess this could be achieved other ways; for example, with a different scope kind, or maybe we could just reuse `Scope::Opaque`. But this seems a bit more verbose. I'm open to feedback anyways.

Fixes #131535
Fixes #131637
Fixes #132530

I opted to remove those crashes tests ^ without adding them as regular tests, since they're basically triggering uninteresting late-bound ICEs far off in the trait solver, and the reason that existing tests such as `tests/ui/type-alias-impl-trait/non-lifetime-binder-in-constraint.rs` don't ICE are kinda just coincidental (i.e. due to a missing impl block). I don't really feel motivated to add random permutations to tests just to exercise non-lifetime binders.

r? cjgillot

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -177,6 +177,7 @@ enum Scope<'a> {
     LateBoundary {
         s: ScopeRef<'a>,
         what: &'static str,
+        deny_late_regions: bool,
     },
 
     Root {
@@ -234,9 +235,11 @@ impl<'a> fmt::Debug for TruncatedScopeDebug<'a> {
                 .field("s", &"..")
                 .finish(),
             Scope::TraitRefBoundary { s: _ } => f.debug_struct("TraitRefBoundary").finish(),
-            Scope::LateBoundary { s: _, what } => {
+            Scope::LateBoundary { s: _, what, deny_late_regions } => f
-                f.debug_struct("LateBoundary").field("what", what).finish()
+                .debug_struct("LateBoundary")
-            }
+                .field("what", what)
+                .field("deny_late_regions", deny_late_regions)
+                .finish(),
             Scope::Root { opt_parent_item } => {
                 f.debug_struct("Root").field("opt_parent_item", &opt_parent_item).finish()
             }
@@ -573,17 +576,11 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
             // give, we will reverse the IndexMap after early captures.
             let mut late_depth = 0;
             let mut scope = self.scope;
-            let mut crossed_late_boundary = None;
             let mut opaque_capture_scopes = vec![(opaque.def_id, &captures)];
             loop {
                 match *scope {
                     Scope::Binder { ref bound_vars, scope_type, s, .. } => {
                         for (&original_lifetime, &def) in bound_vars.iter().rev() {
-                            if let ResolvedArg::LateBound(..) = def
-                                && crossed_late_boundary.is_some()
-                            {
-                                continue;
-                            }
                             if let DefKind::LifetimeParam = self.tcx.def_kind(original_lifetime) {
                                 let def = def.shifted(late_depth);
                                 let ident = lifetime_ident(original_lifetime);
@@ -624,12 +621,8 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
 
                     Scope::ObjectLifetimeDefault { s, .. }
                     | Scope::Supertrait { s, .. }
-                    | Scope::TraitRefBoundary { s, .. } => {
+                    | Scope::TraitRefBoundary { s, .. }
-                        scope = s;
+                    | Scope::LateBoundary { s, .. } => {
-                    }
-
-                    Scope::LateBoundary { s, what, .. } => {
-                        crossed_late_boundary = Some(what);
                         scope = s;
                     }
                 }
@@ -640,7 +633,16 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
         let scope = Scope::Opaque { captures: &captures, def_id: opaque.def_id, s: self.scope };
         self.with(scope, |this| {
             let scope = Scope::TraitRefBoundary { s: this.scope };
-            this.with(scope, |this| intravisit::walk_opaque_ty(this, opaque))
+            this.with(scope, |this| {
+                let scope = Scope::LateBoundary {
+                    s: this.scope,
+                    what: "nested `impl Trait`",
+                    // We can capture late-bound regions; we just don't duplicate
+                    // lifetime or const params, so we can't allow those.
+                    deny_late_regions: false,
+                };
+                this.with(scope, |this| intravisit::walk_opaque_ty(this, opaque))
+            })
         });
 
         let captures = captures.into_inner().into_iter().collect();
@@ -997,9 +999,12 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
     }
 
     fn visit_anon_const(&mut self, c: &'tcx hir::AnonConst) {
-        self.with(Scope::LateBoundary { s: self.scope, what: "constant" }, |this| {
+        self.with(
-            intravisit::walk_anon_const(this, c);
+            Scope::LateBoundary { s: self.scope, what: "constant", deny_late_regions: true },
-        });
+            |this| {
+                intravisit::walk_anon_const(this, c);
+            },
+        );
     }
 
     fn visit_generic_param(&mut self, p: &'tcx GenericParam<'tcx>) {
@@ -1291,8 +1296,10 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                     scope = s;
                 }
 
-                Scope::LateBoundary { s, what } => {
+                Scope::LateBoundary { s, what, deny_late_regions } => {
-                    crossed_late_boundary = Some(what);
+                    if deny_late_regions {
+                        crossed_late_boundary = Some(what);
+                    }
                     scope = s;
                 }
             }
@@ -1508,7 +1515,7 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                     scope = s;
                 }
 
-                Scope::LateBoundary { s, what } => {
+                Scope::LateBoundary { s, what, deny_late_regions: _ } => {
                     crossed_late_boundary = Some(what);
                     scope = s;
                 }

tests/crashes/131535.rs

@@ -1,4 +0,0 @@
-//@ known-bug: #131535
-#![feature(non_lifetime_binders)]
-trait v0<> {}
-fn kind  :(v0<'_, > impl for<v4> v0<'_, v2 = impl v0<v4> + '_>) {}

tests/crashes/131637.rs

@@ -1,7 +0,0 @@
-//@ known-bug: #121637
-#![feature(non_lifetime_binders)]
-trait Trait<Type> {
-    type Type;
-
-    fn method(&self) -> impl for<T> Trait<impl Trait<T>>;
-}

tests/crashes/132530.rs

@@ -1,9 +0,0 @@
-//@ known-bug: #132530
-
-#![feature(non_lifetime_binders)]
-
-trait Trait<'a, A> {
-    type Assoc<'a> = i32;
-}
-
-fn a() -> impl for<T> Trait<Assoc = impl Trait<T>> {}

tests/ui/type-alias-impl-trait/non-lifetime-binder-in-constraint.rs

@@ -7,6 +7,7 @@ fn produce() -> impl for<T> Trait<(), Assoc = impl Trait<T>> {
     //~^ ERROR associated type `Assoc` not found for `Trait`
     //~| ERROR associated type `Assoc` not found for `Trait`
     //~| the trait bound `{integer}: Trait<()>` is not satisfied
+    //~| ERROR cannot capture late-bound type parameter in nested `impl Trait`
     16
 }
 

tests/ui/type-alias-impl-trait/non-lifetime-binder-in-constraint.stderr

@@ -1,3 +1,9 @@
+error: cannot capture late-bound type parameter in nested `impl Trait`
+  --> $DIR/non-lifetime-binder-in-constraint.rs:6:58
+   |
+LL | fn produce() -> impl for<T> Trait<(), Assoc = impl Trait<T>> {
+   |                          - parameter defined here        ^
+
 error[E0220]: associated type `Assoc` not found for `Trait`
   --> $DIR/non-lifetime-binder-in-constraint.rs:6:39
    |
@@ -27,7 +33,7 @@ help: this trait has no implementations, consider adding one
 LL | trait Trait<T: ?Sized> {}
    | ^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors
 
 Some errors have detailed explanations: E0220, E0277.
 For more information about an error, try `rustc --explain E0220`.

tests/ui/type-alias-impl-trait/non-lifetime-binder.rs

@@ -5,6 +5,7 @@ trait Trait<T> {}
 
 fn f() -> impl for<T> Trait<impl Trait<T>> {}
 //~^ ERROR nested `impl Trait` is not allowed
-//~| ERROR the trait bound `(): Trait<impl Trait<T>>` is not satisfied
+//~| ERROR the trait bound `(): Trait<impl Trait<{type error}>>` is not satisfied
+//~| ERROR cannot capture late-bound type parameter in nested `impl Trait`
 
 fn main() {}

tests/ui/type-alias-impl-trait/non-lifetime-binder.stderr

@@ -7,11 +7,19 @@ LL | fn f() -> impl for<T> Trait<impl Trait<T>> {}
    |           |                 nested `impl Trait` here
    |           outer `impl Trait`
 
-error[E0277]: the trait bound `(): Trait<impl Trait<T>>` is not satisfied
+error: cannot capture late-bound type parameter in nested `impl Trait`
+  --> $DIR/non-lifetime-binder.rs:6:40
+   |
+LL | fn f() -> impl for<T> Trait<impl Trait<T>> {}
+   |                    -                   ^
+   |                    |
+   |                    parameter defined here
+
+error[E0277]: the trait bound `(): Trait<impl Trait<{type error}>>` is not satisfied
   --> $DIR/non-lifetime-binder.rs:6:11
    |
 LL | fn f() -> impl for<T> Trait<impl Trait<T>> {}
-   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait<impl Trait<T>>` is not implemented for `()`
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait<impl Trait<{type error}>>` is not implemented for `()`
    |
 help: this trait has no implementations, consider adding one
   --> $DIR/non-lifetime-binder.rs:4:1
@@ -19,7 +27,7 @@ help: this trait has no implementations, consider adding one
 LL | trait Trait<T> {}
    | ^^^^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0277, E0666.
 For more information about an error, try `rustc --explain E0277`.