Fix v0 symbol mangling bug

compiler/rustc_symbol_mangling/src/v0.rs

@@ -485,9 +485,39 @@ impl Printer<'tcx> for SymbolMangler<'tcx> {
         mut self,
         predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
     ) -> Result<Self::DynExistential, Self::Error> {
-        for predicate in predicates {
-            self = self.in_binder(&predicate, |mut cx, predicate| {
-                match predicate {
+        // Okay, so this is a bit tricky. Imagine we have a trait object like
+        // `dyn for<'a> Foo<'a, Bar = &'a ()>`. When we mangle this, the
+        // output looks really close to the syntax, where the `Bar = &'a ()` bit
+        // is under the same binders (`['a]`) as the `Foo<'a>` bit. However, we
+        // actually desugar these into two separate `ExistentialPredicate`s. We
+        // can't enter/exit the "binder scope" twice though, because then we
+        // would mangle the binders twice. (Also, side note, we merging these
+        // two is kind of difficult, because of potential HRTBs in the Projection
+        // predicate.)
+        //
+        // Also worth mentioning: imagine that we instead had
+        // `dyn for<'a> Foo<'a, Bar = &'a ()> + Send`. In this case, `Send` is
+        // under the same binders as `Foo`. Currently, this doesn't matter,
+        // because only *auto traits* are allowed other than the principal trait
+        // and all auto traits don't have any generics. Two things could
+        // make this not an "okay" mangling:
+        // 1) Instead of mangling only *used*
+        // bound vars, we want to mangle *all* bound vars (`for<'b> Send` is a
+        // valid trait predicate);
+        // 2) We allow multiple "principal" traits in the future, or at least
+        // allow in any form another trait predicate that can take generics.
+        //
+        // Here we assume that predicates have the following structure:
+        // [<Trait> [{<Projection>}]] [{<Auto>}]
+        // Since any predicates after the first one shouldn't change the binders,
+        // just put them all in the binders of the first.
+        self = self.in_binder(&predicates[0], |mut cx, _| {
+            for predicate in predicates.iter() {
+                // It would be nice to be able to validate bound vars here, but
+                // projections can actually include bound vars from super traits
+                // because of HRTBs (only in the `Self` type). Also, auto traits
+                // could have different bound vars *anyways*.
+                match predicate.as_ref().skip_binder() {
                     ty::ExistentialPredicate::Trait(trait_ref) => {
                         // Use a type that can't appear in defaults of type parameters.
                         let dummy_self = cx.tcx.mk_ty_infer(ty::FreshTy(0));
@@ -504,9 +534,10 @@ impl Printer<'tcx> for SymbolMangler<'tcx> {
                         cx = cx.print_def_path(*def_id, &[])?;
                     }
                 }
-                Ok(cx)
-            })?;
-        }
+            }
+            Ok(cx)
+        })?;
+
         self.push("E");
         Ok(self)
     }

compiler/rustc_typeck/src/astconv/mod.rs

@@ -1386,11 +1386,13 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let auto_trait_predicates = auto_traits.into_iter().map(|trait_ref| {
             ty::Binder::dummy(ty::ExistentialPredicate::AutoTrait(trait_ref.trait_ref().def_id()))
         });
+        // N.b. principal, projections, auto traits
+        // FIXME: This is actually wrong with multiple principals in regards to symbol mangling
         let mut v = regular_trait_predicates
-            .chain(auto_trait_predicates)
             .chain(
                 existential_projections.map(|x| x.map_bound(ty::ExistentialPredicate::Projection)),
             )
+            .chain(auto_trait_predicates)
             .collect::<SmallVec<[_; 8]>>();
         v.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
         v.dedup();

src/test/ui/symbol-names/trait-objects.rs

@@ -0,0 +1,48 @@
+// Ensure that trait objects don't include more than one binder. See #83611
+
+// build-fail
+// revisions: v0
+//[v0]compile-flags: -Z symbol-mangling-version=v0
+//[v0]normalize-stderr-test: "Cs.*?_" -> "CRATE_HASH"
+//[v0]normalize-stderr-test: "core\[.*?\]" -> "core[HASH]"
+
+#![feature(rustc_attrs)]
+
+trait Bar {
+    fn method(&self) {}
+}
+
+impl Bar for &dyn FnMut(&u8) {
+    #[rustc_symbol_name]
+    //[v0]~^ ERROR symbol-name
+    //[v0]~| ERROR demangling
+    //[v0]~| ERROR demangling-alt
+    fn method(&self) {}
+}
+
+trait Foo {
+    fn method(&self) {}
+}
+
+impl Foo for &(dyn FnMut(&u8) + for<'b> Send) {
+    #[rustc_symbol_name]
+    //[v0]~^ ERROR symbol-name
+    //[v0]~| ERROR demangling
+    //[v0]~| ERROR demangling-alt
+    fn method(&self) {}
+}
+
+trait Baz {
+    fn method(&self) {}
+}
+
+impl Baz for &(dyn for<'b> Send + FnMut(&u8)) {
+    #[rustc_symbol_name]
+    //[v0]~^ ERROR symbol-name
+    //[v0]~| ERROR demangling
+    //[v0]~| ERROR demangling-alt
+    fn method(&self) {}
+}
+
+fn main() {
+}

src/test/ui/symbol-names/trait-objects.v0.stderr

@@ -0,0 +1,56 @@
+error: symbol-name(_RNvXCRATE_HASH13trait_objectsRDG_INtNtNtCRATE_HASH4core3ops8function5FnMutTRL0_hEEp6OutputuEL_NtB2_3Bar6method)
+  --> $DIR/trait-objects.rs:16:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: demangling(<&dyn for<'a> core[HASH]::ops::function::FnMut<(&'a u8,), Output = ()> as trait_objects[17891616a171812d]::Bar>::method)
+  --> $DIR/trait-objects.rs:16:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: demangling-alt(<&dyn for<'a> core::ops::function::FnMut<(&'a u8,), Output = ()> as trait_objects::Bar>::method)
+  --> $DIR/trait-objects.rs:16:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: symbol-name(_RNvXs_CRATE_HASH13trait_objectsRDG_INtNtNtCRATE_HASH4core3ops8function5FnMutTRL0_hEEp6OutputuNtNtBI_6marker4SendEL_NtB4_3Foo6method)
+  --> $DIR/trait-objects.rs:28:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: demangling(<&dyn for<'a> core[HASH]::ops::function::FnMut<(&'a u8,), Output = ()> + core[HASH]::marker::Send as trait_objects[17891616a171812d]::Foo>::method)
+  --> $DIR/trait-objects.rs:28:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: demangling-alt(<&dyn for<'a> core::ops::function::FnMut<(&'a u8,), Output = ()> + core::marker::Send as trait_objects::Foo>::method)
+  --> $DIR/trait-objects.rs:28:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: symbol-name(_RNvXs0_CRATE_HASH13trait_objectsRDG_INtNtNtCRATE_HASH4core3ops8function5FnMutTRL0_hEEp6OutputuNtNtBJ_6marker4SendEL_NtB5_3Baz6method)
+  --> $DIR/trait-objects.rs:40:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: demangling(<&dyn for<'a> core[HASH]::ops::function::FnMut<(&'a u8,), Output = ()> + core[HASH]::marker::Send as trait_objects[17891616a171812d]::Baz>::method)
+  --> $DIR/trait-objects.rs:40:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: demangling-alt(<&dyn for<'a> core::ops::function::FnMut<(&'a u8,), Output = ()> + core::marker::Send as trait_objects::Baz>::method)
+  --> $DIR/trait-objects.rs:40:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 9 previous errors
+