Auto merge of #111754 - lcnr:recursion-depth, r=matthewjasper

fix recursion depth handling after confirmation

fixes #111729

I think having to use `Obligation::with_depth` correctly everywhere is very hard because e.g. the nested obligations from `eq` currently do not have the correct obligation depth.

The new solver [completely removes `recursion_depth` from obligations](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/traits/solve/struct.Goal.html) and instead tracks the depth in the solver itself which is far easier to get right. Moving the old solver towards this shouldn't be that hard but is probably somewhat annoying.

r? `@matthewjasper`

5 files changed, 101 insertions, 4 deletions

diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
index 2f0b07d4c71..5b0b40cbfd9 100644
--- a/compiler/rustc_middle/src/traits/mod.rs
+++ b/compiler/rustc_middle/src/traits/mod.rs
@@ -701,9 +701,9 @@ impl<'tcx, N> ImplSource<'tcx, N> {
     }
 
     pub fn borrow_nested_obligations(&self) -> &[N] {
-        match &self {
-            ImplSource::UserDefined(i) => &i.nested[..],
-            ImplSource::Param(n, _) => &n,
+        match self {
+            ImplSource::UserDefined(i) => &i.nested,
+            ImplSource::Param(n, _) => n,
             ImplSource::Builtin(i) => &i.nested,
             ImplSource::AutoImpl(d) => &d.nested,
             ImplSource::Closure(c) => &c.nested,
@@ -717,6 +717,23 @@ impl<'tcx, N> ImplSource<'tcx, N> {
         }
     }
 
+    pub fn borrow_nested_obligations_mut(&mut self) -> &mut [N] {
+        match self {
+            ImplSource::UserDefined(i) => &mut i.nested,
+            ImplSource::Param(n, _) => n,
+            ImplSource::Builtin(i) => &mut i.nested,
+            ImplSource::AutoImpl(d) => &mut d.nested,
+            ImplSource::Closure(c) => &mut c.nested,
+            ImplSource::Generator(c) => &mut c.nested,
+            ImplSource::Future(c) => &mut c.nested,
+            ImplSource::Object(d) => &mut d.nested,
+            ImplSource::FnPointer(d) => &mut d.nested,
+            ImplSource::TraitAlias(d) => &mut d.nested,
+            ImplSource::TraitUpcasting(d) => &mut d.nested,
+            ImplSource::ConstDestruct(i) => &mut i.nested,
+        }
+    }
+
     pub fn map<M, F>(self, f: F) -> ImplSource<'tcx, M>
     where
         F: FnMut(N) -> M,
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index de023501f9e..0d9f55d4c2e 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -132,6 +132,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
         };
 
+        // The obligations returned by confirmation are recursively evaluated
+        // so we need to make sure they have the correct depth.
+        for subobligation in impl_src.borrow_nested_obligations_mut() {
+            subobligation.set_depth_from_parent(obligation.recursion_depth);
+        }
+
         if !obligation.predicate.is_const_if_const() {
             // normalize nested predicates according to parent predicate's constness.
             impl_src = impl_src.map(|mut o| {
diff --git a/tests/ui/traits/cycle-cache-err-60010.stderr b/tests/ui/traits/cycle-cache-err-60010.stderr
index 2ff16b4af38..aee41c43aef 100644
--- a/tests/ui/traits/cycle-cache-err-60010.stderr
+++ b/tests/ui/traits/cycle-cache-err-60010.stderr
@@ -1,9 +1,25 @@
-error[E0275]: overflow evaluating the requirement `RootDatabase: RefUnwindSafe`
+error[E0275]: overflow evaluating the requirement `SalsaStorage: RefUnwindSafe`
   --> $DIR/cycle-cache-err-60010.rs:27:13
    |
 LL |     _parse: <ParseQuery as Query<RootDatabase>>::Data,
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
+note: required because it appears within the type `PhantomData<SalsaStorage>`
+  --> $SRC_DIR/core/src/marker.rs:LL:COL
+note: required because it appears within the type `Unique<SalsaStorage>`
+  --> $SRC_DIR/core/src/ptr/unique.rs:LL:COL
+note: required because it appears within the type `Box<SalsaStorage>`
+  --> $SRC_DIR/alloc/src/boxed.rs:LL:COL
+note: required because it appears within the type `Runtime<RootDatabase>`
+  --> $DIR/cycle-cache-err-60010.rs:23:8
+   |
+LL | struct Runtime<DB: Database> {
+   |        ^^^^^^^
+note: required because it appears within the type `RootDatabase`
+  --> $DIR/cycle-cache-err-60010.rs:20:8
+   |
+LL | struct RootDatabase {
+   |        ^^^^^^^^^^^^
 note: required for `RootDatabase` to implement `SourceDatabase`
   --> $DIR/cycle-cache-err-60010.rs:44:9
    |
diff --git a/tests/ui/traits/solver-cycles/cycle-via-builtin-auto-trait-impl.rs b/tests/ui/traits/solver-cycles/cycle-via-builtin-auto-trait-impl.rs
new file mode 100644
index 00000000000..d37943b929a
--- /dev/null
+++ b/tests/ui/traits/solver-cycles/cycle-via-builtin-auto-trait-impl.rs
@@ -0,0 +1,34 @@
+//~ ERROR overflow
+// A regression test for #111729 checking that we correctly
+// track recursion depth for obligations returned by confirmation.
+use std::panic::RefUnwindSafe;
+
+trait Database {
+    type Storage;
+}
+trait Query<DB> {
+    type Data;
+}
+struct ParseQuery;
+struct RootDatabase {
+    _runtime: Runtime<RootDatabase>,
+}
+
+impl<T: RefUnwindSafe> Database for T {
+    type Storage = SalsaStorage;
+}
+impl Database for RootDatabase {
+    type Storage = SalsaStorage;
+}
+
+struct Runtime<DB: Database> {
+    _storage: Box<DB::Storage>,
+}
+struct SalsaStorage {
+    _parse: <ParseQuery as Query<RootDatabase>>::Data,
+}
+
+impl<DB: Database> Query<DB> for ParseQuery {
+    type Data = RootDatabase;
+}
+fn main() {}
diff --git a/tests/ui/traits/solver-cycles/cycle-via-builtin-auto-trait-impl.stderr b/tests/ui/traits/solver-cycles/cycle-via-builtin-auto-trait-impl.stderr
new file mode 100644
index 00000000000..8f9ce3ef1e9
--- /dev/null
+++ b/tests/ui/traits/solver-cycles/cycle-via-builtin-auto-trait-impl.stderr
@@ -0,0 +1,24 @@
+error[E0275]: overflow evaluating the requirement `Runtime<RootDatabase>: RefUnwindSafe`
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`cycle_via_builtin_auto_trait_impl`)
+note: required because it appears within the type `RootDatabase`
+  --> $DIR/cycle-via-builtin-auto-trait-impl.rs:13:8
+   |
+LL | struct RootDatabase {
+   |        ^^^^^^^^^^^^
+note: required for `RootDatabase` to implement `Database`
+  --> $DIR/cycle-via-builtin-auto-trait-impl.rs:17:24
+   |
+LL | impl<T: RefUnwindSafe> Database for T {
+   |         -------------  ^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+note: required because it appears within the type `Runtime<RootDatabase>`
+  --> $DIR/cycle-via-builtin-auto-trait-impl.rs:24:8
+   |
+LL | struct Runtime<DB: Database> {
+   |        ^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0275`.