Explicitly specify type parameter on FromResidual impls in stdlib.

zachs18 · zachs18 · commit aa854485cea4 · 2024-08-12T12:54:18.000-05:00
To work around coherence issue. Also adds regression test.
diff --git a/core/src/ops/control_flow.rs b/core/src/ops/control_flow.rs
@@ -116,7 +116,9 @@ impl<B, C> ops::Try for ControlFlow<B, C> {
 }
 
 #[unstable(feature = "try_trait_v2", issue = "84277")]
-impl<B, C> ops::FromResidual for ControlFlow<B, C> {
+// Note: manually specifying the residual type instead of using the default to work around
+// https://github.com/rust-lang/rust/issues/99940
+impl<B, C> ops::FromResidual<ControlFlow<B, convert::Infallible>> for ControlFlow<B, C> {
     #[inline]
     fn from_residual(residual: ControlFlow<B, convert::Infallible>) -> Self {
         match residual {
diff --git a/core/src/option.rs b/core/src/option.rs
@@ -2495,7 +2495,9 @@ impl<T> ops::Try for Option<T> {
 }
 
 #[unstable(feature = "try_trait_v2", issue = "84277")]
-impl<T> ops::FromResidual for Option<T> {
+// Note: manually specifying the residual type instead of using the default to work around
+// https://github.com/rust-lang/rust/issues/99940
+impl<T> ops::FromResidual<Option<convert::Infallible>> for Option<T> {
     #[inline]
     fn from_residual(residual: Option<convert::Infallible>) -> Self {
         match residual {
diff --git a/core/tests/ops.rs b/core/tests/ops.rs
@@ -1,4 +1,5 @@
 mod control_flow;
+mod from_residual;
 
 use core::ops::{
     Bound, Deref, DerefMut, Range, RangeFrom, RangeFull, RangeInclusive, RangeTo, RangeToInclusive,
diff --git a/core/tests/ops/from_residual.rs b/core/tests/ops/from_residual.rs
@@ -0,0 +1,26 @@
+//! Regression test that Option and ControlFlow can have downstream FromResidual impls.
+//! cc https://github.com/rust-lang/rust/issues/99940,
+//! This does NOT test that issue in general; Option and ControlFlow's FromResidual
+//! impls in core were changed to not be affected by that issue.
+
+use core::ops::{ControlFlow, FromResidual};
+
+struct Local;
+
+impl<T> FromResidual<Local> for Option<T> {
+    fn from_residual(_: Local) -> Option<T> {
+        unimplemented!()
+    }
+}
+
+impl<B, C> FromResidual<Local> for ControlFlow<B, C> {
+    fn from_residual(_: Local) -> ControlFlow<B, C> {
+        unimplemented!()
+    }
+}
+
+impl<T, E> FromResidual<Local> for Result<T, E> {
+    fn from_residual(_: Local) -> Result<T, E> {
+        unimplemented!()
+    }
+}

Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,5 @@`
`1`	`1`	`mod control_flow;`
	`2`	`+mod from_residual;`
`2`	`3`
`3`	`4`	`use core::ops::{`
`4`	`5`	`Bound, Deref, DerefMut, Range, RangeFrom, RangeFull, RangeInclusive, RangeTo, RangeToInclusive,`