Rollup merge of rust-lang#128954 - zachs18:fromresidual-no-default, r…

…=scottmcm

Explicitly specify type parameter on FromResidual for Option and ControlFlow.

~~Remove type parameter default `R = <Self as Try>::Residual` from `FromResidual`~~ _Specify default type parameter on `FromResidual` impls in the stdlib_ to work around rust-lang#99940 / rust-lang#87350 ~~as mentioned in rust-lang#84277 (comment).

This does not completely fix the issue, but works around it for `Option` and `ControlFlow` specifically (`Result` does not have the issue since it already did not use the default parameter of `FromResidual`).

~~(Does this need an ACP or similar?)~~ ~~This probably needs at least an FCP since it changes the API described in [the RFC](rust-lang/rfcs#3058). Not sure if T-lang, T-libs-api, T-libs, or some combination (The tracking issue is tagged T-lang, T-libs-api).~~ This probably doesn't need T-lang input, since it is not changing the API of `FromResidual` from the RFC? Maybe needs T-libs-api FCP?

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 {

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 {

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,

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!()
+    }
+}