From e5803fceed33ce5bb2d0f29f8fd0db6a70496326 Mon Sep 17 00:00:00 2001
From: Scott McMurray <scottmcm@users.noreply.github.com>
Date: Wed, 12 Nov 2025 21:59:11 -0800
Subject: [PATCH] Move `into_try_type` to a free function

---
 compiler/rustc_hir/src/lang_items.rs          |  2 +-
 library/core/src/ops/try_trait.rs             | 20 +++++++++++--------
 ...on_traits.PreCodegen.after.panic-abort.mir |  2 +-
 ...n_traits.PreCodegen.after.panic-unwind.mir |  2 +-
 4 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/compiler/rustc_hir/src/lang_items.rs b/compiler/rustc_hir/src/lang_items.rs
index 0b236cebdcaa..391fba8395ce 100644
--- a/compiler/rustc_hir/src/lang_items.rs
+++ b/compiler/rustc_hir/src/lang_items.rs
@@ -372,7 +372,7 @@ language_item_table! {
     TryTraitFromOutput,      sym::from_output,         from_output_fn,             Target::Method(MethodKind::Trait { body: false }), GenericRequirement::None;
     TryTraitBranch,          sym::branch,              branch_fn,                  Target::Method(MethodKind::Trait { body: false }), GenericRequirement::None;
     TryTraitFromYeet,        sym::from_yeet,           from_yeet_fn,               Target::Fn,             GenericRequirement::None;
-    ResidualIntoTryType,     sym::into_try_type,       into_try_type_fn,           Target::Method(MethodKind::Trait { body: true }), GenericRequirement::None;
+    ResidualIntoTryType,     sym::into_try_type,       into_try_type_fn,           Target::Fn,             GenericRequirement::None;
 
     CoercePointeeValidated, sym::coerce_pointee_validated, coerce_pointee_validated_trait, Target::Trait,     GenericRequirement::Exact(0);
 
diff --git a/library/core/src/ops/try_trait.rs b/library/core/src/ops/try_trait.rs
index 374e59e5d75c..f68782c804cd 100644
--- a/library/core/src/ops/try_trait.rs
+++ b/library/core/src/ops/try_trait.rs
@@ -364,15 +364,19 @@ pub const trait Residual<O>: Sized {
     /// The "return" type of this meta-function.
     #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
     type TryType: [const] Try<Output = O, Residual = Self>;
+}
 
-    /// Here for convenience in the `?` desugaring.
-    /// Probably should not be stabilized, as it should never be overridden.
-    /// (without a `final fn` of some form, cc RFC#3678)
-    #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
-    #[lang = "into_try_type"]
-    fn into_try_type(self) -> Self::TryType {
-        FromResidual::from_residual(self)
-    }
+/// Used in `try {}` blocks so the type produced in the `?` desugaring
+/// depends on the residual type `R` and the output type of the block `O`,
+/// but importantly not on the contextual type the way it would be if
+/// we called `<_ as FromResidual>::from_residual(r)` directly.
+#[unstable(feature = "try_trait_v2_residual", issue = "91285")]
+// needs to be `pub` to avoid `private type` errors
+#[expect(unreachable_pub)]
+#[inline] // FIXME: force would be nice, but fails -- see #148915
+#[lang = "into_try_type"]
+pub fn residual_into_try_type<R: Residual<O>, O>(r: R) -> <R as Residual<O>>::TryType {
+    FromResidual::from_residual(r)
 }
 
 #[unstable(feature = "pub_crate_should_not_need_unstable_attr", issue = "none")]
diff --git a/tests/mir-opt/pre-codegen/option_bubble_debug.option_traits.PreCodegen.after.panic-abort.mir b/tests/mir-opt/pre-codegen/option_bubble_debug.option_traits.PreCodegen.after.panic-abort.mir
index 94c430ea2df3..55f44d954d80 100644
--- a/tests/mir-opt/pre-codegen/option_bubble_debug.option_traits.PreCodegen.after.panic-abort.mir
+++ b/tests/mir-opt/pre-codegen/option_bubble_debug.option_traits.PreCodegen.after.panic-abort.mir
@@ -34,7 +34,7 @@ fn option_traits(_1: Option<u32>) -> Option<u32> {
     }
 
     bb3: {
-        _0 = <Option<Infallible> as Residual<u32>>::into_try_type(const Option::<Infallible>::None) -> [return: bb4, unwind unreachable];
+        _0 = ops::try_trait::residual_into_try_type::<Option<Infallible>, u32>(const Option::<Infallible>::None) -> [return: bb4, unwind unreachable];
     }
 
     bb4: {
diff --git a/tests/mir-opt/pre-codegen/option_bubble_debug.option_traits.PreCodegen.after.panic-unwind.mir b/tests/mir-opt/pre-codegen/option_bubble_debug.option_traits.PreCodegen.after.panic-unwind.mir
index 43058051c32c..b42aea38d3e3 100644
--- a/tests/mir-opt/pre-codegen/option_bubble_debug.option_traits.PreCodegen.after.panic-unwind.mir
+++ b/tests/mir-opt/pre-codegen/option_bubble_debug.option_traits.PreCodegen.after.panic-unwind.mir
@@ -34,7 +34,7 @@ fn option_traits(_1: Option<u32>) -> Option<u32> {
     }
 
     bb3: {
-        _0 = <Option<Infallible> as Residual<u32>>::into_try_type(const Option::<Infallible>::None) -> [return: bb4, unwind continue];
+        _0 = ops::try_trait::residual_into_try_type::<Option<Infallible>, u32>(const Option::<Infallible>::None) -> [return: bb4, unwind continue];
     }
 
     bb4: {