WIP: smart pointer (try_)map #144420

Qelxiros · 2025-07-24T21:13:53Z

Tracking issue: #144419

rustbot · 2025-07-25T00:12:56Z

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Qelxiros · 2025-07-25T00:12:59Z

As far as I can tell, the ICE here is because of the function signature of Box::try_map. The return type is equivalent to ChangeOutputType<...> from the ACP, but that definition is in core, so I implemented it manually here. However, when I add the same type alias to alloc, the ICE persists. Is there a workaround here that remains in the spirit of the ACP? Is this a known compiler bug?

joboet

I have no clue about the ICE – perhaps try filing an issue?

View changes since this review

library/alloc/src/boxed.rs

joboet · 2025-09-02T21:57:33Z

library/alloc/src/boxed.rs

+        if size_of::<T>() == size_of::<U>() && align_of::<T>() == align_of::<U>() {
+            let ptr = &raw const *this;
+            unsafe {
+                let new = f(ptr.read());


If f panics, this will lead to a double-free.

Ok, I think this iteration works. I'm still wrapping my head around working with closures that can panic, so thanks for bearing with me.

joboet · 2025-09-03T21:58:03Z

library/alloc/src/boxed.rs

+
+        impl<T> Drop for Guard<T> {
+            fn drop(&mut self) {
+                let _box = unsafe { Box::from_raw(self.0) };


This will still drop the Box's contents and hence result in a double-free.

joboet · 2025-09-03T22:10:04Z

library/alloc/src/boxed.rs

+            let guard = Guard(Box::into_raw(this));
+            unsafe {
+                let new = f(guard.0.read());
+                let ptr = guard.0;
+                mem::forget(guard);
+                ptr.cast::<U>().write(new);
+                Box::from_raw(ptr.cast::<U>())
+            }


I'd recommend converting the Box<T> to a Box<MaybeUninit> before calling f. This ensures that the allocation is freed, but won't drop the old value on panic. After reading from that, you can use Box::write to write the final value back.

Suggested change

let guard = Guard(Box::into_raw(this));

unsafe {

let new = f(guard.0.read());

let ptr = guard.0;

mem::forget(guard);

ptr.cast::().write(new);

Box::from_raw(ptr.cast::())

}

let (allocation, value) = unsafe {

let ptr = Box::into_raw(this);

let value = ptr.read();

let allocation = Box::from_raw(ptr.cast::<MaybeUninit());

(allocation, value)

};

Box::write(allocation, f(value))

Qelxiros · 2025-09-04T13:55:27Z

I'm not sure how to move forward given the ICE (issue #146174).

@rustbot label +S-blocked

…iler-errors fix ICE when suggesting `::new` fixes rust-lang#146174 This code suggests to write `Foo::new(...)` when the user writes `Foo(...)` or `Foo { ... }` and the constructor is private, where `new` is some associated function that returns `Self`. When checking that the return type of `new` is `Self`, we need to instantiate the parameters of `new` with infer vars, so we don't end up with a type like `Box<$param(0)>` in a context that doesn't have any parameters. But then we can't use `normalize_erasing_late_bound_regions` anymore because that goes though a query that can't deal with infer vars. Since this is diagnostic-only code that is supposed to check for exactly `-> Self`, I think it's fine to just skip normalizing here, especially since The Correct WayTM would involve a probe and make this code even more complicated. Also, the code here does almost the same thing, and these suggestions can probably be unified in the future: https://github.com/rust-lang/rust/blob/4ca8078d37c53ee4ff8fb32b4453b915116f25b8/compiler/rustc_hir_typeck/src/method/suggest.rs#L2123-L2129 r? `@compiler-errors` cc `@Qelxiros` -- this should unblock rust-lang#144420

…iler-errors fix ICE when suggesting `::new` fixes rust-lang#146174 This code suggests to write `Foo::new(...)` when the user writes `Foo(...)` or `Foo { ... }` and the constructor is private, where `new` is some associated function that returns `Self`. When checking that the return type of `new` is `Self`, we need to instantiate the parameters of `new` with infer vars, so we don't end up with a type like `Box<$param(0)>` in a context that doesn't have any parameters. But then we can't use `normalize_erasing_late_bound_regions` anymore because that goes though a query that can't deal with infer vars. Since this is diagnostic-only code that is supposed to check for exactly `-> Self`, I think it's fine to just skip normalizing here, especially since The Correct WayTM would involve a probe and make this code even more complicated. Also, the code here does almost the same thing, and these suggestions can probably be unified in the future: https://github.com/rust-lang/rust/blob/4ca8078d37c53ee4ff8fb32b4453b915116f25b8/compiler/rustc_hir_typeck/src/method/suggest.rs#L2123-L2129 r? ```@compiler-errors``` cc ```@Qelxiros``` -- this should unblock rust-lang#144420

…iler-errors fix ICE when suggesting `::new` fixes rust-lang#146174 This code suggests to write `Foo::new(...)` when the user writes `Foo(...)` or `Foo { ... }` and the constructor is private, where `new` is some associated function that returns `Self`. When checking that the return type of `new` is `Self`, we need to instantiate the parameters of `new` with infer vars, so we don't end up with a type like `Box<$param(0)>` in a context that doesn't have any parameters. But then we can't use `normalize_erasing_late_bound_regions` anymore because that goes though a query that can't deal with infer vars. Since this is diagnostic-only code that is supposed to check for exactly `-> Self`, I think it's fine to just skip normalizing here, especially since The Correct WayTM would involve a probe and make this code even more complicated. Also, the code here does almost the same thing, and these suggestions can probably be unified in the future: https://github.com/rust-lang/rust/blob/4ca8078d37c53ee4ff8fb32b4453b915116f25b8/compiler/rustc_hir_typeck/src/method/suggest.rs#L2123-L2129 r? ````@compiler-errors```` cc ````@Qelxiros```` -- this should unblock rust-lang#144420

Rollup merge of #146217 - lukas-code:suggest-new-ice, r=compiler-errors fix ICE when suggesting `::new` fixes #146174 This code suggests to write `Foo::new(...)` when the user writes `Foo(...)` or `Foo { ... }` and the constructor is private, where `new` is some associated function that returns `Self`. When checking that the return type of `new` is `Self`, we need to instantiate the parameters of `new` with infer vars, so we don't end up with a type like `Box<$param(0)>` in a context that doesn't have any parameters. But then we can't use `normalize_erasing_late_bound_regions` anymore because that goes though a query that can't deal with infer vars. Since this is diagnostic-only code that is supposed to check for exactly `-> Self`, I think it's fine to just skip normalizing here, especially since The Correct WayTM would involve a probe and make this code even more complicated. Also, the code here does almost the same thing, and these suggestions can probably be unified in the future: https://github.com/rust-lang/rust/blob/4ca8078d37c53ee4ff8fb32b4453b915116f25b8/compiler/rustc_hir_typeck/src/method/suggest.rs#L2123-L2129 r? ````@compiler-errors```` cc ````@Qelxiros```` -- this should unblock #144420

rustbot · 2025-09-07T20:04:28Z

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jul 24, 2025

Qelxiros mentioned this pull request Jul 24, 2025

Tracking Issue for smart pointer (try_)map #144419

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Qelxiros marked this pull request as ready for review July 25, 2025 00:12

rustbot assigned thomcc Jul 25, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 25, 2025

joboet reviewed Aug 30, 2025

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Qelxiros force-pushed the smart_pointer_try_map branch from d5d5fea to 929d326 Compare September 2, 2025 20:53

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Qelxiros force-pushed the smart_pointer_try_map branch from 929d326 to 6a69f39 Compare September 2, 2025 20:57

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joboet reviewed Sep 2, 2025

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Qelxiros force-pushed the smart_pointer_try_map branch from 6a69f39 to 64b65c1 Compare September 3, 2025 19:47

Qelxiros mentioned this pull request Sep 3, 2025

Complex function signature in Box method causes ICE #146174

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joboet reviewed Sep 3, 2025

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Qelxiros force-pushed the smart_pointer_try_map branch from 64b65c1 to bfc61d6 Compare September 4, 2025 00:21

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rustbot added the S-blocked Status: Blocked on something else such as an RFC or other implementation work. label Sep 4, 2025

lukas-code mentioned this pull request Sep 4, 2025

fix ICE when suggesting ::new #146217

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Qelxiros force-pushed the smart_pointer_try_map branch from bfc61d6 to 0c65df4 Compare September 7, 2025 20:04

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Qelxiros force-pushed the smart_pointer_try_map branch from 0c65df4 to 59acdbc Compare September 8, 2025 12:44

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Qelxiros force-pushed the smart_pointer_try_map branch from 59acdbc to 2ee0f1d Compare September 8, 2025 14:54

add Box::(try_)map

dd0846e

Qelxiros force-pushed the smart_pointer_try_map branch from 2ee0f1d to dd0846e Compare September 8, 2025 15:01

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