WIP: smart pointer (try_)map #144420
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WIP: smart pointer (try_)map #144420
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As far as I can tell, the ICE here is because of the function signature of |
joboet
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Aug 30, 2025
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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. |
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library/alloc/src/boxed.rs
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| if size_of::<T>() == size_of::<U>() && align_of::<T>() == align_of::<U>() { | ||
| let ptr = &raw const *this; | ||
| unsafe { | ||
| let new = f(ptr.read()); |
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If f panics, this will lead to a double-free.
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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.
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library/alloc/src/boxed.rs
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| impl<T> Drop for Guard<T> { | ||
| fn drop(&mut self) { | ||
| let _box = unsafe { Box::from_raw(self.0) }; |
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This will still drop the Box's contents and hence result in a double-free.
library/alloc/src/boxed.rs
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| 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>()) | ||
| } |
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I'd recommend converting the Box<T> to a Box<MaybeUninit<U>> 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::<U>().write(new); | |
| Box::from_raw(ptr.cast::<U>()) | |
| } | |
| let (allocation, value) = unsafe { | |
| let ptr = Box::into_raw(this); | |
| let value = ptr.read(); | |
| let allocation = Box::from_raw(ptr.cast::<MaybeUninit<U>()); | |
| (allocation, value) | |
| }; | |
| Box::write(allocation, f(value)) |
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The job Click to see the possible cause of the failure (guessed by this bot) |
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…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<$param(0)>` in a context that doesn't have any parameters. But then we can't use `normalize_erasing_late_bound_regions` anymore because that goes though a query that can't deal with infer vars. Since this is diagnostic-only code that is supposed to check for exactly `-> Self`, I think it's fine to just skip normalizing here, especially since The Correct Way<sup>TM</sup> would involve a probe and make this code even more complicated. Also, the code here does almost the same thing, and these suggestions can probably be unified in the future: https://github.com/rust-lang/rust/blob/4ca8078d37c53ee4ff8fb32b4453b915116f25b8/compiler/rustc_hir_typeck/src/method/suggest.rs#L2123-L2129 r? `@compiler-errors` cc `@Qelxiros` -- this should unblock rust-lang#144420
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…iler-errors fix ICE when suggesting `::new` fixes rust-lang#146174 This code suggests to write `Foo::new(...)` when the user writes `Foo(...)` or `Foo { ... }` and the constructor is private, where `new` is some associated function that returns `Self`. When checking that the return type of `new` is `Self`, we need to instantiate the parameters of `new` with infer vars, so we don't end up with a type like `Box<$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 Way<sup>TM</sup> 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
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…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<$param(0)>` in a context that doesn't have any parameters. But then we can't use `normalize_erasing_late_bound_regions` anymore because that goes though a query that can't deal with infer vars. Since this is diagnostic-only code that is supposed to check for exactly `-> Self`, I think it's fine to just skip normalizing here, especially since The Correct Way<sup>TM</sup> would involve a probe and make this code even more complicated. Also, the code here does almost the same thing, and these suggestions can probably be unified in the future: https://github.com/rust-lang/rust/blob/4ca8078d37c53ee4ff8fb32b4453b915116f25b8/compiler/rustc_hir_typeck/src/method/suggest.rs#L2123-L2129 r? ````@compiler-errors```` cc ````@Qelxiros```` -- this should unblock rust-lang#144420
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Rollup merge of #146217 - lukas-code:suggest-new-ice, r=compiler-errors fix ICE when suggesting `::new` fixes #146174 This code suggests to write `Foo::new(...)` when the user writes `Foo(...)` or `Foo { ... }` and the constructor is private, where `new` is some associated function that returns `Self`. When checking that the return type of `new` is `Self`, we need to instantiate the parameters of `new` with infer vars, so we don't end up with a type like `Box<$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 Way<sup>TM</sup> 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
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Tracking issue: #144419