split try trait in twain

[bend-n]

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https://rust-lang.zulipchat.com/#narrow/channel/219381-t-libs/topic/on.20.60Try.60.20for.20.60.26Option.60/with/582974682

currently Try for &Option is impossible, because from_output would need to return &Option from &Self::Output. but i think there should be some way to do Option::as_ref in a generic setting, and i think this is a good cause to split the traits up a bit, like so:

pub trait Branch {
    type Output;
    type Residual;
    fn branch(self) -> ControlFlow<Self::Residual, Self::Output>;
}
pub trait FromOutput<O = <Self as Branch>::Output>: Branch {
    fn from_output(output: O) -> Self;
}
pub trait FromResidual<R = <Self as Branch>::Residual>: Branch {
    fn from_residual(residual: R) -> Self;
}
pub trait Try = Branch + FromResidual + FromOutput;
pub trait Residual<O>: Sized {
    type TryType: Try<Output = O, Residual = Self>;
}

which then allows (this is not implemented in this PR.)

impl<'a, T, E> Branch for &'a Result<T, E> {
    type Output = &'a T;
    type Residual = Result<Infallible, &'a E>;

    fn branch(self) -> ControlFlow<Self::Residual, Self::Output> { … }
}

which then allows

fn as_ref<R>(x: R) -> <R::Residual as Residual<<R as Branch>::Output>>::TryType
where
    R: Branch<Residual: Residual<<R as Branch>::Output>>
 { …  }

which then allows

let x: Result<&i32, &()> = as_ref(&Ok::<i32, ()>(1));

which i think is a great boon to Try.

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rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

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r? @jhpratt

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[jhpratt]

@rustbot reroll

[rustbot]

This PR was rebased onto a different main 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.

[bend-n]

@rustbot reroll

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

28    |
29 LL |     let res: () = try { };
30    |                         ^ could not wrap the final value of the block as `()` doesn't implement `Try`
+    |
+    = help: the nightly-only, unstable trait `FromOutput<_>` is not implemented for `()`
31 
32 error[E0277]: a `try` block must return `Result` or `Option` (or another type that implements `Try`)
33   --> $DIR/try-block-bad-type.rs:17:25

42    |
---
To only update this specific test, also pass `--test-args try-block/try-block-bad-type.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/try-block/try-block-bad-type.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/try-block/try-block-bad-type" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "--edition=2018"
stdout: none
--- stderr -------------------------------
error[E0308]: mismatched types
##[error]  --> /checkout/tests/ui/try-block/try-block-bad-type.rs:7:9
   |
LL |         Err("")?; //~ ERROR mismatched types
   |         ^^^^^^^^ expected `Result<u32, TryFromSliceError>`, found `Result<_, &str>`
   |
   = note: expected enum `Result<u32, TryFromSliceError>`
              found enum `Result<_, &str>`
help: consider using `Result::expect` to unwrap the `Result<_, &str>` value, panicking if the value is a `Result::Err`
   |
LL |         Err("")?.expect("REASON"); //~ ERROR mismatched types
   |                 +++++++++++++++++

error[E0271]: type mismatch resolving `<Result<i32, i32> as Branch>::Output == &str`
##[error]  --> /checkout/tests/ui/try-block/try-block-bad-type.rs:12:9
   |
LL |         "" //~ ERROR type mismatch
   |         ^^ expected `&str`, found `i32`

error[E0271]: type mismatch resolving `<Result<i32, i32> as Branch>::Output == ()`
##[error]  --> /checkout/tests/ui/try-block/try-block-bad-type.rs:15:39
   |
LL |     let res: Result<i32, i32> = try { }; //~ ERROR type mismatch
   |                                       ^ expected `()`, found `i32`

error[E0277]: a `try` block must return `Result` or `Option` (or another type that implements `Try`)
##[error]  --> /checkout/tests/ui/try-block/try-block-bad-type.rs:17:25
   |
LL |     let res: () = try { };
   |                         ^ could not wrap the final value of the block as `()` doesn't implement `Try`
   |
   = help: the nightly-only, unstable trait `FromOutput<_>` is not implemented for `()`

error[E0277]: a `try` block must return `Result` or `Option` (or another type that implements `Try`)
##[error]  --> /checkout/tests/ui/try-block/try-block-bad-type.rs:17:25
   |
LL |     let res: () = try { };
   |                         ^ could not wrap the final value of the block as `()` doesn't implement `Try`
   |
   = help: the nightly-only, unstable trait `Branch` is not implemented for `()`

error[E0277]: a `try` block must return `Result` or `Option` (or another type that implements `Try`)
##[error]  --> /checkout/tests/ui/try-block/try-block-bad-type.rs:21:26
   |
LL |     let res: i32 = try { 5 }; //~ ERROR a `try` block must return `Result` or `Option`
   |                          ^ could not wrap the final value of the block as `i32` doesn't implement `Try`
   |
   = help: the nightly-only, unstable trait `FromOutput<_>` is not implemented for `i32`

error: aborting due to 6 previous errors

---
29    |
30 LL |     let res = try bikeshed () { };
31    |                                 ^ could not wrap the final value of the block as `()` doesn't implement `Try`
+    |
+    = help: the nightly-only, unstable trait `FromOutput<_>` is not implemented for `()`
32 
33 error[E0277]: a `try` block must return `Result` or `Option` (or another type that implements `Try`)
34   --> $DIR/try-block-bad-type-heterogeneous.rs:17:33

43    |
---
To only update this specific test, also pass `--test-args try-block/try-block-bad-type-heterogeneous.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/try-block/try-block-bad-type-heterogeneous.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/try-block/try-block-bad-type-heterogeneous" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "--edition=2018"
stdout: none
--- stderr -------------------------------
error[E0277]: `?` couldn't convert the error to `TryFromSliceError`
##[error]  --> /checkout/tests/ui/try-block/try-block-bad-type-heterogeneous.rs:7:16
   |
LL |         Err("")?; //~ ERROR `?` couldn't convert the error
   |         -------^ the trait `From<&str>` is not implemented for `TryFromSliceError`
   |         |
   |         this can't be annotated with `?` because it has type `Result<_, &str>`
   |
   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
help: the trait `From<&str>` is not implemented for `TryFromSliceError`
      but trait `From<Infallible>` is implemented for it
  --> /rustc/FAKE_PREFIX/library/core/src/array/mod.rs:198:0
   = help: for that trait implementation, expected `Infallible`, found `&str`

error[E0271]: type mismatch resolving `<Result<i32, i32> as Branch>::Output == &str`
##[error]  --> /checkout/tests/ui/try-block/try-block-bad-type-heterogeneous.rs:12:9
   |
LL |         "" //~ ERROR type mismatch
   |         ^^ expected `&str`, found `i32`

error[E0271]: type mismatch resolving `<Result<i32, i32> as Branch>::Output == ()`
##[error]  --> /checkout/tests/ui/try-block/try-block-bad-type-heterogeneous.rs:15:47
   |
LL |     let res = try bikeshed Result<i32, i32> { }; //~ ERROR type mismatch
   |                                               ^ expected `()`, found `i32`

error[E0277]: a `try` block must return `Result` or `Option` (or another type that implements `Try`)
##[error]  --> /checkout/tests/ui/try-block/try-block-bad-type-heterogeneous.rs:17:33
   |
LL |     let res = try bikeshed () { };
   |                                 ^ could not wrap the final value of the block as `()` doesn't implement `Try`
   |
   = help: the nightly-only, unstable trait `FromOutput<_>` is not implemented for `()`

error[E0277]: a `try` block must return `Result` or `Option` (or another type that implements `Try`)
##[error]  --> /checkout/tests/ui/try-block/try-block-bad-type-heterogeneous.rs:17:33
   |
LL |     let res = try bikeshed () { };
   |                                 ^ could not wrap the final value of the block as `()` doesn't implement `Try`
   |
   = help: the nightly-only, unstable trait `Branch` is not implemented for `()`

error[E0277]: a `try` block must return `Result` or `Option` (or another type that implements `Try`)
##[error]  --> /checkout/tests/ui/try-block/try-block-bad-type-heterogeneous.rs:21:34
   |
LL |     let res = try bikeshed i32 { 5 }; //~ ERROR a `try` block must return `Result` or `Option`
   |                                  ^ could not wrap the final value of the block as `i32` doesn't implement `Try`
   |
   = help: the nightly-only, unstable trait `FromOutput<_>` is not implemented for `i32`

error: aborting due to 6 previous errors

---
Saved the actual stderr to `/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/try-block/try-block-in-while/try-block-in-while.stderr`
diff of stderr:

3    |
4 LL |     while try { false } {}
5    |                 ^^^^^ could not wrap the final value of the block as `bool` doesn't implement `Try`
+    |
+    = help: the nightly-only, unstable trait `FromOutput<_>` is not implemented for `bool`
6 
7 error: aborting due to 1 previous error
8 
---
To only update this specific test, also pass `--test-args try-block/try-block-in-while.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/try-block/try-block-in-while.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/try-block/try-block-in-while" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "--edition=2018"
stdout: none
--- stderr -------------------------------
error[E0277]: a `try` block must return `Result` or `Option` (or another type that implements `Try`)
##[error]  --> /checkout/tests/ui/try-block/try-block-in-while.rs:6:17
   |
LL |     while try { false } {}
   |                 ^^^^^ could not wrap the final value of the block as `bool` doesn't implement `Try`
   |
   = help: the nightly-only, unstable trait `FromOutput<_>` is not implemented for `bool`

error: aborting due to 1 previous error

[Mark-Simulacrum]

I know there has been a bunch of discussion about improvements to Try (e.g., something about GATs making the API friendlier), but it's not obvious to me that we want to try to further generalize it. I think it would make sense for T-libs to discuss this and figure out what the right path forward is before we spend more time on review / iteration on the specifics of the PR.

cc @scottmcm @Amanieu

[rust-bors]

☔ The latest upstream changes (presumably #157208) made this pull request unmergeable. Please resolve the merge conflicts.