From b32c37ef870e3b0eab2eadfc7d4279204ace628b Mon Sep 17 00:00:00 2001 From: Amanieu d'Antras Date: Sat, 4 May 2024 00:31:31 +0100 Subject: [PATCH 1/6] Supertrait item shadowing v2 --- text/0000-supertrait-item-shadowing-v2.md | 77 +++++++++++++++++++++++ 1 file changed, 77 insertions(+) create mode 100644 text/0000-supertrait-item-shadowing-v2.md diff --git a/text/0000-supertrait-item-shadowing-v2.md b/text/0000-supertrait-item-shadowing-v2.md new file mode 100644 index 00000000000..78e1e941e82 --- /dev/null +++ b/text/0000-supertrait-item-shadowing-v2.md @@ -0,0 +1,77 @@ +- Feature Name: `supertrait_item_shadowing` +- Start Date: 2024-05-04 +- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000) +- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000) + +# Summary +[summary]: #summary + +When name resolution encounters an ambiguity between 2 trait methods, if one trait is a sub-trait of the other then select that method instead of reporting an ambiguity error. + +# Motivation +[motivation]: #motivation + + +The libs-api team would like to stabilize `Iterator::intersperse` but has a problem. The `itertools` crate already has: + +```rust +// itertools +trait Itertools: Iterator { + fn intersperse(self, element: Self::Item) -> Intersperse; +} +``` + +This method is used in crates with code similar to the following: + +```rust +use core::iter::Iterator; // Implicit import from prelude + +use itertools::Itertools as _; + +fn foo() -> impl Iterator { + "1,2,3".split(",").intersperse("|") + // ^ This is ambiguious: it could refer to Iterator::intersperse or Itertools::intersperse +} +``` + +This code actually works today because `intersperse` is an unstable API, which works because the compiler already has [logic](https://github.com/rust-lang/rust/pull/48552) to prefer stable methods over unstable methods when an amiguity occurs. + +Attempts to stabilize `intersperse` have failed with a large number of regressions [reported by crater](https://github.com/rust-lang/rust/issues/88967) which affect many popular crates. Even if these were to be manually corrected (since ambiguity is considered allowed breakage) we would have to go through this whole process again every time a method from `itertools` is uplifted to the standard library. + +# Proposed solution +[proposed-solution]: #proposed-solution + +This RFC proposes to change name resolution to resolve the ambiguity in the following specific circumstances: +- All method candidates are trait methods. (Inherent methods are already prioritized over trait methods) +- One trait is transitively a sub-trait of all other traits in the candidate list. + +When this happens, the sub-trait method is selected instead of reporting an ambiguity error. + +# Drawbacks +[drawbacks]: #drawbacks + +This behavior can be surprising: adding a method to a sub-trait can change which function is called in unrelated code. A lint could be emitted to warn users about the potential ambiguity. + +# Rationale and alternatives +[rationale-and-alternatives]: #rationale-and-alternatives + +If we choose not to accept this RFC then there doesn't seem to be a reasonable path for adding new methods to the `Iterator` trait if such methods are already provided by `itertools` without a lot of ecosystem churn. + +# Prior art +[prior-art]: #prior-art + +### RFC 2845 + +RFC 2845 was a previous attempt to address this problem, but it has several drawbacks: +- It doesn't fully address the problem since it only changes name resolution when trait methods are resolved due to generic bounds. In practice, most of the amiguity from stabilizing `intersperse` comes from non-generic code. +- It adds a lot of complexity because name resolution depends on the specific trait bounds that have been brought into scope. + +# Unresolved questions +[unresolved-questions]: #unresolved-questions + +None + +# Future possibilities +[future-possibilities]: #future-possibilities + +None From de6a4cfdc83edd41c0e412b2924e782cb6b75055 Mon Sep 17 00:00:00 2001 From: Amanieu d'Antras Date: Mon, 6 May 2024 11:09:27 +0200 Subject: [PATCH 2/6] Clarify that this only triggers when both traits are in scope --- text/0000-supertrait-item-shadowing-v2.md | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/text/0000-supertrait-item-shadowing-v2.md b/text/0000-supertrait-item-shadowing-v2.md index 78e1e941e82..7a4a0bb6a91 100644 --- a/text/0000-supertrait-item-shadowing-v2.md +++ b/text/0000-supertrait-item-shadowing-v2.md @@ -6,7 +6,7 @@ # Summary [summary]: #summary -When name resolution encounters an ambiguity between 2 trait methods, if one trait is a sub-trait of the other then select that method instead of reporting an ambiguity error. +When name resolution encounters an ambiguity between 2 trait methods when both traits are in scope, if one trait is a sub-trait of the other then select that method instead of reporting an ambiguity error. # Motivation [motivation]: #motivation @@ -47,6 +47,8 @@ This RFC proposes to change name resolution to resolve the ambiguity in the foll When this happens, the sub-trait method is selected instead of reporting an ambiguity error. +Note that this only happens when *both* traits are in scope since this is required for the ambiguity to occur in the first place. + # Drawbacks [drawbacks]: #drawbacks From 95594f76884586ab578a691eef26d008f7cbbba3 Mon Sep 17 00:00:00 2001 From: Amanieu d'Antras Date: Tue, 28 May 2024 23:21:03 +0100 Subject: [PATCH 3/6] Add more alternatives --- text/0000-supertrait-item-shadowing-v2.md | 52 +++++++++++++++++++++++ 1 file changed, 52 insertions(+) diff --git a/text/0000-supertrait-item-shadowing-v2.md b/text/0000-supertrait-item-shadowing-v2.md index 7a4a0bb6a91..51a911f5c44 100644 --- a/text/0000-supertrait-item-shadowing-v2.md +++ b/text/0000-supertrait-item-shadowing-v2.md @@ -59,6 +59,58 @@ This behavior can be surprising: adding a method to a sub-trait can change which If we choose not to accept this RFC then there doesn't seem to be a reasonable path for adding new methods to the `Iterator` trait if such methods are already provided by `itertools` without a lot of ecosystem churn. +## Only doing this for specific traits + +One possible alternative to a general change to the name resolution rules would be to only do so on a case-by-case basis for specific methods in standard library traits. This could be done by using a perma-unstable `#[shadowable]` attribute specifically on methods like `Iterator::intersperse`. + +There are both advantages and inconvenients to this approach. While it allows most Rust users to avoid having to think about this issue for most traits, it does make the `Iterator` trait more "magical" in that it doesn't follow the same rules as the rest of the language. Having a consistent rule for how name resolution works is easier to teach people. + +## Preferring the supertrait method instead + +In cases of ambiguity between a subtrait method and a supertrait method, there are 2 ways of resolving the ambiguity. This RFC proposes to resolve in favor of the subtrait since this is most likely to avoid breaking changes in practice. + +Consider this situation: + +- library A has trait `Foo` +- crate B, depending on A, has trait `FooExt` with `Foo` as a supertrait +- A adds a new method to `Foo`, but it has a default implementation so it's not breaking. B has a pre-existing method with the same name. + +In this general case, the reason this cannot be resolved in favor of the supertrait is that the method signatures are not necessarily compatible. + +[In code](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=b3919f7a8480c445d40b18a240936a07): + +```rust +#![allow(unused)] + +mod a { + pub trait Int { + // fn call(&self) -> u32 { + // 0 + // } + } + impl Int for () {} +} + +mod b { + pub trait Int: super::a::Int { + fn call(&self) -> u8 { + 0 + } + } + impl Int for () {} +} + +use a::Int as _; +use b::Int as _; + +fn main() { + let val = ().call(); + println!("{}", std::any::type_name_of_val(&val)); +} +``` + +Resolving in favor of `a` is a breaking change; in favor of `b` is not. The only other option is the status quo: not compiling. `a` simply cannot happen lest we violate backwards compatibility and the status quo is not ideal. + # Prior art [prior-art]: #prior-art From d205e1b49be4d7eba62d0689d06ece6b6748f31b Mon Sep 17 00:00:00 2001 From: Amanieu d'Antras Date: Sat, 1 Jun 2024 18:53:14 +0100 Subject: [PATCH 4/6] Update text/0000-supertrait-item-shadowing-v2.md Co-authored-by: Andres O. Vela --- text/0000-supertrait-item-shadowing-v2.md | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/text/0000-supertrait-item-shadowing-v2.md b/text/0000-supertrait-item-shadowing-v2.md index 51a911f5c44..3a00d7a8451 100644 --- a/text/0000-supertrait-item-shadowing-v2.md +++ b/text/0000-supertrait-item-shadowing-v2.md @@ -30,7 +30,7 @@ use itertools::Itertools as _; fn foo() -> impl Iterator { "1,2,3".split(",").intersperse("|") - // ^ This is ambiguious: it could refer to Iterator::intersperse or Itertools::intersperse + // ^ This is ambiguous: it could refer to Iterator::intersperse or Itertools::intersperse } ``` From 0eb09edab5d8e516284c396c84606372652cf9ee Mon Sep 17 00:00:00 2001 From: Amanieu d'Antras Date: Tue, 23 Jul 2024 19:40:02 +0100 Subject: [PATCH 5/6] Add lint and add type inference example --- text/0000-supertrait-item-shadowing-v2.md | 23 ++++++++++++++++++++++- 1 file changed, 22 insertions(+), 1 deletion(-) diff --git a/text/0000-supertrait-item-shadowing-v2.md b/text/0000-supertrait-item-shadowing-v2.md index 3a00d7a8451..2fa8f599c20 100644 --- a/text/0000-supertrait-item-shadowing-v2.md +++ b/text/0000-supertrait-item-shadowing-v2.md @@ -49,10 +49,31 @@ When this happens, the sub-trait method is selected instead of reporting an ambi Note that this only happens when *both* traits are in scope since this is required for the ambiguity to occur in the first place. +When an ambiguity is resolved in this way, a lint warning is also emitted to warn the user about the potential ambiguity. The aim of this lint is to discourage reliance on this mechanism in normal code usage: it should only be used for backwards-compatibilty and the lint can be silenced by having users change their code. We can always later change this lint to be allowed by default if we consider that there are valid use cases for this feature other than backwards-compatiblity. + +### Type inference + +This change happens during name resolution and specifically doesn't interact with type inference. Consider this example: + +```rust +trait Foo { fn method(&self) {} } +trait Bar: Foo { fn method(&self) {} } +impl Foo for Vec { } +impl Bar for Vec { } + +fn main() { + let x = vec![]; + x.method(); // which to call? + x.push(Box::new(22)); // oh, looks like `Foo` +} +``` + +Today that example will give an ambiguity error because `method` is provided by multiple traits in scope. With this RFC, it will instead always resolve to the sub-trait method and then compilation will fail because `Vec` does not implement the `Copy` trait required by `Bar::method`. + # Drawbacks [drawbacks]: #drawbacks -This behavior can be surprising: adding a method to a sub-trait can change which function is called in unrelated code. A lint could be emitted to warn users about the potential ambiguity. +This behavior can be surprising: adding a method to a sub-trait can change which function is called in unrelated code. This is mitigated by the which warns users about the potential ambiguity. # Rationale and alternatives [rationale-and-alternatives]: #rationale-and-alternatives From 5b030b0d8618b5cd9385dc2456b7377fd8e20758 Mon Sep 17 00:00:00 2001 From: Amanieu d'Antras Date: Wed, 24 Jul 2024 12:08:13 +0100 Subject: [PATCH 6/6] Update text/0000-supertrait-item-shadowing-v2.md Co-authored-by: Travis Cross --- text/0000-supertrait-item-shadowing-v2.md | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/text/0000-supertrait-item-shadowing-v2.md b/text/0000-supertrait-item-shadowing-v2.md index 2fa8f599c20..12f63aba4e7 100644 --- a/text/0000-supertrait-item-shadowing-v2.md +++ b/text/0000-supertrait-item-shadowing-v2.md @@ -73,7 +73,7 @@ Today that example will give an ambiguity error because `method` is provided by # Drawbacks [drawbacks]: #drawbacks -This behavior can be surprising: adding a method to a sub-trait can change which function is called in unrelated code. This is mitigated by the which warns users about the potential ambiguity. +This behavior can be surprising: adding a method to a sub-trait can change which function is called in unrelated code. This is mitigated by the lint which warns users about the potential ambiguity. # Rationale and alternatives [rationale-and-alternatives]: #rationale-and-alternatives