Importing an enum variant can create a conflict with a new prelude type

[joshtriplett]

The experiment in #125107 uncovered an interesting source of version breakage, which occurs if the standard library prelude attempts to introduce an additional type.

Suppose std adds the type Cell to the prelude. This will break code that looks like this:

use Cell::A;

// The derive here is important but it doesn't particularly matter which trait is being derived.
#[derive(PartialEq)]
pub enum Cell {
    A
}

Per the comment at #125107 (comment) , this generates an error of the form:

error[E0659]: `Cell` is ambiguous
  --> src/puzzle.rs:5:5
   |
5  | use Cell::{Empty, Unknown, ValA, ValB, ValC, ValD};
   |     ^^^^ ambiguous name
   |
   = note: ambiguous because of a conflict between a macro-expanded name and a less macro-expanded name from outer scope during import or macro resolution
note: `Cell` could refer to the enum defined here
  --> src/puzzle.rs:8:1
   |
8  | / pub enum Cell {
9  | |     ValA,
10 | |     ValB,
11 | |     ValC,
...  |
14 | |     Unknown,
15 | | }
   | |_^
   = help: use `self::Cell` to refer to this enum unambiguously
note: `Cell` could also refer to a struct from prelude
  --> /rustc/9130c02509ce15f69dc5da6359bb9d140d41d4ac/library/std/src/prelude/mod.rs:148:13

This makes it a potentially breaking change to add new types to the prelude. We could still potentially add types that create no actual breakage (e.g. because no extant Rust code declares its own conflicting type), but this nonetheless introduces a potential source of breakage that could block introducing new types until an edition boundary (and creates one more thing people have to deal with when migrating to the new edition).

It seems worth exploring whether we can, or should, attempt to fix this, such as by allowing the type to shadow in this case. Note that it does properly shadow if there's no derive on the enum.

Nominating for lang discussion on the "should we" question: should we consider doing something to eliminate or mitigate this source of conflict? This kind of conflict applies both to the current problem with the standard library prelude, as well as any potential future features for letting other libraries have preludes.

Nominating for compiler discussion on the "can we" question: to find out how feasible it would be to deprioritize standard library prelude types and make them lower priority than local enums in this case, or to otherwise mitigate the issue that seems to specifically arise when using a derive macro on such types.

[Noratrieb]

@petrochenkov do you know whether this is possible? (I suspect the people in the compiler meeting will not know)

[petrochenkov]

@Nilstrieb
What exactly, removing the "Cell is ambiguous" error?

It is a part of the "time travel prevention" machinery in macro/import resolution that ensures that resolution results are independent of things like order of items in the crate, or some specific expansion order used by the compiler.
The issue is not related to derive specifically, it would also appear if enum Cell were emitted by any other macro.

In the specific example above, I don't think the error can be avoided.
The use of Cell (use Cell::A) does materialize earlier than its definition (enum Cell) emitted by the derive macro, so we must restrict shadowing.

[joshtriplett]

@petrochenkov I really appreciate the pointer to that writeup, thank you. I think I understand why we restrict shadowing in that case.

So, if I'm understanding correctly, the ordering problem is roughly:

1. We encounter Cell in the prelude.
2. We encounter use Cell::A;.
3. We encounter a macro resolution (for the derive).
4. In this first pass, we tentatively assume the use Cell::A; will refer to the Cell we've seen, from the prelude.
5. Also in the first pass, we expand the macro, which produces a definition of Cell, which would shadow the one in the prelude.
6. In a second pass, we notice that use Cell::A; could refer to the definition just produced by the macro expansion in the first pass, so we error.

Is that an accurate description?

And, in the case where we don't have a derive, both definitions come from the first pass, so we allow one to shadow the other?

I understand that we can't fix the fully general case of that. Would it be possible to mitigate this issue in the common case of a derive? We know that a derive (even a proc-macro derive) applied to a top-level item X can't change that top-level item, it can only add additional things. So, given that, could we arrange to know in the first pass that we have a Cell, even if it has a derive attached to it? That would then let us fall into the same case as if we didn't have the proc macro, where all the names showed up in the first pass alongside the use.

[petrochenkov]

Is that an accurate description?

Yes.
When we see use Cell::A we have two choices

• either to use the definition of Cell that we see right now (the prelude one), even if it can be potentially shadowed later
  • that's what we do right now
  • if the name does gets shadowed some time later, after expanding an arbitrary number of macros, then we produce a restricted shadowing error, like one in the example above
• or block and wait until all macros are expanded and we are sure that it certainly won't be shadowed
  • this is not a viable choice for resolution in "current scope", because it will make resolution stuck pretty much always
  • however, it's a viable choice when resolving in a specific module (like specific_module::Cell), then we can block and wait instead of possibly producing restricted shadowing errors later

[petrochenkov]

Would it be possible to mitigate this issue in the common case of a derive?

Only if derive is turned from a macro into a built-in syntax.
Basically, the main requirement for this is that derive should not go through name resolution.

• If on seeing #[derive] we immediately know that it is indeed the well known https://doc.rust-lang.org/stable/std/prelude/rust_2024/attr.derive.html, then we can immediately expand it and produce enum Cell in the "first pass".
• If we need to go and find derive in std::prelude::rust_2024 where it normally resides, or in some other place if it's shadowed, then it can potentially send the derive's expansion to "second/third/etc pass" even if it ends up being the standard well known derive eventually.

[petrochenkov]

It may be possible to amend macro expansion algorithm with "immediate expansion" attempts.

If we see a macro invocation (that includes #[derive]) and an immediate attempt at resolving it succeeds, then we immediately expand it as well, instead of putting it into the usual queue.
This should lift some restrictions from code emitted by such macro, including shadowing restrictions like in the example above.

In practice most derive invocations will be able to be promoted to "immediate expansions".

Not sure how viable this idea is, someone needs to try prototyping.

[petrochenkov]

If ... an immediate attempt at resolving it succeeds

On a second thought, results of a probing like this are going depend on the specific expansion order, and that's what we are trying to avoid now.

[joshtriplett]

@petrochenkov

Would it be possible to mitigate this issue in the common case of a derive?

Only if derive is turned from a macro into a built-in syntax. Basically, the main requirement for this is that derive should not go through name resolution.

• If on seeing #[derive] we immediately know that it is indeed the well known https://doc.rust-lang.org/stable/std/prelude/rust_2024/attr.derive.html, then we can immediately expand it and produce enum Cell in the "first pass".

That sounds potentially feasible, and seems unlikely to create conflicts. I'd be genuinely surprised if anything in the ecosystem is overriding the name derive.