(Modules) Tracking issue for Picking a Module Path System variant #53130

Centril · 2018-08-06T20:50:11Z

This is a sub-tracking issue for the RFC "Clarify and streamline paths and visibility" (rust-lang/rfcs#2126)
as well as the internals post Relative Paths in Rust 2018.

The issue deals with revamping paths and making a decision about whether we should pick:

The uniform_paths variant.
Proposed in Relative Paths in Rust 2018.
The anchored_use_paths variant.
You have to use self::, super::, and crate::, or an external crate name explicitly....
See RFC: Clarify and streamline paths and visibility rfcs#2126).

Implement all the things.
- uniform_paths, #[feature(uniform_paths)]: allow use x::y; to resolve through self::x, not just ::x. #52923
- anchored_use_paths
Make both variants available to test in Edition Preview 2 with appropriate documentation on how to test.
Adjust documentation (see instructions on forge)
Pick a variant
Stabilization PR (see instructions on forge)

Unresolved questions:

Which variant do we pick?

The text was updated successfully, but these errors were encountered:

sanmai-NL · 2018-08-07T10:33:27Z

Could you give more details about what has been implemented concretely, and what renaming operations are needed exactly? It comes down to: how can I start testing anything right now?

A priori, I vote for anchored_use_paths, for being the most explicit/teachable. One suggestion: I find the crate keyword a bit odd since it does not seem related to visibility at all. I suppose this has been debated before but it would be better if it becomes pubcrate or whatever.

Centril · 2018-08-08T08:54:29Z

cc @eddyb re. @sanmai-NL's question.

@cramertj

#[feature(uniform_paths)]: allow `use x::y;` to resolve through `self::x`, not just `::x`. _Branch originally by @cramertj, based on @petrochenkov's [description on the internals forum](https://internals.rust-lang.org/t/relative-paths-in-rust-2018/7883/30?u=petrochenkov)._ _(note, however, that the approach has significantly changed since)_ Implements `#[feature(uniform_paths)]` from #53130, by treating unqualified `use` paths as maybe-relative. That is, `use x::y;`, where `x` is a plain identifier (not a keyword), is no longer the same as `use ::x::y;`, and before picking an external crate named `x`, it first looks for an item named `x` in the same module (i.e. `self::x`) and prefers that local item instead. Such a "maybe-relative" `x` can only resolve to an external crate if it's listed in "`extern_prelude`" (i.e. `core` / `std` and all the crates passed to `--extern`; the latter includes Cargo dependencies) - this is the same condition as being able to refer to the external crate from an unqualified, non-`use` path. All other crates must be explicitly imported with an absolute path, e.g. `use ::x::y;` To detect an ambiguity between the external crate and the local item with the same name, a "canary" import (e.g. `use self::x as _;`), tagged with the `is_uniform_paths_canary` flag, is injected. As the initial implementation is not sophisticated enough to handle all possible ways in which `self::x` could appear (e.g. from macro expansion), this also guards against accidentally picking the external crate, when it might actually get "shadowed" later. Also, more canaries are injected for each block scope around the `use`, as `self::x` cannot resolve to any items named `x` in those scopes, but non-`use` paths can, and that could be confusing or even backwards-incompatible. Errors are emitted only if the main "canary" import succeeds while an external crate exists (or if any of the block-scoped ones succeed at all), and ambiguities have custom error reporting, e.g.: ```rust #![feature(uniform_paths)] pub mod foo { use std::io; pub mod std { pub mod io {} } } ``` ```rust error: import from `std` is ambiguous --> test.rs:3:9 | 3 | use std::io; | ^^^ could refer to external crate `::std` 4 | pub mod std { pub mod io {} } | ----------------------------- could also refer to `self::std` | = help: write `::std` or `self::std` explicitly instead = note: relative `use` paths enabled by `#![feature(uniform_paths)]` ``` Another example, this time with a block-scoped item shadowing a module-scoped one: ```rust #![feature(uniform_paths)] enum Foo { A, B } fn main() { enum Foo {} use Foo::*; } ``` ```rust error: import from `Foo` is ambiguous --> test.rs:5:9 | 4 | enum Foo {} | ----------- shadowed by block-scoped `Foo` 5 | use Foo::*; | ^^^ | = help: write `::Foo` or `self::Foo` explicitly instead = note: relative `use` paths enabled by `#![feature(uniform_paths)]` ``` Additionally, this PR, because replacing "the `finalize_import` hack" was a blocker: * fixes #52140 * fixes #52141 * fixes #52705 cc @aturon @joshtriplett

eddyb · 2018-08-14T09:28:29Z

@sanmai-NL Now that #52923 has landed, the next nightly should have both options available:

anchored_use_paths has been the Rust 2018 default for some(?) time now
uniform_paths you get by being on Rust 2018 and adding #![feature(uniform_paths)]

I'd suggesting trying the latter after upgrading to the edition, and report back if the ambiguity errors end up being too excessive.

AFAIK, the upgrade involves taking a Rust 2015 crate, adding #![feature(rust_2018_preview)] without changing the edition, running rustfix to apply the suggestions it gives for making same-crate paths explicit (i.e. prepending all paths that don't start with a crate name, with crate::), and only then do you switch the edition (usually in Cargo.toml) to Rust 2018.
You can remove #![feature(rust_2018_preview)] afterwards, since it's redundant.

joshtriplett · 2018-08-14T22:12:58Z

rust-lang/edition-guide#85

archer884 · 2018-08-16T06:53:35Z

I absolutely despise the "anchored paths variant," and I am very excited to see that we're considering an alternative. Any alternative, honestly. >.>

...That said, uniform paths look great. :) I'd much prefer to let people use absolute paths (the anchored version) if they like, but not force people to do it.

(No idea where the appropriate place to provide feedback for this would be, so I just left it here. Sorry.)

kohensu · 2018-08-16T07:50:50Z

Using uniform_paths, there is an error I find confusing.
Considering:

#![feature(rust_2018_preview)]
#![feature(uniform_paths)]
use hex;
fn main() {
    println!("hex {}", hex::encode("01"));
}

I get this error:

error: import from `hex` is ambiguous
 --> src/main.rs:3:5
  |
3 | use hex;
  |     ^^^
  |     |
  |     could refer to external crate `::hex`
  |     could also refer to `self::hex`
  |
  = help: write `::hex` or `self::hex` explicitly instead
  = note: relative `use` paths enabled by `#![feature(uniform_paths)]`

whereas self::hex doesn't exist in the module.

Because the code is working without use hex maybe suggesting to remove it will be more clear.

Nemo157 · 2018-08-16T08:03:42Z

@kohensu see #53408, I think the plan is to either special case allow that or improve the error message to just suggest removing it.

kohensu · 2018-08-16T08:14:37Z

@Nemo157
Sorry, I did not see that.
Thanks for the reference to the issue.

sanmai-NL · 2018-08-16T09:42:47Z

@archer884: What’s more interesting is your motivation to feel that way?

Xymist · 2018-08-16T11:37:27Z

As a user (who perhaps hasn't been paying as much attention as I should) I think the uniform_paths option feels strictly better. Special case sugar for when there's no possible ambiguity would be nice, but by no means necessary; optimising for clarity is definitely a greater win.

eddyb · 2018-08-16T13:52:28Z

Special case sugar for when there's no possible ambiguity would be nice

Not sure what you mean. Does #53427 (by treating each namespace independently) solve that for you?

archer884 · 2018-08-16T15:50:59Z

@sanmai-NL: Either Eric Lippert or Anders Hejlsberg had an article on this awhile back: people want loud syntax for things at first, but then as time goes on they want the syntax to be unobtrusive and elegant instead. If we make long, annoying imports the standard (and ONLY) option, we will absolutely regret it. "Maybe not today, maybe not tomorrow, but someday." :)

sanmai-NL · 2018-08-17T11:12:58Z

That’s an argument from authority, and a very aspecific one. Conciseness is a criterion independent from explicitness and also from simplicity.

rfcbot · 2018-09-16T19:35:24Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

aturon · 2018-11-02T17:35:33Z

See also: #53130 proposing to stabilize on uniform paths.

Centril · 2018-11-02T18:05:09Z

@aturon haha; came here to link to that issue as well =P

You linked to the current issue tho ;)
Here's the right issue: #55618.

@ghost

[beta] resolve: Implement uniform paths 2.0 With this PR paths in imports on 2018 edition are resolved as relative in the scope in which they are written, similarly to any other paths. The previous implementation worked differently - it tried to resolve the import in the current module (`self::import`) and in the "crate universe" (`::import`), and then merge these two resolutions if possible. The difference is that the new scheme can refer to strictly larger set of names in scope - names from unnamed blocks, names from all kinds of preludes, including macros imported with `#[macro_use] extern crate`, built-in types/macros, macros introduced with `macro_rules`. This means strictly more potential ambiguities and therefore ambiguity errors, since we keep the rule that any two different candidate names in scope conflict with each other during import resolution. So this is a breaking change for 2018 edition, but it should be relatively minor. All paths that don't start with an extern crate are also gated with the `uniform_paths` feature, paths that refer to extern crates are not gated (so we effectively get something like "future-proofed anchored paths" on stable). Another difference is treatment of paths in visibilities (`pub(in path)`). Few people remember about paths in visibilities, so before this PR they still used the old 2015 rules even on 2018 edition. Namely, paths in visibilities were crate-relative, analogous to 2015 edition imports. This PR resolves paths in visibilities as uniform as well, or rather future proofs them in this direction. Paths in visibilities are restricted to parent modules, so relative paths almost never make sense there, and `pub(in a)` needs to be rewritten as `pub(in crate::a)` in the uniform scheme, de-facto cementing the discouraged status of non-`pub(crate)` and non-`pub(super)` fine-grained visibilities. This is clearly a breaking change for 2018 edition as well, but also a minor one. The in-scope resolution strategy for import paths mirrors what is currently done for macro paths on stable (on both editions), so it will continue working even if the "ambiguity always means error" restriction is relaxed in the future. This PR also significantly improves diagnostics for all kinds of resolution ambiguities, from the newly introduced import ones to pretty old "glob vs glob" conflicts. (That's probably what I've spent most of the time on.) Why beta: - This is a breaking change on 2018 edition. - This is a large PR, it's less risky to forward-port it to nightly, than back-port to beta. cc #55618 cc #53130 cc rust-lang/rfcs#1289 Closes #18084 Closes #54525 Fixes #54390 Fixes #55668 r? @ghost

@ghost

[beta] resolve: Implement uniform paths 2.0 With this PR paths in imports on 2018 edition are resolved as relative in the scope in which they are written, similarly to any other paths. The previous implementation worked differently - it tried to resolve the import in the current module (`self::import`) and in the "crate universe" (`::import`), and then merge these two resolutions if possible. The difference is that the new scheme can refer to strictly larger set of names in scope - names from unnamed blocks, names from all kinds of preludes, including macros imported with `#[macro_use] extern crate`, built-in types/macros, macros introduced with `macro_rules`. This means strictly more potential ambiguities and therefore ambiguity errors, since we keep the rule that any two different candidate names in scope conflict with each other during import resolution. So this is a breaking change for 2018 edition, but it should be relatively minor. All paths that don't start with an extern crate are also gated with the `uniform_paths` feature, paths that refer to extern crates are not gated (so we effectively get something like "future-proofed anchored paths" on stable). Another difference is treatment of paths in visibilities (`pub(in path)`). Few people remember about paths in visibilities, so before this PR they still used the old 2015 rules even on 2018 edition. Namely, paths in visibilities were crate-relative, analogous to 2015 edition imports. This PR resolves paths in visibilities as uniform as well, or rather future proofs them in this direction. Paths in visibilities are restricted to parent modules, so relative paths almost never make sense there, and `pub(in a)` needs to be rewritten as `pub(in crate::a)` in the uniform scheme, de-facto cementing the discouraged status of non-`pub(crate)` and non-`pub(super)` fine-grained visibilities. This is clearly a breaking change for 2018 edition as well, but also a minor one. The in-scope resolution strategy for import paths mirrors what is currently done for macro paths on stable (on both editions), so it will continue working even if the "ambiguity always means error" restriction is relaxed in the future. This PR also significantly improves diagnostics for all kinds of resolution ambiguities, from the newly introduced import ones to pretty old "glob vs glob" conflicts. (That's probably what I've spent most of the time on.) Why beta: - This is a breaking change on 2018 edition. - This is a large PR, it's less risky to forward-port it to nightly, than back-port to beta. cc #55618 cc #53130 cc rust-lang/rfcs#1289 Closes #18084 Closes #54525 Fixes #54390 Fixes #55668 r? @ghost

petrochenkov · 2018-12-01T18:29:29Z

Some follow-up issues:

Stabilize uniform paths on Rust 2018 (technical details) #56417 Stabilize uniform paths on Rust 2018 (technical details)
Ambiguity errors in 2018 uniform import paths are not technically necessary #56414 Ambiguity errors in 2018 uniform import paths are not technically necessary
Known deviations of macro paths and 2018 import paths from the "uniform path" model #56413 Known deviations of macro paths and 2018 import paths from the "uniform path" model

Stabilize `uniform_paths` Address all the things described in #56417. Assign visibilities to `macro_rules` items - `pub` to `macro_export`-ed macros and `pub(crate)` to non-exported macros, these visibilities determine how far these macros can be reexported with `use`. Prohibit use of reexported inert attributes and "tool modules", after renaming (e.g. `use inline as imported_inline`) their respective tools and even compiler passes won't be able to recognize and properly check them. Also turn use of uniform paths in 2015 macros into an error, I'd prefer to neither remove nor stabilize them right away because I still want to make some experiments in this area (uniform path fallback was added to 2015 macros used on 2018 edition in #56053 (comment)). UPDATE: The last commit also stabilizes the feature `uniform_paths`. Closes #53130 Closes #55618 Closes #56326 Closes #56398 Closes #56417 Closes #56821 Closes #57252 Closes #57422

Centril added B-RFC-approved Blocker: Approved by a merged RFC but not yet implemented. T-lang Relevant to the language team, which will review and decide on the PR/issue. C-tracking-issue Category: A tracking issue for an RFC or an unstable feature. labels Aug 6, 2018

Centril added this to the Rust 2018 RC milestone Aug 6, 2018

Centril changed the title ~~(Modules) Tracking issue for Picking one of the two proposed Module Path Systems~~ (Modules) Tracking issue for Picking one of the two Module Path Systems Aug 6, 2018

Centril changed the title ~~(Modules) Tracking issue for Picking one of the two Module Path Systems~~ (Modules) Tracking issue for Picking a Module Path System variant Aug 6, 2018

Centril mentioned this issue Aug 6, 2018

Tracking issue for RFC 2126: Clarify and streamline paths and visibility #44660

Closed

11 tasks

sanmai-NL mentioned this issue Aug 7, 2018

(Modules) Tracking issue for (use) crate_name:: paths without extern crate #53128

Closed

3 tasks

petrochenkov mentioned this issue Aug 7, 2018

std-using paths work just fine in 2018 edition #![no_std] #53166

Closed

eddyb mentioned this issue Aug 9, 2018

#[feature(uniform_paths)]: allow use x::y; to resolve through self::x, not just ::x. #52923

Merged

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rfcbot added finished-final-comment-period The final comment period is finished for this PR / Issue. and removed final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. labels Sep 16, 2018

steveklabnik mentioned this issue Sep 18, 2018

Update to reflect module system as-is rust-lang/edition-guide#62

Closed

petrochenkov mentioned this issue Sep 21, 2018

use proc_macro regressed from nightly-2018-09-14 to nightly 2018-09-15 in edition 2018 #54418

Closed

Nemo157 mentioned this issue Sep 21, 2018

alloc not added to extern prelude #54392

Closed

petrochenkov mentioned this issue Oct 5, 2018

Add extern crate items to extern prelude #54658

Merged

Centril mentioned this issue Oct 7, 2018

Ability to refer to a path relative to the current scope in use declarations rust-lang/rfcs#959

Closed

stephaneyfx mentioned this issue Oct 29, 2018

Fix extern crate alias example for anchored paths rust-lang/edition-guide#118

Merged

withoutboats mentioned this issue Nov 2, 2018

Stabilize uniform paths on Rust 2018 #55618

Closed

petrochenkov mentioned this issue Nov 12, 2018

[beta] resolve: Implement uniform paths 2.0 #55884

Merged

petrochenkov mentioned this issue Dec 27, 2018

Stabilize uniform_paths #56759

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cramertj mentioned this issue Jan 2, 2019

Don't require extern crate #57288

Open

bors closed this as completed in #56759 Jan 12, 2019

niklasad1 mentioned this issue Feb 4, 2019

fix(osx and windows builds): bump parity-daemonize openethereum/parity-ethereum#10291

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neachdainn mentioned this issue Apr 3, 2019

Change Edition from 2018 to 2015 jeikabu/nng-rust#10

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Compling issue ecumene/rust-sloth#10

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BryantLam mentioned this issue May 21, 2019

Local Module Search Paths chapel-lang/chapel#12923

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Fix path of imports for Rust 2018 mmtk/mmtk-core#36

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(Modules) Tracking issue for Picking a Module Path System variant #53130

(Modules) Tracking issue for Picking a Module Path System variant #53130

Centril commented Aug 6, 2018 •

edited by eddyb

sanmai-NL commented Aug 7, 2018

Centril commented Aug 8, 2018

eddyb commented Aug 14, 2018

joshtriplett commented Aug 14, 2018

archer884 commented Aug 16, 2018 •

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petrochenkov commented Dec 1, 2018

(Modules) Tracking issue for Picking a Module Path System variant #53130

(Modules) Tracking issue for Picking a Module Path System variant #53130

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Centril commented Aug 6, 2018 • edited by eddyb

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