# 2022-01-20

## Today's plan

### Idea

* Niko explains some interesting things (15 minutes)
* Experimental period (15 minutes):
* Come up with a working theory for how things work
* Come up with 2 or 3 interesting examples
* Sharing period (30 minutes):
* Read people's examples
* Niko picks one and
* Reading period (15 minutes):
* Try to trace through the code for that example
* Can use a pernosco link that Niko prepares

### Next session

* Resume from one of these examples
* Trace through the code
* Niko will have a pernosco link prepared

## Notes

### Niko

<details>
<summary>all</summary>

A: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=0676b4a1a40fff205dd7ebe2282f0b2d -- when/where/how do we detect this error?

B: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=1e5229e795f2bde67e2afe6332f64b60

C: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=2e3eeb0c8542bf7cd036ca16b3232ba5

glob + macro-generated item that relies on glob errors out at `use b::foo`:

D: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=9fe12f62c17dbdd91b63a00a50ca7580

explicit use + macro-generated item that relies on that path *also* errors out at `use b::foo`:

E: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=d34fea87c08b62054c8ab7d13915715f

above examples turn out to be hygiene related:

F: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=238538ccc02fbc55af3d30e9ebf5c711

obvious error:

G: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=1b32ae8863106bd86fdf1d12b1c41da2

explicit use + macro-generated item resolves to `b::foo`:

H: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=0139155e189738fabee23b1029d580c7

glob + macro-generated item *also* resolves to `b::foo`:

I: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=964e01a30dbb0a8ceef08630243dfce1

macros generating macros:

J: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=bcbd9d8dd8fc46a37a8f0b4126211e12

injecting a use:

K: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=d4002eedab5de91d9af146b275e6dc8d

injecting a use, no shadow:

L: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=8c4d640f80a72d50a343a6b1da08503a

use of a (private) use

M: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=21a02f8ca3412eee4acb74b43f30f38b

</details>

(below samples are taken from above character labelled list)

use of a (private) use

M: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=21a02f8ca3412eee4acb74b43f30f38b

* Note from pnkfelix: I don't think the linked example is complete. It references an `inject_use!` macro that is not in the given source code, presumably the one from the example labelled 'K'

glob + macro-generated item *also* resolves to `b::foo`:

I: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=964e01a30dbb0a8ceef08630243dfce1

injecting a use:

K: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=d4002eedab5de91d9af146b275e6dc8d

### metajack

- are there multiple priority levels? or just explicit and low
- if all resolutions are ambiguous, it is an error:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=4ed9794cbcb1b67e2dec7711b518a54a
- why doesn't this work?
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=76e62533017556662afce760b47107ba
or this:
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=dc8447b001da15401e1ea7ecac04dcc4

- ambiguity kind is GlobVsGlob: https://github.com/rust-lang/rust/blob/master/compiler/rustc_resolve/src/lib.rs#L749
- error comes from https://github.com/rust-lang/rust/blob/master/compiler/rustc_resolve/src/diagnostics.rs#L1224 `report_ambiguity_error()` called from `report_errors()` https://github.com/rust-lang/rust/blob/master/compiler/rustc_resolve/src/lib.rs#L2964 called from `resolve_crate()`
- ambiguity errors are created at https://github.com/rust-lang/rust/blob/master/compiler/rustc_resolve/src/lib.rs#L1692 inside `record_use()`
- the ambiguity is stored in `NameBinding.ambiguity`
- `record_use()` called from `resolve_main()` and `extern_prelude_get()`.
- what calls `extern_prelude_get()`?

### forest

- Walking through this "loop", how can I think of precedence of macros vs imports? <-- note that you can import from crates on play

I got one to compile!
(deeper nesting of crates)
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=8638adbaac70b8e0ee7a1eeb4612e477

Same as above, but I get to import everything
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=7b4226c03d5ab5f69bc31d5e55c840f6

Trying to just import in modules directly
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=7e87a8662967caca5ff63837605e705d

Surprisingly, my original code works when I call all functions foo?
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=5571c06a7e43b9ba6dee0ed84ecd6243

Trying to clean stuff up, but it's not working just yet (too many glob imports)
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=0318519b807f1e033885c11e039c3042

Cleaned up
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=2b1f8911ec5a91d873a23e777a831113

**key examples**

Using crates inside of the macro (cleaned up version):
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=54a0f01c3832f9c370bebda0c4475ba8

### Olivia
Seems to resolve the latest use https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=7303f57de07e65c78b826adc37ef7621

Traits collide with structs, kind of what I expected
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=8b4d838c3952c11e005883014e17f31b

Functions and structs/traits do not collide, not what I expected
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=728b89c02bb39aced1731b15d53aaabd
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=6c4837d58f5147ddd1c1f42dacdb2f10

what happens when you use two globs
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=5f1159a9a064b38a5c7061cb6ba0f5a1

### Santiago Pastorino

Was expecting ambiguous and indeed we get ambiguous ...
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=c9c4938e035a1176d3f4bb3b235625cd

Same thing using macros ...
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=6990c9a5da0238047b0146e5cd8789f7

It works if you're explicit ...
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=68d2c804bbb8dc53f02eb49bdfd14c55

What should happen here? ...
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=55de697cb4a28f7f5fbfb52b8f741fe6

### rémy

The macros namespace is kinda "last come first served" https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=73416ad63ddb9ea890ccb0cfd94c48ce

but not when the duplicate names come from macro expansion ? https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=b99bbf676dc9d64d8ea651e2252e4651 is now ambiguous, but the "amount" of expansion matters here only.

Modules seem to have the same glob vs explicit import priority mechanism https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=dc301bb7689f92baab65683382512586

### Sean

`b::foo` resolves based on what is passed into `crate::c::m!`?

pub mod b {
    crate::c::m!(foo);
}

This fails to compile:

pub mod b {
    crate::c::m!(blah);
}

Ah, never mind, it makes sense. It was just weird to me at first that the module was resolving based off of what is being passed into the macro.

## Niko's plan

Start with this simple case of ambiguity:

N1: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=ab7fab835eb45e899af54b7aaf3cf00f

Add in macro-rules:

N2: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=a848d5f22fcaf5fc9c2766b2f6e04043

Add in pub macro instead:

N3: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=e8bbf0dfe6e444d9ff99e5959e067eda

Add in hygiene:

N4: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=5066bd6e137e7624401d1bd2ef6f1b11

## Do the read

* Find where the error is generated
* `RUST_BACKTRACE=1 rustc +nightly -Ztreat-err-as-bug foo.rs`
* Read the context and note a few things that you find interesting
* e.g., a hunch at how the data structure works
* something about the names
* etc

### Niko

* Interesting points:
* [`record_use`](https://github.com/rust-lang/rust/blob/74fbbefea8d13683cca5eee62e4740706cb3144a/compiler/rustc_resolve/src/lib.rs#L1685)
* So where is the call where `used_binding.ambiguity` is some?
* Pernosco would be helpful!
* probably `try_resolve_as_non_binding`... no, this is not a pattern
* probably [`record_use`](https://github.com/rust-lang/rust/blob/74fbbefea8d13683cca5eee62e4740706cb3144a/compiler/rustc_resolve/src/lib.rs#L1685)...wait what
* I have no idea :)
* probably [`try_resolve_as_non_binding`](https://github.com/rust-lang/rust/blob/74fbbefea8d13683cca5eee62e4740706cb3144a/compiler/rustc_resolve/src/late.rs#L1803)

https://rustc-dev-guide.rust-lang.org/building/suggested.html#configuring-rust-analyzer-for-rustc

niko's vscode settings:

> cat .vscode/settings.json
{
    "rust-analyzer.checkOnSave.overrideCommand": [
        "vscode.sh"
    ],
    "rust-analyzer.rustfmt.overrideCommand": [
        "./build/x86_64-unknown-linux-gnu/stage0/bin/rustfmt",
        "--edition=2018"
    ],
    "editor.formatOnSave": true,
    "rust-analyzer.cargo.runBuildScripts": false,
    "rust-analyzer.rustcSource": "./Cargo.toml",
    "rust-analyzer.procMacro.enable": false
}

> cat $(which vscode.sh)
#!/bin/bash
mkdir -p build/vscode
cd build/vscode
../../x.py check --json-output

### rémy
- backtrace: not great
- `configure_and_expand`
- `rustc_resolve::diagnostics::report_ambiguity_error`
- `rustc_resolve::record_use`
-> emits error per `used_binding.ambiguity`
- `rustc_resolve::imports::ambiguity`
- `rustc_resolve::imports::try_define`

[`// Define the name or return the existing binding if there is a collision.`](https://github.com/rust-lang/rust/blob/74fbbefea8d13683cca5eee62e4740706cb3144a/compiler/rustc_resolve/src/imports.rs#L492-L560)

### olivia
error code leads to https://github.com/rust-lang/rust/blob/48e89b00caa94829a5f07e0f1ecb33bf37431244/compiler/rustc_resolve/src/lib.rs
GlobVsGlob - import.rs - try_define