Transitioning a 3rd party crate (conrod) to 2018 edition

Hello!

I am trying to migrate https://github.com/PistonDevelopers/conrod to 2018 edition.

https://rust-lang-nursery.github.io/edition-guide/editions/transitioning-an-existing-project-to-a-new-edition.html states that:

It's our intention that the migration to new editions is as smooth an experience as possible. If it's difficult for you to upgrade to Rust 2018, we consider that a bug. If you run into problems with this process, please file a bug. Thank you!

So, here I am, although this may very well be a layer 8 problem.

What I did: I checked out conrod and first migrated conrod_derive via

cargo fix --edition

But it turned out to be quite tedious until I found out about:

cargo fix --prepare-for 2018 (any reason this is not more prominently mentioned in the guide?)

Then all the low hanging fruits were done automagically - great! (My compliments/gratitude here!)

I then proceeded enabling edition = 2018 in the Cargo.toml file and then the nightmare started:

error[E0412]: cannot find type Theme in the crate root
--> src/widget/toggle.rs:38:50
|
38 | #[derive(Copy, Clone, Debug, Default, PartialEq, WidgetStyle_)]
| ^^^^^^^^^^^^ not found in the crate root
help: possible candidate is found in another module, you can import it into scope
|
3 | use crate::theme::Theme;

Appears a few dozen times for different files! (and if it's not WidgetStyle_ then it's the WidgetCommon_ macro)

use crate::theme::Theme; does not help at all, sadly.

When I try to use use conrod_derive::WidgetStyle_ nothing changes.

When I use: use conrod_derive::style::WidgetStyle_; I get a lot more errors, now saying:

error[E0658]: The attribute conrod is currently unknown to the compiler and may have meaning added to it in the future (see issue #29642)
--> src/widget/toggle.rs:49:5
|
49 | #[conrod(default = "theme.border_color")]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= help: add #![feature(custom_attribute)] to the crate attributes to enable

if I follow that hint, I get the same errors as before adding the crate attribute with the addition of

error[E0432]: unresolved import conrod_derive::style
--> src/widget/toggle.rs:18:20
|
18 | use conrod_derive::style::WidgetStyle_;
| ^^^^^ Could not find style in conrod_derive

because style is not public... but I made it public already and the same error is still present.

Here is where I give up and let someone smarter than me point me in the right direction, because obviously I am doing something wrong.

Sorry for bothering the bugtracker with what is probably my fault and mine alone, but it was suggested and I am kind of desperate to understand this now (how else can I improve?)

[jebrosen]

I only took a cursory glance, but I think I can explain this. Conrod's _-suffixed macros are for use within conrod itself, where "extern crate conrod"; can't be used. At https://github.com/PistonDevelopers/conrod/blob/master/conrod_derive/src/style.rs#L28 , the "path to conrod itself" is set to "None", so #crate_tokens::Theme becomes ::Theme at https://github.com/PistonDevelopers/conrod/blob/master/conrod_derive/src/style.rs#L62 . This is probably wrong for the 2018 edition and should be something like crate::Theme instead, and the same for other similar macros.

use conrod_derive::style::WidgetStyle_; looks wrong, it should be conrod_derive::WidgetStyle_. Alternatively, #[macro_use] extern crate conrod_derive; already imports WidgetStyle_ for use from anywhere within conrod.

Unfortunately, errors in proc_macro-generated code often don't point to anything useful in the original source.

[petrochenkov]

so #crate_tokens::Theme becomes ::Theme

Then it looks like unimplemented edition hygiene for absolute paths (including imports) - #50999.

If macro defined by 2015 edition crate produces path ::foo, then it's supposed to be interpreted according to 2015 edition rules (i.e. crate-relative).

[jebrosen]

I then proceeded enabling edition = 2018 in the Cargo.toml file and then the nightmare started:

The macro is defined by the 2018 edition, so the 2018 import rules should apply.

[ehuss]

cargo fix --prepare-for 2018 (any reason this is not more prominently mentioned in the guide?)

This is functionally identical to cargo fix --edition, there should be no difference.

Hm, so anyone knows how to fix this? I tried changing the let crate_tokens= expression but to no avail.
e.g. let crate_tokens = Some(syn::Ident::new("crate", proc_macro2::Span::call_site()));

I also tried some other tricks (like matching on the crate_tokens: Option<syn::Ident>), but nothing worked.

nevermind, the matching worked, I just wonder what's the right solution to get a ident for "crate"

it's like a box of worms. Whatever I fix, there is still something else to fix. I am hereby giving up.

[rylev]

Closing as this doesn't seem to be a specific issue.