no_std build with pest_derive fails

[jzbor]

Describe the bug
I get the following error, when trying to build with no_std:

   Compiling lash v0.3.1 (/home/jzbor/Programming/Rust/lash)
error: cannot find macro `format` in this scope
  --> src/typing.rs:48:13
   |
48 |             format!("{}", ('A' as usize + n) as u8 as char)
   |             ^^^^^^

error: cannot find macro `format` in this scope
  --> src/typing.rs:50:13
   |
50 |             format!("T{}", n)
   |             ^^^^^^

error: cannot find macro `format` in this scope
   --> src/typing.rs:154:58
    |
154 |             Variable(x) => Ok(self.inst(env.get(x).ok_or(format!("unable to instantiate {}", x))?.clone())),
    |                                                          ^^^^^^

error: cannot find macro `format` in this scope
   --> src/typing.rs:126:13
    |
126 |         Err(format!("unable to unify {} with {}", t1, t2))
    |             ^^^^^^

error: cannot find macro `format` in this scope
  --> src/typing.rs:77:21
   |
77 |                 Err(format!("occurs check failed - {} versus {}", tv.borrow(), tv2.borrow()))
   |                     ^^^^^^

error: cannot find macro `format` in this scope
  --> src/typing.rs:65:21
   |
65 |                 Err(format!("occurs check failed - {} == {}", tv.borrow(), tv2.borrow()))
   |                     ^^^^^^

error[E0433]: failed to resolve: could not find `std` in the list of imported crates
  --> src/parsing.rs:17:10
   |
17 | #[derive(Parser)]
   |          ^^^^^^ could not find `std` in the list of imported crates
   |
   = note: this error originates in the derive macro `Parser` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0433]: failed to resolve: could not find `std` in the list of imported crates
  --> src/parsing.rs:17:10
   |
17 | #[derive(Parser)]
   |          ^^^^^^ could not find `std` in the list of imported crates
   |
   = note: this error originates in the derive macro `Parser` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider importing this module
   |
3  + use core::result;
   |

For more information about this error, try `rustc --explain E0433`.
error: could not compile `lash` (lib) due to 8 previous errors

To Reproduce
Clone https://github.com/jzbor/lash/tree/pest
Run cargo build --lib --no-default-features

Expected behavior
From what I understand this is a supported setup, so it should just build and use core and alloc where necessary.

[tomtau]

For pest and pest_derive: https://github.com/jzbor/lash/blob/pest/Cargo.toml#L23

they should be specified with

pest = { version = "2.7.14", default-features = false }
pest_derive = { version = "2.7.14", default-features = false }

I believe when one calls cargo build --lib --no-default-features, it only applies that --no-default-features to the crate that's being built, not recursively to its dependencies. In the features, it should also specify this:

[features]
default = ["std"]
std = [ "rustyline", "clap", "colored", "humantime", "pest/std", "pest_generator/std" ]

so that the std feature is on by default for pest dependencies in that crate.

[jzbor]

Maybe I have to build alloc in the crate, I'll see...

[jzbor]

Hm I tried with my OS crate, which builds alloc, but I get the same issue

[tomtau]

Maybe cargo needs some extra flags to be passed? e.g.

 cargo build -j1 -Z build-std=core,alloc --lib --no-default-features

instead of just cargo build --lib --no-default-features

[jzbor]

Hey, sorry for taking so long. I tried to run that command on the respective branch and it failed with:

error[E0433]: failed to resolve: could not find `alloc` in the list of imported crates
  --> src/parsing.rs:16:10
   |
16 | #[derive(Parser)]
   |          ^^^^^^ could not find `alloc` in the list of imported crates
   |
   = note: this error originates in the derive macro `Parser` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0433`.
error: could not compile `lash` (lib) due to 1 previous error

You can checkout that branch here

[jzbor]

I believe this StackOverflow-Thread may be related

[jzbor]

@tomtau are there any updates on this? I do believe it is an actual issue with pest, so it would be great if you could either provide a solution or convert this back to an issue so it can be tracked and found by others.

[tomtau]

I wasn't able to reproduce it before (it build ok with cargo build -j1 -Z build-std=core,alloc --lib --no-default-features); I'll try again in a fresh environment

[tomtau]

@jzbor

1. I checked out the branch you posted: https://github.com/jzbor/lash/tree/pest
2. I added extern crate alloc to src/lib.rs

diff --git a/src/lib.rs b/src/lib.rs
index 605d50c..ae0daa5 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,5 +1,5 @@
 #![cfg_attr(not(feature = "std"), no_std)]
-
+extern crate alloc;
 pub mod debruijn;
 pub mod environment;
 pub mod error;

3. it builds fine:

$ cargo build -j1 -Z build-std=core,alloc --lib --no-default-features
   Compiling lash v0.3.1 (/workspaces/lash)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.22s

[jzbor]

Oh, I am very sorry, if I have wasted your time. I was not aware that extern crate alloc; was also required at the crate root.