Error building ICU4X on a no_std platform

[ghost]

I'm following Using ICU4X from C++ to compile a c++ static library. We will be building it on a no_std platform, more specifically thumbv7em-none-eabi, so I used the command listed under Embedded Platforms to compile. However, I'm getting errors like

   Compiling icu_capi v1.5.0
error[E0152]: found duplicate lang item `panic_impl`
  --> /usr/local/google/home/jiyuntu/.cargo/registry/src/index.crates.io-6f17d22bba15001f/icu_capi-1.5.0/src/lib.rs:51:1
   |
51 | / fn panic(_info: &core::panic::PanicInfo) -> ! {
52 | |     loop {}
53 | | }
   | |_^
   |
   = note: the lang item is first defined in crate `std` (which `serde` depends on)
   = note: first definition in `std` loaded from /usr/local/google/home/jiyuntu/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-d5189b81a4fa4d36.rlib
   = note: second definition in the local crate (`icu_capi`)

For more information about this error, try `rustc --explain E0152`.
error: could not compile `icu_capi` (lib) due to 1 previous error

if the target is not specified (I guess it's using default stable-x86_64-unknown-linux-gnu) and much more errors if the target is set to thumbv7em-none-eabi.

It seems like the errors come from the dependency "serde", which is dependent on "std". So I tried specifying serde = { version = "1.0", default-features = false } in Cargo.toml, but that doesn't help. How would you suggest to compile ICU4X on a no_std platform?

[Manishearth]

I'm unable to reproduce this, could you share the cargo command and the Cargo.toml you're using?

[Manishearth]

Ah, I'm able to repro now

[Manishearth]

So the first problem is that you're not invoking it with the right --target flag

Try

cargo rustc --release -p icu_capi --crate-type staticlib --features icu_capi/default_components,icu_capi/buffer_provider,icu_capi/looping_panic_handler,icu_capi/libc_alloc --target thumbv7em-none-eabi

However, that's also broken.

For now if you run cargo rustc --release -p icu_capi --crate-type staticlib --features icu_capi/default_components,icu_capi/buffer_provider,icu_capi/looping_panic_handler,icu_capi/libc_alloc --no-default-features --target thumbv7em-none-eabi from ICU4X's tree with #5119 applied, it should mostly work?

[Manishearth]

@sffc So our tutorial's way of building icu_capi is weird, we use cargo rustc -p icu_capi with a fake crate

With the tutorial steps, and the following command, I still get a problem:

$ cargo rustc --release -p icu_capi --crate-type staticlib --features icu_capi/default_components,icu_capi/buffer_provider,icu_capi/looping_panic_handler,icu_capi/libc_alloc  --target thumbv7em-none-eabi 2>&1 | less
warning: unused manifest key: resolver
   Compiling serde v1.0.203
error[E0463]: can't find crate for `std`
  |
  = note: the `thumbv7em-none-eabi` target may not support the standard library
  = note: `std` is required by `serde` because it does not declare `#![no_std]`

However, cargo tree -e features -p icu_capi does not show up serde anywhere except under diplomat, which is a proc macro.

[Manishearth]

Got it, we need resolver = 2. Preparing a docs fix now

[Manishearth]

Fixing the docs: #5122

I'm also going to fix diplomat: rust-diplomat/diplomat#510

There should be patch releases for these things available soon.

[Manishearth]

The docs are being updated in #5122

diplomat_core was released at 0.8.1 with this patch: rust-diplomat/diplomat#510 . This will make it work with the older docs, but it should already work with the new docs, barring the icu_capi bug below.

I also discovered a bug in icu_capi that prevents this build from working, this has been fixed in #5123 and patch-released as icu_capi 1.5.1.

Thanks for reporting this to us!

[ghost]

@Manishearth Thank you so much. The compilation was successful!