Empty no_std project for esp32 fails to compile

[jcard0na]

I just tried to build a no_std project and it failed to compile. std project compiles and runs successfully.

Project created as:

[javier@t420 dev]$ cargo generate --git https://github.com/esp-rs/esp-idf-template cargo
⚠️   Unable to load config file: /home/javier/.cargo/cargo-generate.toml
🤷   Project Name : meh
🔧   Generating template ...
✔ 🤷   Rust toolchain (beware: nightly works only for esp32c3!) · esp
✔ 🤷   STD support · false
✔ 🤷   MCU · esp32
? 🤷   ESP-IDF native build version (v4.3.2 = previous stable, v4.4 = stable, v5.0 = development; NOTE: applicable only with `cargo build --features✔ 🤷   ESP-IDF native build version (v4.3.2 = previous stable, v4.4 = stable, v5.0 = development; NOTE: applicable only with `cargo build --features native`) · v4.4
[1/9]   Done: .cargo/config.toml
[2/9]   Done: .cargo
[3/9]   Done: .gitignore
[4/9]   Done: Cargo.toml
[5/9]   Done: build.rs
[6/9]   Done: rust-toolchain.toml
[7/9]   Done: sdkconfig.defaults
[8/9]   Done: src/main.rs
[9/9]   Done: src
🔧   Moving generated files into: `/opt/javier/dev/meh`...
💡   Initializing a fresh Git repository
✨   Done! New project created /opt/javier/dev/meh

Then built as...

[javier@t420 meh]$ cargo build
    Updating crates.io index
   Compiling compiler_builtins v0.1.53
   Compiling proc-macro2 v1.0.36
(...)
error[E0433]: failed to resolve: use of undeclared crate or module `std`
 --> /home/javier/.cargo/registry/src/github.com-1ecc6299db9ec823/esp-idf-svc-0.37.0/src/eventloop.rs:7:5
  |
7 | use std::ptr::NonNull;
  |     ^^^ use of undeclared crate or module `std`

error[E0433]: failed to resolve: use of undeclared crate or module `std`
 --> /home/javier/.cargo/registry/src/github.com-1ecc6299db9ec823/esp-idf-svc-0.37.0/src/mqtt/client.rs:5:5
  |
5 | use std::mem::ManuallyDrop;
  |     ^^^ use of undeclared crate or module `std`

error[E0433]: failed to resolve: use of undeclared type `NonNull`
   --> /home/javier/.cargo/registry/src/github.com-1ecc6299db9ec823/esp-idf-svc-0.37.0/src/eventloop.rs:168:13
    |
168 |             NonNull::dangling().as_ptr() as *const P
    |             ^^^^^^^ not found in this scope
    |
help: consider importing one of these items
    |
1   | use core::ptr::NonNull;
    |
1   | use crate::eventloop::ptr::NonNull;
    |

error[E0412]: cannot find type `Box` in this scope
   --> /home/javier/.cargo/registry/src/github.com-1ecc6299db9ec823/esp-idf-svc-0.37.0/src/mqtt/client.rs:166:28
    |
166 | struct UnsafeCallback(*mut Box<dyn FnMut(esp_mqtt_event_handle_t)>);
    |                            ^^^ not found in this scope
    |
help: consider importing this struct
    |
1   | use alloc::boxed::Box;
    |

error[E0412]: cannot find type `Box` in this scope
   --> /home/javier/.cargo/registry/src/github.com-1ecc6299db9ec823/esp-idf-svc-0.37.0/src/mqtt/client.rs:169:25
    |
169 |     fn from(boxed: &mut Box<Box<dyn FnMut(esp_mqtt_event_handle_t)>>) -> Self {
    |                         ^^^ not found in this scope
    |
help: consider importing this struct
    |
1   | use alloc::boxed::Box;
    |

error[E0412]: cannot find type `Box` in this scope
   --> /home/javier/.cargo/registry/src/github.com-1ecc6299db9ec823/esp-idf-svc-0.37.0/src/mqtt/client.rs:169:29
    |
169 |     fn from(boxed: &mut Box<Box<dyn FnMut(esp_mqtt_event_handle_t)>>) -> Self {
    |                             ^^^ not found in this scope
    |
help: consider importing this struct
    |
1   | use alloc::boxed::Box;
    |

error[E0412]: cannot find type `Box` in this scope
   --> /home/javier/.cargo/registry/src/github.com-1ecc6299db9ec823/esp-idf-svc-0.37.0/src/mqtt/client.rs:190:5
    |
190 |     Box<dyn FnMut(esp_mqtt_event_handle_t)>,
    |     ^^^ not found in this scope
    |
help: consider importing this struct
    |
1   | use alloc::boxed::Box;
    |

error[E0433]: failed to resolve: use of undeclared type `Box`
   --> /home/javier/.cargo/registry/src/github.com-1ecc6299db9ec823/esp-idf-svc-0.37.0/src/mqtt/client.rs:213:13
    |
213 |             Box::new(move |event_handle| EspMqttConnection::post(&client_connection, event_handle)),
    |             ^^^ not found in this scope
    |
help: consider importing this struct
    |
1   | use alloc::boxed::Box;
    |

error[E0433]: failed to resolve: use of undeclared type `Box`
   --> /home/javier/.cargo/registry/src/github.com-1ecc6299db9ec823/esp-idf-svc-0.37.0/src/mqtt/client.rs:231:13
    |
231 |             Box::new(move |event_handle| {
    |             ^^^ not found in this scope
    |
help: consider importing this struct
    |
1   | use alloc::boxed::Box;
    |

error[E0433]: failed to resolve: use of undeclared type `ManuallyDrop`
   --> /home/javier/.cargo/registry/src/github.com-1ecc6299db9ec823/esp-idf-svc-0.37.0/src/mqtt/client.rs:235:37
    |
235 |                     let mut event = ManuallyDrop::new(Some(EspMqttMessage::new_event(event, None)));
    |                                     ^^^^^^^^^^^^ not found in this scope
    |
help: consider importing this struct
    |
1   | use core::mem::ManuallyDrop;
    |

error[E0433]: failed to resolve: use of undeclared type `ManuallyDrop`
   --> /home/javier/.cargo/registry/src/github.com-1ecc6299db9ec823/esp-idf-svc-0.37.0/src/mqtt/client.rs:240:25
    |
240 |                         ManuallyDrop::drop(&mut event);
    |                         ^^^^^^^^^^^^ not found in this scope
    |
help: consider importing this struct
    |
1   | use core::mem::ManuallyDrop;
    |

error[E0412]: cannot find type `Box` in this scope
   --> /home/javier/.cargo/registry/src/github.com-1ecc6299db9ec823/esp-idf-svc-0.37.0/src/mqtt/client.rs:252:23
    |
252 |         raw_callback: Box<dyn FnMut(esp_mqtt_event_handle_t)>,
    |                       ^^^ not found in this scope
    |
help: consider importing this struct
    |
1   | use alloc::boxed::Box;
    |

error[E0433]: failed to resolve: use of undeclared type `Box`
   --> /home/javier/.cargo/registry/src/github.com-1ecc6299db9ec823/esp-idf-svc-0.37.0/src/mqtt/client.rs:257:38
    |
257 |         let mut boxed_raw_callback = Box::new(raw_callback);
    |                                      ^^^ not found in this scope
    |
help: consider importing this struct
    |
1   | use alloc::boxed::Box;
    |

Some errors have detailed explanations: E0412, E0433.
For more information about an error, try `rustc --explain E0412`.
error: could not compile `esp-idf-svc` due to 13 previous errors
warning: build failed, waiting for other jobs to finish...
error: build failed

Same toolchain, std project builds fine...

[javier@t420 rust-esp32-std-demo]$ cargo build
   Compiling rust-esp32-std-demo v0.22.8 (/opt/javier/dev/rust-esp32-std-demo)
    Finished dev [optimized + debuginfo] target(s) in 13.40s

[ivmarkov]

Should be fixed now.

[ivmarkov]

By the way - and I'm not sure what you are trying to achieve - but - using no_std on top of ESP-IDF is pretty much pointless. It only exists "because we can", and "just in case".

Rust support for ESP-IDF is a STD compatible environment anyway, so you are not winning anything by living in no_std land, and losing a lot. As in how do you plan to e.g. use networking or spin threads?

With that said, no_std has value for library crates that depend only on embedded-hal and embedded-svc, as this makes these portable across various MCUs. But you should not be using the esp-idf-template for library MCU-agnostic crates anyway.

[jcard0na]

Thanks a lot for the clarification @ivmarkov.
Mi project does not use networking, nor spin threads. I only use an epd display on waveshare's esp32 driver board. I was looking to use no_std because I'm migrating a no_std project from a different target and did not want to inadvertently introduce std dependencies that would prevent me to migrate back. For my use case, the ideal environment would be to depend exclusively on embedded-hal and embedded-svc and not use ESP-IDF at all, but I do not think that is possible with this target, is that right? Cheers,

[ivmarkov]

With or without this target, there is no chance to not depend on something which is MCU specific. I mean, e.g. instantiating the SPI and pins is always MCU specific and not covered by e.g. embedded-hal which is just a bunch of traits. Regardless of the MCU.

If you want to split your code to MCU-independent part and MCU-dependent part, you should use two crates: a library crate which only depends on embedded-hal, and then a binary crate that depends on the MCU specific hal stuff and also on the library crate.

As for embedded-svc - given that you just want to draw on an epd display, you don't need that at all. And, it currently has an implementation for the ESP32 anyway (in esp-idf-svc). With a PC (and RPI) implementation likely to follow sometime this year.