Same type, but compiler says it's different types.

[amab8901]

I tried this code:

Cargo.toml

cargo-features = ["codegen-backend"]

[profile.dev]
codegen-backend = "cranelift"

[workspace]
resolver = "2"
members = ["nice"]

nice/Cargo.toml:

[package]
name = "nice"

[dependencies]

nice/src/kiwi/kiwi_file.rs

#![allow(clippy::let_and_return)]

#[derive(Debug, Clone, PartialEq)]
pub struct Hello {
    pub field: String,
}

pub fn hello_func() -> Hello {
    let hello = Hello {
        field: String::default(),
    };
    hello
}

nice/src/kiwi.rs

pub mod kiwi_file;

nice/src/lib.rs

pub mod kiwi;

nice/src/main.rs:

extern crate nice;

use nice::kiwi::kiwi_file::hello_func;

use crate::kiwi::kiwi_file::Hello;

pub mod kiwi;

fn main() {
    let func_hello = hello_func();

    let direct_hello = Hello {
        field: String::default(),
    };

    assert_eq!(func_hello, direct_hello);
}

I expected to see this happen:

cargo check works without throwing any errors

Instead, this happened:

cargo check produces this error:

error[E0308]: mismatched types
  --> nice/src/main.rs:16:28
   |
16 |     assert_eq!(func_hello, direct_hello);
   |                            ^^^^^^^^^^^^ expected `nice::kiwi::kiwi_file::Hello`, found `kiwi::kiwi_file::Hello`
   |
   = note: `kiwi::kiwi_file::Hello` and `nice::kiwi::kiwi_file::Hello` have similar names, but are actually distinct types
note: `kiwi::kiwi_file::Hello` is defined in the current crate
  --> nice/src/kiwi/kiwi_file.rs:4:1
   |
4  | pub struct Hello {
   | ^^^^^^^^^^^^^^^^
note: `nice::kiwi::kiwi_file::Hello` is defined in crate `nice`
  --> /home/amir-abdin/src/nice/nice/src/kiwi/kiwi_file.rs:4:1
   |
4  | pub struct Hello {
   | ^^^^^^^^^^^^^^^^
   = note: the crate `nice` is compiled multiple times, possibly with different configurations

Meta

rustc --version --verbose:

rustc 1.77.0-nightly (d5fd09972 2024-01-22)
binary: rustc
commit-hash: d5fd0997291ca0135401a39dff25c8a9c13b8961
commit-date: 2024-01-22
host: x86_64-unknown-linux-gnu
release: 1.77.0-nightly
LLVM version: 17.0.6

[saethlin]

The error is correct, but a bit unhelpful. There are two crates in this build called nice, one is a lib crate whose root is src/lib.rs and the other is a bin crate whose root is src/main.rs.

You can fix this problem by removing the mod declaration from src/main.rs and using everything through nice:: (which is the lib crate) not crate:: (which is the bin crate).

I'd prefer to see this diagnostic mention that the two builds are as a bin crate and a lib crate. It would be awesome if it could detect that the two crates are declaring the same module, because I don't see any good reason to do that.