Code:

// src/lib.rs
#[inline] #[no_mangle] pub extern "C" fn foo() { }

# Cargo.toml
[package]
name = "foo"
version = "0.0.1"
edition = "2018"

[lib]
name = "foo"
crate-type = ["cdylib"]

cargo build --release

If the function is marked as #[inline], it doesn't get compiled into the dll file:

strings target/release/libfoo.so | grep foo

The reason a function may be marked as both #[inline] and #[no_mangle] is so that it gets inlined when compiling the crate as a rust library, but not inlined when compiling the crate as a cdylib.

There should be a warning to use something like #[cfg_attr(not(crate_type="cdylib"), inline] instead.

This issue has been assigned to @DoctorN via this comment.