I always expect that cargo test basically runs a cargo build before doing anything test specific. But that's apparently not the case.

Minimal example:

Cargo.toml

[package]
name = "foo"
version = "0.1.0"
authors = ["me"]
edition = "2018"

src/lib.rs

pub fn foo() {}

tests/main.rs

use foo::foo;

#[test]
fn main() {
    foo();
}

Executing cargo test will compile the foo crate and execute tests. Fine. BUT: cargo clean && cargo test won't create the file target/debug/libfoo.rlib while cargo clean && cargo build does create that file. This is very unexpected.

Tests on rustc 1.31.0-nightly (5597ee8a6 2018-10-03)