"Test" and "Clippy" dead code warnings for code with main and tests

In this playground

fn main() {
    println!("hello world");
}

or similar, using "Test" or "Clippy" gives dead code warnings:

   Compiling playground v0.0.1 (/playground)
warning: function is never used: `main`
 --> src/lib.rs:1:4
  |
1 | fn main() {
  |    ^^^^
  |
  = note: `#[warn(dead_code)]` on by default

    Finished test [unoptimized + debuginfo] target(s) in 0.73s

Looks like the code is put in lib.rs instead of main.rs. Pull Request #461 appears to have added support for passing the crate type to the backend, but I can't find anything in the interface to set the crate type — is it set heuristically?

Reproduce the warning with the linked Rust Playground example using Test or Clippy, then inspect how the playground places code in src/lib.rs versus main.rs. Read the crate-type handling added in PR #461 and find the interface used by the backend. Done means these modes no longer report a spurious dead_code warning for a valid main function.