todo! and unimplemented! should generate rustc warnings when returning used values

I tried this code:

pub fn some_num() -> i32 {
    todo!();
}

I expected to see this happen:

A warning noting that a panic would occur (either showing the line of the todo! or the line of the function call making use of the todo!, for instance). I suppose even fancier ergonomics could be added, so that if the return type is (), a () could could be supplied - but this is still not good IMO, since that implies an effect should happen, but isn't happening. So even in that case, I think a warning should be given.

Instead, this happened:

Currently (nightly) rustc will not warn if some_num is used, e.g., from main: println!("{}", some_num()); - this will compile without warning, but will panic:

$ ./target/debug/sfwtools
thread 'main' panicked at 'not yet implemented', src/sfwtools.rs:63:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Meta

rustc --version --verbose:

rustc 1.50.0-nightly (0edce6f4b 2020-12-24)
binary: rustc
commit-hash: 0edce6f4bbb4514482537f569f0b8ef48e71e0a0
commit-date: 2020-12-24
host: x86_64-unknown-linux-gnu
release: 1.50.0-nightly

Start by reproducing the examples with the nightly rustc version described in the issue, including a used return value and the unit-return case. Trace the compiler behavior for todo!() and unimplemented!(); done means the intended cases emit a rustc warning while preserving the existing panic behavior.