Check for missing features in Cargo.toml

[elichai]

Hi,
Right not as I understand it there's no real communication between rustc to cargo about what cfg features exist in the code, only the opposite(cargo passes --cfg flags to rustc).

But it would be a great feature IMHO to check somehow if there are any cfg features in the code that don't exist in the toml and produce a warning.

Example:
toml:

[package]
name = "Example"
version = "0.1.0"
authors = ["Elichai Turkel <elichai.turkel@gmail.com>"]
edition = "2018"
# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
[dependencies]

main.rs:

#[cfg(feature = "binary")]
fn main() {
    println!("Hello, world!");
}

Real world example: rust-bitcoin/rust-secp256k1#199 A typo in a feature name, in cargo toml it was defined as external-symbols but in rust it was external_symbols

[heisen-li]

I'm sorry, I don't understand this very well. This is also checked for syntax or cfg in cargo? But as far as I know, the rust compiler does a check at compile time. What is the point of doing this?

[ehuss]

I think the point is to check for typos and other mistakes, where the author writes the wrong cfg expressions.

I'm going to close this, as there is an accepted RFC to implemented this, see tracking issue rust-lang/rust#82450.