This crate requires a nightly (unstable, experimental) compiler version. To switch to it, you must have Rust installed via Rustup, and run:

rustup default nightly

Run rustup default stable to switch back.

Installing `cargo-whynot` `whynot` executables

Assuming you have Rust/Cargo installed, run this command in a terminal:

cargo install cargo-whynot

It will make cargo-whynot whynot commands available in your PATH if you've allowed the PATH to be modified when installing Rust. cargo uninstall cargo-whynot uninstalls.

Back to the crate overview.

Readme

Why Not?

Cargo subcommand to discover why a function is unsafe.

Requires a recent enough nightly rust toolchain.

# Make sure you have the necessary components installed

$ rustup component add rustc-dev llvm-tools-preview --toolchain nightly

# Install `cargo-whynot` with the nightly toolchain

$ cargo  nightly install cargo-whynot

# Invoke the tool!

$ cargo whynot safe foo

What is unsafety?

Nomicon definition

Dereference raw pointers
Call unsafe functions (including C functions, compiler intrinsics, and the raw allocator)
Implement unsafe traits
Mutate statics
Access fields of unions

Why this tool?

Because it's a fun experiment, hooking into rustc to query the drivers. You should not use this tool because unsafe code is generally bad (it's not), but you can use it to figure out if there is an opportunity to make a function "safe".

Examples

With the following code

pub use unsafe_mod::unsafety;

pub unsafe fn foo() {
    let a = unsafety();
    eprintln!("a: {}", a);
}

pub mod unsafe_mod {
    pub unsafe fn unsafety() -> u32 {
        let mut a = 1;
        let a = std::ptr::addr_of_mut!(a);
        // this is the unsafe part
        let b = *a;
        b
    }
}

cargo whynot safe foo

will report

note: Function is unsafe
  ┌─ src/lib.rs:3:1
  │
3 │ pub unsafe fn foo() {
  │ ^^^^^^^^^^^^^^^^^^^ function is unsafe because:
4 │     let a = unsafety();
  │             ---------- call to unsafe function `unsafe_mod::unsafety`

help:
   ┌─ src/lib.rs:9:5
   │
 9 │     pub unsafe fn unsafety() -> u32 {
   │     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ function is unsafe because:
   ·
13 │         let b = *a;
   │                 ^^ dereference of raw pointer
   │
   = this function does a fundamentally unsafe operation

License

^{Licensed under either of Apache License, Version
2.0 or MIT license at your option.}
_{Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
be dual licensed as above, without any additional terms or conditions.}

Installing cargo-whynot whynot executables

Installing `cargo-whynot` `whynot` executables