Unreachable code warning even when no code follows

[tamird]

#85556 introduced an unreachable code warning after uninhabited expressions; unfortunately that warning fires even when no code follows the expression.

This makes it impossible to use patterns such as:

fn foo() -> Result<std::convert::Infallible, u64> {
    Err(5)
}

fn main() { 
    match foo().expect("should") {}
}

where the match is being used in the manner suggested by @nikomatsakis in a blog post.

[Mark-Simulacrum]

The match expression in this case I think is the one being linted against, and it is indeed dead in some sense. We might want to special case empty matches as sort of a "is uninhabited" assertion in code, I suppose...

[darakshan]

Here's a simple case that doesn't involve match. And code does follow

fn main() {
    if false {
        stop().unwrap();
    }
    println!("Hello, world!");
}

fn stop() -> Result<std::convert::Infallible, u64> {
    Err(5)
}

The compiler warns that the println!() statement is unreachable, but the program works and prints as expected. I'm using stable-aarch64-apple-darwin rustc 1.56.0 (09c42c4 2021-10-18). The code that uncovered this contains a call to nix::unistd::execv.

[Mark-Simulacrum]

Yes, that seems like a genuine bug. Thanks!

My guess is we're seeing that the println! is unreachable if the code path goes through the if expression, but of course, that can't actually happen here.

cc @FabianWolff

[a-ba]

I have a case where all branches are reachable (appeared with rust 1.56)

enum Never {}

fn terminate() -> Never
{
    std::process::exit(1);
}

fn main() {
    if std::env::args().count() > 1 {
        println!("terminate");
        terminate();
    }
    println!("Hello, world!");
}

/src/tmp/unreachable$ cargo run 
warning: unreachable expression
  --> src/main.rs:13:5
   |
11 |         terminate();
   |         ----------- any code following this expression is unreachable
12 |     }
13 |     println!("Hello, world!");
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ unreachable expression
   |
   = note: `#[warn(unreachable_code)]` on by default
note: this expression has type `Never`, which is uninhabited
  --> src/main.rs:11:9
   |
11 |         terminate();
   |         ^^^^^^^^^^^
   = note: this warning originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)

warning: `unreachable` (bin "unreachable") generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
     Running `target/debug/unreachable`
Hello, world!

[fmease]

#89779 (comment) and #89779 (comment) no longer trigger unreachable_code. Closing as fixed then (assuming we rule the reproducer from the issue description as wont-fix). Please comment if I should reopen this issue.