Off-by-one in println! diagnostic

[steveklabnik]

This program:

fn main() {
    let name = "foo";
    
    println!("{} {}", name);
}

Gives this error:

error: invalid reference to argument `1` (there is 1 argument)

Both things having 1 here is confusing, it'd be nicer if we didn't compare a zero-indexed and a one-indexed number.

Additionally, a note could make this easier to understand overall too.

[estebank]

Mentoring instructions

Modify verify_arg_type to instead of outputting positional argument errors, to keep track of them (bubbling them up to the callee of verify_piece) so that the following cases are handled:

• println!("{} {} {}", name);: All arguments are positional, estate the difference clearly:
  error: 3 positional arguments in format string, while only 1 was supplied
• println!("{} {value} {}", name, value=2); There're named arguments in the string, keep current output:
  error: invalid reference to positional argument 3 (there are 3 arguments)
• println!("{name} {value} {} {} {} {} {}", 0, name=1, value=2); There are multiple missing arguments, make a list and present only one diagnostic:
  error: invalid reference to positional arguments 4and5 (there are 3 arguments)

The following case are already handled in other parts of the code:

• println!("{}", name, surname);

[BrunoWilkinson]

Hi,

I was wondering if this PR is still ongoing? I've started learning Rust and I was wondering if you think I can tackle this, I'm very curious and driven to learn more about the language and also how programming languages are built.

Thank you for your feedback.

[estebank]

@BrunoWilkinson go ahead! People are available at https://gitter.im/rust-impl-period/WG-compiler-errors and on IRC for quick questions as well as on this issue for longer questions. Once you create a PR we can continue the conversation there. There are instructions to build the compiler, and if you want to use debug statements, you can use RUST_LOG=debug ./path/to/rustc file.rs and debug!() statements in the code will be printed out to stderr.

[BrunoWilkinson]

@estebank thank you, I'm gonna check all this out and give it a shot!

[bagedevimo]

Hey @BrunoWilkinson - are you still looking at this? If not, I'd like to have a go.

[BrunoWilkinson]

Hey @bagedevimo, currently no I couldn't find the time to tackle the issue, In parallel, I'm still learning Rust. But I would love to see the solution to this issue like I did spend some time on it.

[tommyip]

Hey @bagedevimo are you working on this?

[bagedevimo]

Yeah, but not having much luck. If you're super keen and beat me to it, I won't be upset. :)

…

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[tommyip]

Currently

let name = "foo";
println!("{} {value} {}", name, value=2);

produces the output foo 2 2, is this the intended behavior and do we want to change it?

[estebank]

@tommyip I believe that is expected behavior, let's keep it and handle the other cases laid down in the mentoring instructions.