Print message to help user manage false positives

[tgnottingham]

When unused dependencies are found, print a message that tells the user how to manage false positives.

Example output:

Analyzing dependencies of crates in this directory...
cargo-machete found the following unused dependencies in /home/tgnottingham/example:
example -- /home/tgnottingham/example/Cargo.toml:
        fxhash

If you believe cargo-machete has detected an unused dependency incorrectly, you may add it
to the list of dependencies to ignore in the `[package.metadata.cargo-machete]` section of
the appropriate Cargo.toml file. For example:

[package.metadata.cargo-machete]
ignored = ["prost"]

Done!

[bnjbvr]

Thanks, that's a nice touch. Do you think it'd make sense to publicize the --with-metadata option too, as it could find other unused dependencies based on their public names?

[tgnottingham]

Sure, that makes sense to me.

When user doesn't pass --with-metadata:

Analyzing dependencies of crates in this directory...
cargo-machete found the following unused dependencies in /home/tgnottingham/example:
example -- /home/tgnottingham/example/Cargo.toml:
        fxhash
        xml-rs

If you believe cargo-machete has detected an unused dependency incorrectly,
you can try running it with the `--with-metadata` flag for better accuracy,
though this may modify your Cargo.lock files.

You can also add the dependency to the list of dependencies to ignore in the
`[package.metadata.cargo-machete]` section of the appropriate Cargo.toml file.
For example:

[package.metadata.cargo-machete]
ignored = ["prost"]

Done!

When user does pass --with-metadata (here, xml-rs turned out to be used):

Analyzing dependencies of crates in this directory...
cargo-machete found the following unused dependencies in /home/tgnottingham/example:
example -- /home/tgnottingham/example/Cargo.toml:
        fxhash

If you believe cargo-machete has detected an unused dependency incorrectly,
you can add the dependency to the list of dependencies to ignore in the
`[package.metadata.cargo-machete]` section of the appropriate Cargo.toml file.
For example:

[package.metadata.cargo-machete]
ignored = ["prost"]

Done!

[bnjbvr]

Nice! I think we can avoid a bit of text duplication, and this should be good to go 👍

[bnjbvr]

nit: we could use a raw string here:

println!(r#"my

long

string");

[tgnottingham]

I tried, but the formatting was very unpleasant with a raw string, especially after cargo fmt got its hands on it.

[bnjbvr]

Could we use constant str to avoid the code duplication here? This would also help spot that we're showing almost the same text, and that would highlight what the differences are too.

[tgnottingham]

I gave something like that a shot, but the result reads "inside-out", which is awkward:

    if has_unused_dependencies {
        let middle = if !args.with_metadata {
            "you can try running it with the `--with-metadata` flag for better accuracy,\n\
            though this may modify your Cargo.lock files.\n\
            \n\
            You can also add the dependency to the list of dependencies to ignore in the\n"
        } else {
            "you can add the dependency to the list of dependencies to ignore in the\n"
        };

        println!(
            "\n\
            If you believe cargo-machete has detected an unused dependency incorrectly,\n\
            {middle}\
            `[package.metadata.cargo-machete]` section of the appropriate Cargo.toml.\n\
            For example:\n\
            \n\
            [package.metadata.cargo-machete]\n\
            ignored = [\"prost\"]"
        );
    }

Maybe I'm missing an obvious way to make this less contorted.

In the end, I moved the part which is only applicable sometimes -- the --with-metadata suggestion -- to the end. That simplifies things, though it obviously means the --with-metadata part is mentioned second, which I originally didn't like as much. But it seems fine. Anyway, let me know what you prefer here.

[tgnottingham]

After reducing some duplication, without --with-metadata:

Analyzing dependencies of crates in this directory...
cargo-machete found the following unused dependencies in /home/tgnottingham/example:
example -- /home/tgnottingham/example/Cargo.toml:
        ahash

If you believe cargo-machete has detected an unused dependency incorrectly,
you can add the dependency to the list of dependencies to ignore in the
`[package.metadata.cargo-machete]` section of the appropriate Cargo.toml.
For example:

[package.metadata.cargo-machete]
ignored = ["prost"]

You can also try running it with the `--with-metadata` flag for better accuracy,
though this may modify your Cargo.lock files.

Done!

With --with-metadata:

Analyzing dependencies of crates in this directory...
cargo-machete found the following unused dependencies in /home/tgnottingham/example:
example -- /home/tgnottingham/example/Cargo.toml:
        ahash

If you believe cargo-machete has detected an unused dependency incorrectly,
you can add the dependency to the list of dependencies to ignore in the
`[package.metadata.cargo-machete]` section of the appropriate Cargo.toml.
For example:

[package.metadata.cargo-machete]
ignored = ["prost"]

Done!

[bnjbvr]

Nice, thank you!