Correct default cargo new edition

[gauntface]

I think this is a small typo in the docs but feel free to close this PR if I'm mistaken

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[ehuss]

Hm, this has come up several times before (#8952, #7926). The default if the edition is not specified is 2015. cargo new fills in the edition field with the most recent one. Perhaps there is some way to make that a little clearer?

[gauntface]

Ah I see what you mean now. I've changed my commit to be what would have made this more obvious for me but let me know if you have something else in mind.

Rationales for suggestions:

1. Moving the example toml above the caveat sentence suggests that the final sentence is something different from cargo new's behavior
2. Alter the wording to suggest that the default 2015 edition is used in specific scenarios. Originally I read the statement as the default value cargo new uses is 2015, which then was contradicted by the example.

As an FYI, this is me just going through the "Getting Started" sections of the docs and got curious about the edition field, so please feel free to close this PR if you'd prefer to follow up on this in a different forum with other folks. I just wanted to give some suggestions that would have helped me parse the docs as intended (or what I think was intended).

[ehuss]

Thanks! This looks clearer to me!

@bors r+

[bors]

📌 Commit d1c28a5 has been approved by ehuss

[bors]

⌛ Testing commit d1c28a5 with merge cb0e8c3...

[bors]

☀️ Test successful - checks-actions
Approved by: ehuss
Pushing cb0e8c3 to master...

[alexcrichton]

@gauntface out of curiosity, how did you come across this documentation and what led to your desire to fix it? I imagine there was confusion after reading the documentation, but I'm curious how you got on this page and what led you to it in the first place.

[gauntface]

The path that lead me here was:

1. Curious about rust and startef going through the getting started guide on the rust site
2. Cargo new made the project
3. The Cargo.toml file linked to this manifest document and I was curious what the "edition" field was, I read the sentence (briefly)
4. Noticed my manifest had 2018 same as the sample and thought that "default to 2015" was a typo in the sentence above it.

I raised the PR assuming that it was a small thing that may have gone unnoticed.

[alexcrichton]

Ok thanks for the info, that's what we suspected! I wordsmithed this a bit more at #9233 when we got another PR to update the wording, so I'm hoping that helps a bit more here too