Added new lint: reserve_after_initialization

[agroudiev]

Closes #11330.

A new lint that informs the user about a more concise way to create a vector with a known capacity.
Example:

let mut v: Vec<usize> = vec![];
v.reserve(10);

Produces the following help:

  |
2 | /     let mut v: Vec<usize> = vec![];
3 | |     v.reserve(10);
  | |__________________^ help: consider using `Vec::with_capacity(space_hint)`: `let v: Vec<usize> = Vec::with_capacity(10);`
  |

And can be rewritten as:

let v: Vec<usize> = Vec::with_capacity(10);

changelog: new lint [reserve_after_initialization]

[rustbot]

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[y21]

Looks pretty good to me already for a first contribution 👀 but I have one question

[blyxyas]

WOW, really cool for a first lint, just some minor nits, but very very good overall ❤️, I'm really impressed!

[agroudiev]

I added some tests and fixed everything you mentioned except for:

• the use of sym!, since it apparently will not improve performance (I'm glad, I couldn't get it to work properly anyway)
• the macros tests: I have trouble using an external crate while running tests manually, as I explained above

[y21]

Btw I think you can mark things that are done as resolved, that might make it a little easier to read through and see what's left ^^ (saying this because my very first comment is taking up lots of space and that comment is not relevant anymore/already applied, I can't click "resolve" myself it looks like)

[blyxyas]

I just noticed that actually testing with macros isn't really documented anywhere, so I'll do a tutorial here.

In our test suite headers can be used with the "//@" syntax. E.g. "//@run-rustfix" as the first line is a header.
You can use the header //@aux-build:proc_macros.rs:proc-macro to import the auxiliary create just for testing procedural macros and how lints interact with them.

So, after using //@aux-build:proc_macros.rs:proc-macro you can then extern macro proc_macro;, and then use proc_macros::{external, with_span}.

The auxiliary proc macro external! simulates a proc macro that is external to the crate (I think), and the with_span macro simulates a proc macro that changes the span, so you can verify if you're linting the correct expression and such.

We really should document this, #10597
If in the future something isn't in the book or other documentation (and a reviewer isn't online), you can always check the rustc-dev-guide, it's a book mainly for developing rustc and maybe cargo, but it has some things that also apply to Clippy. (if a reviewer is online, you can always ask for help here or on the Zulip server, we all check that chat more or less periodically ❤️ )

[y21]

Just the last few tiny nits I could find ^^. After these it looks good to me 👍

[blyxyas]

Just this mini-nit and it should be good to go! Really impressive for a first contribution ❤️

Edit: @y21 let me know if you also think this is ready, your reviews were very helpful!

[y21]

LGTM!

your reviews were very helpful!

Thanks :D Glad to be of help

[blyxyas]

@bors r=blyxyas,y21

[bors]

📌 Commit f3c5877 has been approved by blyxyas,y21

It is now in the queue for this repository.

[bors]

⌛ Testing commit f3c5877 with merge 4932d05...

[agroudiev]

Thank you so much for your help, @blyxyas and @y21!

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: blyxyas,y21
Pushing 4932d05 to master...