New lint: Suggest ptr.add([usize]) over ptr.offset([usize] as isize).

[frewsxcv]

First part of #3047.

let vec = vec![b'a', b'b', b'c'];
let ptr = vec.as_ptr();
let offset = 1_usize;

unsafe { ptr.offset(offset as isize); }

Could be written:

let vec = vec![b'a', b'b', b'c'];
let ptr = vec.as_ptr();
let offset = 1_usize;

unsafe { ptr.add(offset); }

[frewsxcv]

Is this the right category?

[oli-obk]

It would fit the complexity category. Feels less like a stylistic choice, more like in the scope of "use x.is_empty() instead of x.len() == 0.

[frewsxcv]

feb3e9f

[oli-obk]

That split into many functions <3

Just a few small things. Also: you can probably easily extend your code to also catch wrapping_offset doing the same thing

[oli-obk]

you can use cx.tcx.types.usize to compare directly with the result of expr_ty

[frewsxcv]

a48a6ef

[oli-obk]

You can use the is_unsafe_ptr method instead of destructuring

[frewsxcv]

f7d1ef9

[oli-obk]

if the snippet is not obtainable, instead of reporting an empty snippet, we should not produce any suggestion at all

[frewsxcv]

61c20c1

[frewsxcv]

thanks for the quick review @oli-obk! your comments should be addressed

[frewsxcv]

Just a few small things. Also: you can probably easily extend your code to also catch wrapping_offset doing the same thing

oh missed this, let me add it in

[oli-obk]

Now your code can be simplified to

let receiver = utils::snippet_opt(cx, receiver_expr.span)?;
let cast_lhs = utils::snippet_opt(cx, cast_lhs_expr.span)?;
Some(format!("{}.add({})", receiver, cast_lhs))

[frewsxcv]

oh right, ? with Option is now a thing 💅

[oli-obk]

travis is unhappy (just make your enum Copy).

r=me with travis passing

[mati865]

Travis error:

error: this argument is passed by reference, but would be more efficient if passed by value
   --> clippy_lints/src/ptr_offset_with_cast.rs:136:19
    |
136 |     fn suggestion(&self) -> &'static str {
    |                   ^^^^^ help: consider passing by value instead: `self`
    |
    = note: `-D trivially-copy-pass-by-ref` implied by `-D clippy`
    = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#trivially_copy_pass_by_ref

[frewsxcv]

@bors r=oli-obk

[oli-obk]

I don't think we have bors setup. we totally should though

[frewsxcv]

wasn't sure if i had bors privileges here – looks like someone else will have to approve or merge