New Lint: Iter nth zero

[bradsherman]

Check for the use of iter.nth(0) and encourage iter.next() instead as it is more readable

changelog: add new lint when iter.nth(0) is used

Fixes #4957

[flip1995]

@krishna-veerareddy is correct with his review, that this will lint on every method called nth.

For lints like this, we already have infrastructure. You can just add ["nth", ..] below this statements:

rust-clippy/clippy_lints/src/methods/mod.rs

Lines 1194 to 1195 in c807fbc

	["nth", "iter"] => lint_iter_nth(cx, expr, arg_lists[1], false),
	["nth", "iter_mut"] => lint_iter_nth(cx, expr, arg_lists[1], true),

and implement a function, that checks for the lint. To verify that this function is from the Iterator trait, you can add a check like this:

rust-clippy/clippy_lints/src/methods/mod.rs

Lines 2105 to 2106 in c807fbc

	// lint if caller of skip is an Iterator
	if match_trait_method(cx, expr, &paths::ITERATOR) {

---

So steps to more easily implement the lint and addressing all review comments:

1. add the declare_clippy_lint to `methods/mod.rs
2. add ["nth", ..] to the above mentioned match statement
3. add a new module for this lint in methods and implement your check function, with the above mentioned Iterator check and the check for the second (1) argument for constant 0.

[flip1995]

LGTM overall

[bradsherman]

I made the requested changes, however after rebasing on master sometime yesterday I could not compile clippy_lints:

error[E0107]: wrong number of lifetime arguments: expected 0, found 1
  --> clippy_lints/src/zero_div_zero.rs:29:75
   |
29 |     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr<'_>) {
   |                                                                           ^^ unexpected lifetime argument

This happens in ~900 other places as well.

[bradsherman]

I have replicated the issue even when cloning a new repo from rust-lang:rust-clippy. Here's my rustc version:

rustc 1.42.0-nightly (0de96d37f 2019-12-19)

I'm only pointing this out because I can't build locally in order ensure all the tests pass

[JohnTitor]

@bradsherman It's because there is a difference between nightly and master rustc. Clippy is built for master, not nightly. You can use rustup-toolchain-install-master.

ref. https://github.com/rust-lang/rust-clippy/blob/master/CONTRIBUTING.md#fixing-build-failures-caused-by-rust

[bradsherman]

@JohnTitor thanks, that fixed it. Sorry I missed that in the docs.

[flip1995]

Thanks! LGTM, just a NIT with the documentation

[flip1995]

Thanks! r=me with CI green

[flip1995]

@bors r+ Thanks!

[bors]

📌 Commit ab5ff03 has been approved by flip1995

[bors]

⌛ Testing commit ab5ff03 with merge d9d2013...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: flip1995
Pushing d9d2013 to master...

[bradsherman]

thanks all for helping with my first open source contribution! 😃🎉