Auto merge of #10692 - y21:missing-asserts, r=Alexendoo

new lint: `missing_asserts_for_indexing`

Fixes #8296

This lint looks for repeated slice indexing and suggests adding an `assert!` beforehand that helps LLVM elide bounds checks. The lint documentation has an example.

I'm not really sure what category this should be in. It seems like a nice lint for the `perf` category but I suspect this has a pretty high FP rate, so it might have to be a pedantic lint or something.
I'm also not sure about the name. If someone knows a better name for this lint, I'd be fine with changing it.

changelog: new lint [`missing_asserts_for_indexing`]

CHANGELOG.md

@@ -5132,6 +5132,7 @@ Released 2018-09-13
 [`misnamed_getters`]: https://rust-lang.github.io/rust-clippy/master/index.html#misnamed_getters
 [`misrefactored_assign_op`]: https://rust-lang.github.io/rust-clippy/master/index.html#misrefactored_assign_op
 [`missing_assert_message`]: https://rust-lang.github.io/rust-clippy/master/index.html#missing_assert_message
+[`missing_asserts_for_indexing`]: https://rust-lang.github.io/rust-clippy/master/index.html#missing_asserts_for_indexing
 [`missing_const_for_fn`]: https://rust-lang.github.io/rust-clippy/master/index.html#missing_const_for_fn
 [`missing_docs_in_private_items`]: https://rust-lang.github.io/rust-clippy/master/index.html#missing_docs_in_private_items
 [`missing_enforced_import_renames`]: https://rust-lang.github.io/rust-clippy/master/index.html#missing_enforced_import_renames

clippy_lints/src/declared_lints.rs

@@ -457,6 +457,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::misc_early::ZERO_PREFIXED_LITERAL_INFO,
     crate::mismatching_type_param_order::MISMATCHING_TYPE_PARAM_ORDER_INFO,
     crate::missing_assert_message::MISSING_ASSERT_MESSAGE_INFO,
+    crate::missing_asserts_for_indexing::MISSING_ASSERTS_FOR_INDEXING_INFO,
     crate::missing_const_for_fn::MISSING_CONST_FOR_FN_INFO,
     crate::missing_doc::MISSING_DOCS_IN_PRIVATE_ITEMS_INFO,
     crate::missing_enforced_import_rename::MISSING_ENFORCED_IMPORT_RENAMES_INFO,

clippy_lints/src/lib.rs

@@ -210,6 +210,7 @@ mod misc;
 mod misc_early;
 mod mismatching_type_param_order;
 mod missing_assert_message;
+mod missing_asserts_for_indexing;
 mod missing_const_for_fn;
 mod missing_doc;
 mod missing_enforced_import_rename;
@@ -1100,6 +1101,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|_| Box::new(ignored_unit_patterns::IgnoredUnitPatterns));
     store.register_late_pass(|_| Box::<reserve_after_initialization::ReserveAfterInitialization>::default());
     store.register_late_pass(|_| Box::new(implied_bounds_in_impls::ImpliedBoundsInImpls));
+    store.register_late_pass(|_| Box::new(missing_asserts_for_indexing::MissingAssertsForIndexing));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }