from_iter_instead_of_collect / needless_collect interaction questionable · rust-lang/rust-clippy#6533

2021-01-01T21:11:57.000Z

````rust for _i in Vec::from_iter([1, 2, 3].iter()) {} ```` clippy will suggest to use `.collect()` instead of `from_iter()` here but blindly replacing will not work because `.collect()` will need some kind of type hint. Them again ````rust for _i in [1, 2, 3].iter().collect:: >() {} ```` does not trigger `clippy::needless_collect` (although it should!; neither does the first example) because we can iterate over ````rust for _i in [1, 2, 3].iter() {} ```` directly.

(2 comments) (0 reactions) (0 assignees)Rust (10,406 stars) (1,391 forks)batch import

C-enhancementI-false-negativeL-nurserygood first issue

Description

for _i in Vec::from_iter([1, 2, 3].iter()) {}

clippy will suggest to use .collect() instead of from_iter() here but blindly replacing will not work because .collect() will need some kind of type hint.

Them again

for _i in [1, 2, 3].iter().collect::<Vec<&i32>>() {}

does not trigger clippy::needless_collect (although it should!; neither does the first example) because we can iterate over

for _i in [1, 2, 3].iter() {}

directly.

Contributor guide

Tech stack: rust
Domain: tooling
Issue type: bug
Difficulty: 3
Estimated time: 1-3 hours
Activity status: fresh
Clarity: mostly clear
Prerequisites: Basic understanding of Rust lintsFamiliarity with Clippy's lint structure
Newbie friendliness: 40
Research direction: Investigate the `from iter instead of collect` and `needless collect` lints in Clippy's source code, likely in files like `clippy lints/src/from iter instead of collect.rs` and `clippy lints/src/needless collect.rs`. Determine how they interact and why the suggestion to use `collect()` fails without type hints. Consider adjusting the lint to either suggest a proper type hint or suppress the suggestion when it would cause an error. Also check if `needless collect` should trigger for the `from iter` case.