clippy::unnecessary_join: called `.collect::<Vec<String>>().join("")` on an iterator

What you see

warning: called `.collect::<Vec<String>>().join("")` on an iterator
 --> main.rs:4:60
  |
4 | let output = vector.iter().map(|item| item.to_uppercase()).collect::<Vec<String>>().join("");
  |                                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `collect::<String>()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#unnecessary_join
  = note: `-W clippy::unnecessary-join` implied by `-W clippy::pedantic`

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

.collect::<String>() is more concise and might be more performant

Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.

How to fix it

Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.

Flagged

let vector = vec!["hello",  "world"];
let output = vector.iter().map(|item| item.to_uppercase()).collect::<Vec<String>>().join("");
println!("{}", output);

Write this instead

let vector = vec!["hello",  "world"];
let output = vector.iter().map(|item| item.to_uppercase()).collect::<String>();
println!("{}", output);

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

[lints.clippy]
unnecessary_join = "warn"

#![warn(clippy::unnecessary_join)]

cargo clippy -- -W clippy::unnecessary-join

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.