significant-drop-tightening suggests syntax error stdin.take(40).; #15574
Description
Using the following flags
--force-warn clippy::significant-drop-tightening
this code:
use std::{io::{stdin, BufRead, Read}, process};
fn main() {
//Let's read from stdin
println!("Hello, what's your name?");
let stdin = stdin().lock();
let mut buffer = String::with_capacity(10);
//Here we lock stdin and block to 10 bytes
// Our string is now then only 10 bytes.
//Even if it overflows like expected, it will reallocate.
let mut stdin = stdin.take(40);
if stdin.read_line(&mut buffer).is_err() {
eprintln!("An error has occured while reading.");
return;
} //Now we print the result, our data is safe
println!("Our string has a capacity of {}",buffer.capacity());
println!("Hello {}!",buffer);
//The string is freed automatically.
}
//Now let's try to overflow!! The string was reallocated.caused the following diagnostics:
Checking _snippet_0 v0.1.0 (/tmp/icemaker_global_tempdir.W9mV5T7suTWy/icemaker_clippyfix_tempdir.Z8u9dYrFB4Kw/_snippet_0)
warning: temporary with significant `Drop` can be early dropped
--> src/main.rs:5:9
|
2 | fn main() {
| ___________-
3 | | //Let's read from stdin
4 | | println!("Hello, what's your name?");
5 | | let stdin = stdin().lock();
| | ^^^^^
... |
18 | | }
| |_- temporary `stdin` is currently being dropped at the end of its contained scope
|
= note: this might lead to unnecessary resource contention
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#significant_drop_tightening
= note: requested on the command line with `--force-warn clippy::significant-drop-tightening`
help: merge the temporary construction with its single usage
|
5 ~
6 + let stdin = stdin().lock().take(40);
7 | let mut buffer = String::with_capacity(10);
...
10 | //Even if it overflows like expected, it will reallocate.
11 ~
|
warning: temporary with significant `Drop` can be early dropped
--> src/main.rs:10:13
|
2 | fn main() {
| ___________-
3 | | //Let's read from stdin
4 | | println!("Hello, what's your name?");
5 | | let stdin = stdin().lock();
... |
10 | | let mut stdin = stdin.take(40);
| | ^^^^^
... |
18 | | }
| |_- temporary `stdin` is currently being dropped at the end of its contained scope
|
= note: this might lead to unnecessary resource contention
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#significant_drop_tightening
help: merge the temporary construction with its single usage
|
10 ~
11 + stdin.take(40).;
12 ~ //Now we print the result, our data is safe
|
warning: `_snippet_0` (bin "_snippet_0") generated 2 warnings
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.23s
However after applying these diagnostics, the resulting code:
use std::{io::{stdin, BufRead, Read}, process};
fn main() {
//Let's read from stdin
println!("Hello, what's your name?");
let stdin = stdin().lock();
let mut buffer = String::with_capacity(10);
//Here we lock stdin and block to 10 bytes
// Our string is now then only 10 bytes.
//Even if it overflows like expected, it will reallocate.
stdin.take(40).;
//Now we print the result, our data is safe
println!("Our string has a capacity of {}",buffer.capacity());
println!("Hello {}!",buffer);
//The string is freed automatically.
}
//Now let's try to overflow!! The string was reallocated.no longer compiled:
Checking _snippet_0 v0.1.0 (/tmp/icemaker_global_tempdir.W9mV5T7suTWy/icemaker_clippyfix_tempdir.Z8u9dYrFB4Kw/_snippet_0)
error: unexpected token: `;`
--> src/main.rs:11:20
|
11 | stdin.take(40).;
| ^
error: could not compile `_snippet_0` (bin "_snippet_0" test) due to 1 previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `_snippet_0` (bin "_snippet_0") due to 1 previous error
Version:
rustc 1.91.0-nightly (91ee6a405 2025-08-26)
binary: rustc
commit-hash: 91ee6a4057ce4bf1ab6d2f932cae497488d67c81
commit-date: 2025-08-26
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 21.1.0