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Suggest changing argument mutability #159490
Description
Code
fn main() {
let input = String::new();
std::io::stdin().read_line(&input).unwrap();
}Current output
error[E0308]: mismatched types
--> src/main.rs:5:27
|
5 | io::stdin().read_line(&input).unwrap();
| --------- ^^^^^^ types differ in mutability
| |
| arguments to this method are incorrect
|
= note: expected mutable reference `&mut String`
found reference `&String`
note: method defined here
--> /playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/io/stdio.rs:412:12
|
412 | pub fn read_line(&self, buf: &mut String) -> io::Result<usize> {
| ^^^^^^^^^Desired output
error[E0308]: mismatched types
--> src/main.rs:5:27
|
5 | io::stdin().read_line(&input).unwrap();
| --------- ^^^^^^ types differ in mutability
| |
| arguments to this method are incorrect
|
= note: expected mutable reference `&mut String`
found reference `&String`
note: method defined here
--> /playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/io/stdio.rs:412:12
|
412 | pub fn read_line(&self, buf: &mut String) -> io::Result<usize> {
| ^^^^^^^^^
help: change the borrow to be mutable
|
5 | io::stdin().read_line(&mut input).unwrap();
| +++Rationale and extra context
No response
Other cases
Rust Version
1.97Anything else?
No response
Activity
EvoPot commentedon Aug 5, 2026
Quick question unrelated to the issue, how much do we need to specify where the standard library is in the user's system? I am talking about this diagnostic right here:
Clearly the user shouldn't need to go to the location of the toolchain in their home system. Imagine this for a scenario where people already do know that they should go to somewhere where they have control over the source code like they are going to trace where a file that they are going to change is in
stdin GitHub or something. I personally think thatstdshould have some more levarage as a central part of the language, which it is for many people. I personally think rather than extra verbosity it should be something more like:Of course this raises a different number of questions (What even is
std, what if you are making a fork of the language and change std's name, what if a contributor is debugging with multiple compiler builds (like they have multiple +stage's added to their toolchain) ) but personally I think it is better than giving the end user a full system path for what they shouldn't change. Is there an issue for this? I cannot seem to find one because I lack the expertise in how to find similar issues, but maybe this is a good place to start.Rollup merge of rust-lang#162106 - TirushOne:addr-mutability-suggesti…
Rollup merge of rust-lang#162106 - TirushOne:addr-mutability-suggesti…
Rollup merge of rust-lang#162106 - TirushOne:addr-mutability-suggesti…
Unrolled build for #162106 in rollup 164052