diagnostics: "try using a conversion method: `*i.to_string().to_string()"

[matthiaskrgr]

Example I came up with (tried to reduce from a larger set of code I found this in)

fn main() {
	let mut v = vec!["hello", "this", "is", "a", "test"];

	let v2 = Vec::new();
	
	v.into_iter().map(|s|s.to_owned()).collect::<Vec<_>>();
	
	let mut a = String::new();
	for i in v {
		a = *i.to_string();
		v2.push(a);
	}
}

This gives a rather confusing error message:

error[E0308]: mismatched types
  --> src/main.rs:10:7
   |
10 |         a = *i.to_string();
   |             ^^^^^^^^^^^^^^
   |             |
   |             expected struct `std::string::String`, found str
   |             help: try using a conversion method: `*i.to_string().to_string()`
   |
   = note: expected type `std::string::String`
              found type `str`

meta:

rustc 1.30.0-nightly (d5a448b3f 2018-08-13)
binary: rustc
commit-hash: d5a448b3f47b22c9cb010198bdcc49d4392b090b
commit-date: 2018-08-13
host: x86_64-unknown-linux-gnu
release: 1.30.0-nightly
LLVM version: 7.0

[matthew-russo]

Not sure if this is entirely correct but I'm pretty sure it has to do with precedence of the dereference.

This code has the same issue:

fn main() {
	let hello = String::from("hello");
	let mut vec: Vec<String> = Vec::new();
	vec.push(*hello.to_string());
}

while if you write:

fn main() {
	let hello = String::from("hello");
	let mut vec: Vec<String> = Vec::new();
	vec.push((*hello).to_string());
}

it will compile fine.

@matthiaskrgr Also just to note there were some other issues with the code you posted -- v2 was not mutable so couldn't be pushed to and the v.into_iter()... moved v so the for loop was then invalid due to moved value

[matthew-russo]

And also just wanted to point out that this is a redundant operation to begin with because you're trying to convert a String to a String.

fn main() {
	let v = vec!["hello", "this", "is", "a", "test"];

	let mut v2: Vec<String> = Vec::new();
	
	let owned = v.into_iter().map(|s|s.to_owned()).collect::<Vec<_>>();
	
	for i in owned {
		v2.push(i);
	}
}

This is an equivalent example with strong typing on v2 to see that its still Vec<String>.

or if you still wanted to iterate over v (ignoring the move) you could do:

fn main() {
	let v = vec!["hello", "this", "is", "a", "test"];

	let mut v2: Vec<String> = Vec::new();
	
	for i in v {
		v2.push(i.to_string());
	}
}

[matthiaskrgr]

Yeah, I know the code is quite wonky, :) I was in the middle of a refactoring when I stumbled over the .to_string().to_string() oddity.

[ishowta]

This problem looks essentially the same as #64919 and that has been resolved.
So I think #53406 should be reverted.
It's confusing if there are no hints.