Improve error message in operator expression

[philberty]

Tried this test case: https://github.com/rust-lang/rust/blob/aa3ca1994904f2e056679fce1f185db8c7ed2703/src/test/ui/issue-10401.rs

fn main() {
    let mut a = "a";
    a += { "b" };
    //~^ ERROR: binary assignment operation `+=` cannot be applied
}

We return: https://godbolt.org/z/P9zGT19rs

<source>:3:5: error: cannot apply this operator to types & str and & str
    3 |     a += { "b" };
      |     ^

[anuragcs]

fn main() {
let mut a = String::from("a");
a += "b";
}
concatenation (+=) was added from version 1.6 so if you run your program with version 1.6 it will run just fine.

[CohenArthur]

@anuragcs String and &str are different types, so one does have an implementation for the AddAssign trait which allows the += operation, but not &str. So the error is still present in modern Rust:

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=1c8a3c23539dcee1297e1a12edbdad42

[anuragcs]

fn foo() {
let mut a = String::from("a");
a += "b";
}

fn bar() {
let mut a = String::from("a");
a += "b";
}

[CohenArthur]

Yes, the error should not happen when we are seeing a value of type String, only a value of type &str. So the code you wrote is valid and compiles, but the original code in the issue is invalid and should throw an error - and we want to improve that error message.