Point to let when modifying field of immutable variable

[estebank]

Point at the immutable local variable when trying to modify one of its
fields.

Given a file:

structFoo{pubv:Vec<String>}fnmain(){let f = Foo{v:Vec::new()};
f.v.push("cat".to_string());}

present the following output:

error: cannot borrow immutable field `f.v` as mutable
--> file.rs:7:13
|
6 | let f = Foo { v: Vec::new() };
| - this should be `mut`
7 | f.v.push("cat".to_string());
| ^^^
error: aborting due to previous error

Fix#27593.

[rust-highfive]

r? @eddyb

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[eddyb]

r? @jonathandturner

[Emilgardis]

I think this should read

error: cannot borrow immutable field `f.v` as mutable
--> file.rs:7:13
|
6 | let f = Foo { v: Vec::new() };
| ^ - consider adding `mut` here
7 | f.v.push("cat".to_string());
| ^^^

or

consider changing this to `mut f`

[nagisa]

@Emilgardis I disagree, because with complex patterns there’s multiple ways to make some binding mutable.

IME it should read simply, without suggesting any specific syntax.

6 | let f = Foo { v: Vec::new() };
| - consider making this binding mutable

[nagisa]

Please also add a test for argument binding.

[sophiajt]

@nagisa

6 | let f = Foo { v: Vec::new() };
| - consider making this binding mutable

This is a useful suggestion to more experienced Rust programmers, but one of the nice things about making a suggestion is that the errors can help teach you the language. By teaching them about the mut keyword, they have something they can do to fix the error. Your suggestion has the unfortunate side effect of saying "go read the manual"

[sophiajt]

consider changing this to `mut f`

👍

[estebank]

Please also add a test for argument binding.

Wealreadydothat since #39139, and it isn't affected by this change because here it's checking wether it is an argument to only add this message if it isn't.

This is a useful suggestion to more experienced Rust programmers, but one of the nice things about making a suggestion is that the errors can help teach you the language. By teaching them about the mut keyword, they have something they can do to fix the error. Your suggestion has the unfortunate side effect of saying "go read the manual"

I have to agree with these points. I'll change it, as proposed, to

error: cannot borrow immutable field `f.v` asmutable
--> file.rs:7:13
|
6 | let f = Foo{v:Vec::new()};
| - consider changing this to `mut f`
7 | f.v.push("cat".to_string());
| ^^^

[Emilgardis]

I agree with these changes.

Why did travis fail on only one suite?

[sophiajt]

error: cannot borrow immutable field `f.v` as mutable
--> file.rs:7:13
|
6 | let f = Foo { v: Vec::new() };
| - consider changing this to `mut f`
7 | f.v.push("cat".to_string());
| ^^^

The ^^^ needs a label.

[estebank]

@jonathandturner does the following wording make sense?

error: cannot borrow immutable field `z.x` asmutable
--> $DIR/issue-39544.rs:21:18
|
20 | let z = Z{x:X::Y};
| - consider changing this to `mut z`
21 | let _ = &mut z.x;
| ^^^ cannot borrow mutably field of immutable binding

[sophiajt]

Seems close. How about...

error: cannot borrow immutable field `z.x` as mutable
--> $DIR/issue-39544.rs:21:18
|
20 | let z = Z { x: X::Y };
| - consider changing this to `mut z`
21 | let _ = &mut z.x;
| ^^^ cannot mutably borrow immutable field

[estebank]

@jonathandturner LGTM.

Done.

[estebank]

@jonathandturner ping.

[sophiajt]

@bors r+

[bors]

📌 Commit 9ac628d has been approved by jonathandturner