rustfmt: error[internal]: left behind trailing whitespace on long lines in raw string

System

$ rustfmt --version
rustfmt 1.3.0-stable (d3345024 2019-06-09)

eric@Friendly_Name /cygdrive/c/Users/eric/Downloads/rustfmt_bug_lucidbrot/ex
$ cargo --version
cargo 1.37.0 (9edd08916 2019-08-02)

eric@Friendly_Name /cygdrive/c/Users/eric/Downloads/rustfmt_bug_lucidbrot/ex
$ uname -a
CYGWIN_NT-10.0 Friendly_Name 3.0.7(0.338/5/3) 2019-04-30 18:08 x86_64 Cygwin

Reproduction

rustfmt fails on this code sample.

const CONFIG_STRING: &str = 
    r###"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
"###;
fn main() {
    println!("Hello, world: {}", CONFIG_STRING);
}

The error it prints is this:

eric@Friendly_Name /cygdrive/c/Users/eric/Downloads/rustfmt_bug_lucidbrot/ex
$ cargo fmt
error[internal]: left behind trailing whitespace
 --> \\?\C:\Users\eric\Downloads\rustfmt_bug_lucidbrot\ex\src\main.rs:1:1:27
  |
1 | const CONFIG_STRING: &str =
  |                            ^
  |

warning: rustfmt has failed to format. See previous 1 errors.

This code sample, which has one a less, is successfully formatted:

const CONFIG_STRING: &str = 
    r###"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
"###;
fn main() {
    println!("Hello, world: {}", CONFIG_STRING);
}

The successfully formatted version looks like this:

const CONFIG_STRING: &str =
    r###"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
"###;
fn main() {
    println!("Hello, world: {}", CONFIG_STRING);
}

Another version of this snippet that can successfully be formatted is the following. It has the same number of as as the problematic example, but no trailing space after the equal sign on line 1:

const CONFIG_STRING: &str =
    r###"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
"###;
fn main() {
    println!("Hello, world: {}", CONFIG_STRING);
}

Possibly Related

I don’t know enough to say for sure. That’s why I created a new issue. But these may or may not be related:
https://github.com/rust-lang/rustfmt/issues/2896 (open) https://github.com/rust-lang/rustfmt/issues/3295 (nightly) (closed) https://github.com/rust-lang/rustfmt/issues/2896 (open) (collection thread)

More Examples

Good (Can be formatted):

const CONFIG_STRING: &str =     r###"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
"###;
fn main() {
    println!("Hello, world: {}", CONFIG_STRING);
}

Bad (Can not successfully be formatted):

const CONFIG_STRING: &str = 
r#"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"#;
fn main() {
    println!("Hello, world: {}", CONFIG_STRING);
}

Good:

const CONFIG_STRING: &str = 
"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
fn main() {
    println!("Hello, world: {}", CONFIG_STRING);
}

Bad: Has no whitespace after the equal sign, but a long raw string

const CONFIG_STRING: &str = 
r#"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"#;
fn main() {
    println!("Hello, world: {}", CONFIG_STRING);
}

Good: Just like the sample right above this one, except it has one a less:

const CONFIG_STRING: &str = 
r#"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"#;
fn main() {
    println!("Hello, world: {}", CONFIG_STRING);
}

Bad: no longer a raw string

const CONFIG_STRING: &str = 
    "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
fn main() {
    println!("Hello, world: {}", CONFIG_STRING);
}

Splitting into multiple lines makes the bug disappear. Good:

const CONFIG_STRING: &str = "aaaaaaaaaaaaaaaaa
    aaaaaaaaaaaaaaaaaaaaaaaa
    aaaaaaaaaaaaaaaaaaaa
    aaaaaaaaaaaaaaaaaaaaaaaaaa
    aaaaaaaaaaaaaaaaaaa";
fn main() {
    println!("Hello, world: {}", CONFIG_STRING);
}

Conclusion

It seems like this bug has to do with all of these in some way:

• the space after the equal sign.
• the difference between 90 as vs 89 as (I think the counts are correct?) - or rather, the length of the line as a whole.

When there is no whitespace after the equal sign, the number of as is irrelevant and formatting works.
When there are few enough as, the formatting works even with the whitespace.