Weird newline in string literal

[nrc]

     fn cxx(&self, target: &str) -> &Path {
         match self.cxx.get(target) {
             Some(p) => p.path(),
-            None => panic!("\n\ntarget `{}` is not configured as a host,
-                            only as a target\n\n", target),
+            None => {
+                panic!("\n\ntarget `{}` is not configured as a host,
+                            \
+                        only as a target\n\n",
+                       target)
+            }
         }
     }

[topecongiro]

In 0.8.3 (), rustfmt formarts

impl Foo {
    fn cxx(&self, target: &str) -> &Path {
        match self.cxx.get(target) {
            Some(p) => p.path(),
            None => panic!("\n\ntarget `{}` is not configured as a host,
                            only as a target\n\n", target),
        }
    }
}

to

impl Foo {
    fn cxx(&self, target: &str) -> &Path {
        match self.cxx.get(target) {
            Some(p) => p.path(),
            None => {
                panic!("\n\ntarget `{}` is not configured as a host,
                            only as a target\n\n",
                       target)
            }
        }
    }
}

so the weird newline is gone.

[nrc]

This only reproduces with the format_strings option set.

[topecongiro]

Just setting format_strings does not reproduce the above bug anymore.
However, when the max_width is set to smaller value, the bug gets reproduced.

rustfmt.toml:

format_strings = true
max_width = 50

Output:

impl Foo {
    fn cxx(&self, target: &str) -> &Path {
        match self.cxx.get(target) {
            Some(p) => p.path(),
            None => {
                panic!("\n\ntarget `{}` is not \
                        configured as a host,
                        \
                        only as a target\n\n",
                       target)
            }
        }
    }
}

[scampi]

@nrc @topecongiro I am having a look at this issue and I was wondering if you could clarify the signature of rewrite_string which takes in addition to a format specification an optional max_width: https://github.com/rust-lang-nursery/rustfmt/blob/f711078126f95bd580c69c39af9fc16e3f83c859/src/string.rs#L47-L51

However, it seems that the fmt.width property is enough:

• None is passed as argument: https://github.com/rust-lang-nursery/rustfmt/blob/cc2afeca9eda9d85c088479335c3c664d449a7f4/src/expr.rs#L1394-L1398

• the max_chars value is actually assigned to fmt.width too: https://github.com/rust-lang-nursery/rustfmt/blob/604764abe15c2742af85c1461e6f8055083e3c0b/src/comment.rs#L441-L452

Could it be that argument is a left-over of a previous refactoring ? Could it be removed ?