Supertraits are wrapped at less than `max_width` (sometimes 20+ chars less)

[taylordotfish]

If I format this code with max_width = 50:

// line below is 30 chars
pub(super) unsafe trait A: B {
    fn foo();
}

// line below is 36 chars
pub unsafe trait Trait2: A + B + C {
    fn foo();
}

// line below is 43 chars
pub trait TraitThree: Super1 + Super2 + X {
    fn foo();
}

// line below is 47 chars
trait TraitNumberFour: Super1 + Super2 + Abcd {
    fn foo();
}

// line below is correctly not wrapped
pub const THIS_LINE_IS_50_CHARS: usize = 12345678;

I get this:

// line below is 30 chars
pub(super) unsafe trait A:
    B
{
    fn foo();
}

// line below is 36 chars
pub unsafe trait Trait2:
    A + B + C
{
    fn foo();
}

// line below is 43 chars
pub trait TraitThree:
    Super1 + Super2 + X
{
    fn foo();
}

// line below is 47 chars
trait TraitNumberFour:
    Super1 + Super2 + Abcd
{
    fn foo();
}

// line below is correctly not wrapped
pub const THIS_LINE_IS_50_CHARS: usize = 12345678;

All of the supertraits were moved to a separate line even though none of the original lines exceeded the max width.

The same thing happens with other values of max_width, including the default of 100.

[matthewhughes934]

The same thing happens with other values of max_width, including the default of 100.

🤔 I can reproduce with max_width=50 but not 100:

$ cat <<'EOF' > f.rs
// line below is 30 chars
pub(super) unsafe trait A: B {
    fn foo();
}

// line below is 36 chars
pub unsafe trait Trait2: A + B + C {
    fn foo();
}

// line below is 43 chars
pub trait TraitThree: Super1 + Super2 + X {
    fn foo();
}

// line below is 47 chars
trait TraitNumberFour: Super1 + Super2 + Abcd {
    fn foo();
}

// line below is correctly not wrapped
pub const THIS_LINE_IS_50_CHARS: usize = 12345678;
EOF
$ cargo run --quiet --bin rustfmt -- --config max_width=50 --check f.rs
Diff in /home/mjh/src/rustfmt/f.rs:1:
 // line below is 30 chars
-pub(super) unsafe trait A: B {
+pub(super) unsafe trait A:
+    B
+{
     fn foo();
 }
 
Diff in /home/mjh/src/rustfmt/f.rs:6:
 // line below is 36 chars
-pub unsafe trait Trait2: A + B + C {
+pub unsafe trait Trait2:
+    A + B + C
+{
     fn foo();
 }
 
Diff in /home/mjh/src/rustfmt/f.rs:11:
 // line below is 43 chars
-pub trait TraitThree: Super1 + Super2 + X {
+pub trait TraitThree:
+    Super1 + Super2 + X
+{
     fn foo();
 }
 
Diff in /home/mjh/src/rustfmt/f.rs:16:
 // line below is 47 chars
-trait TraitNumberFour: Super1 + Super2 + Abcd {
+trait TraitNumberFour:
+    Super1 + Super2 + Abcd
+{
     fn foo();
 }
$ cargo run --quiet --bin rustfmt -- --config max_width=100 --check f.rs
# nothing printed

[taylordotfish]

To reproduce the issue with max_width = 100, the lines in the example need to be lengthened by 50 chars:

// line below is 80 chars
pub(super) unsafe trait ALetsMakeThisLineLongerByExactlyFiftyCharacters1234: B {
    fn foo();
}

// line below is 86 chars
pub unsafe trait Trait2LetsMakeThisLineLongerByExactlyFiftyCharacters1234: A + B + C {
    fn foo();
}

// line below is 93 chars
pub trait TraitThreeLetsMakeThisLineLongerByExactlyFiftyCharacters1234: Super1 + Super2 + X {
    fn foo();
}

// line below is 97 chars
trait TraitNumberFourLetsMakeThisLineLongerByExactlyFiftyCharacters1234: Super1 + Super2 + Abcd {
    fn foo();
}

// line below is correctly not wrapped
pub const THIS_LINE_IS_100_CHARS: &str = "12345678901234567890123456789012345678901234567890123456";

After formatting:

// line below is 80 chars
pub(super) unsafe trait ALetsMakeThisLineLongerByExactlyFiftyCharacters1234:
    B
{
    fn foo();
}

// line below is 86 chars
pub unsafe trait Trait2LetsMakeThisLineLongerByExactlyFiftyCharacters1234:
    A + B + C
{
    fn foo();
}

// line below is 93 chars
pub trait TraitThreeLetsMakeThisLineLongerByExactlyFiftyCharacters1234:
    Super1 + Super2 + X
{
    fn foo();
}

// line below is 97 chars
trait TraitNumberFourLetsMakeThisLineLongerByExactlyFiftyCharacters1234:
    Super1 + Super2 + Abcd
{
    fn foo();
}

// line below is correctly not wrapped
pub const THIS_LINE_IS_100_CHARS: &str = "12345678901234567890123456789012345678901234567890123456";

The issue is that instead of wrapping at max_width, the lines are getting wrapped at max_width - N (i.e., N chars “too early”), where N seemingly depends on the amount/length of visibility and safety modifiers. The first trait in the example gets wrapped 20 characters early, while the last one gets wrapped 3 characters early.