Strange formatting of `@`, `|`, and parens patterns

[Centril]

In a rust-lang/rust PR, I saw some pretty strange formatting of pattern matching with @, |, and parens patterns: https://github.com/rust-lang/rust/pull/70906/files#r405165086

The diff was:

-                    category: category @ ConstraintCategory::Return,
+                    category:
+                        category
+                        @
+                        (ConstraintCategory::Return
+                        | ConstraintCategory::CallArgument
+                        | ConstraintCategory::OpaqueType),

which seems odd because it is putting @ on its own line and doing ( on the same line as a pattern and then ) also on the same line as a pattern.

I would have expected:

-                    category: category @ ConstraintCategory::Return,
+                    category: category @ (
+                        ConstraintCategory::Return
+                        | ConstraintCategory::CallArgument
+                        | ConstraintCategory::OpaqueType
+                    ),

[calebcartwright]

Not a duplicate but similar report in #4031. Fixes will likely be similar (if not identical)

[topecongiro]

Did this pattern exist before, or is it something new that has been introduced recently? Either way, we should fix this.

[calebcartwright]

I believe it's a new-ish change, but not sure precisely when it was introduced

[Centril]

category: category @ Record { ... }

has existed since Rust 1.0 I think, so that isn't new, but p0 | ... | pn nested within another pattern is new and currently an unstable feature.

[calebcartwright]

Oops, I misunderstood the question anyway. I don't know definitively but I believe that the poor binding formatting described in #4031 was a change rustfmt started doing relatively recently.

Not sure what, if any, impact that may have on the formatting here

[qm3ster]

Happens equally as root for a match arm:

match head.packet_type()? {
    p
    @
    (PacketType::Connect
    | PacketType::ConnAck
    | PacketType::SubAck
    | PacketType::UnsubAck) => return Err(Error::WrongPacket(p)),

Manually, I would probably format it as:

match head.packet_type()? {
    p @ (
        PacketType::Connect
        | PacketType::ConnAck
        | PacketType::SubAck
        | PacketType::UnsubAck
    ) => return Err(Error::WrongPacket(p)),

Interestingly enough, we still end up with N+2 lines. But moving the output expression to the left is definitely a huge win.

[evanjs]

"Fixing" this by locally by increasing max_width for now (not really a fix...) but this is definitely something that would make rustfmt much less annoying when using nested or patterns.

I wonder if this would be more worthwhile to investigate now that rust-lang/rust#54883 is stabilized (?), etc.

[ytmimi]

This was fixed in #4978, and a test case was added for this issue