malformed `derive` attribute input deletes derive arguments

[ratmice]

Describe the bug
Adding a generic type to a derive macro deletes all derives

To Reproduce

cat <<EOF | rustfmt
#[derive(Debug, Foo<T>)]
enum Bar{}
EOF 

This produces the following output, removing all derive attributes:

#[derive()]
enum Bar {}

Expected behavior
Something less nefarious, like rustc's

error: malformed `derive` attribute input

Meta

• rustfmt version: rustrustfmt 1.4.24-stable (eb894d5 2020-11-05)
• From where did you install rustfmt?: rustup
• How do you run rustfmt: In an editor with undo hopefully 😈

[camelid]

Just noticed this as well:

#[derive(Debug, , Default)]
struct Struct(i32);

turns into this:

#[derive()]
struct Struct(i32);

which is bizarre when you're deleting one of the derives and you save partway through!

[calebcartwright]

This has been fixed in source but not backported to a released version. Will try to do so for the upcoming 1.4.28 release

[calebcartwright]

Closing as a duplicate of #3898, fix has been backported and will be in the next release