We are using PSToml for some Rust release pipelines and found that empty, unreferenced[^unref] arrays are not being serialized.

You can observe this with the latest version of PSToml:

$crate = @'
[package]
name = "example"
version = "0.1.0"

[features]
default = ["foo"]
foo = []
bar = []
'@ | ConvertFrom-Toml

$obj
# Name                           Value
# ----                           -----
# package                        {[name, example], [version, 0.1.0]}
# features                       {[default, System.Object[]], [foo, System.Object[]], [bar, System.Object[]]}

$obj | ConvertTo-Toml -Depth 10
# [package]
# name = "example"
# version = "0.1.0"
# [features]
# default = ["foo"]
# foo = []

Notice that bar is missing. What's odd is that foo is serialized, despite it being referenced by name as a string, as if Cargo.toml has special handling, or perhaps that is a common enough pattern to warrant forcing serialization. But as is also common in Cargo.toml, not ever feature would be refenced.

Deserialized serialized an empty array and that's what should be serialized. I could see if an array is $null, but this may require specific handling apart from PowerShell's "emptyness" (like its "falsy" behavior) such that $null != empty for collections.