Marshalling fails for nested arrays of tables

[x-hgg-x]

Describe the bug
Marshalling produces an error:

marshalling error: unsupported value type *toml.Tree: key = "Control"

To Reproduce

tree, err := toml.Load(`
combinations = [
	[{ key = "Control" }, { key = "Q" }],
	[{ key = "Escape" }],
]
`)

if err != nil {
	fmt.Println("loading error:", err)
}

_, err = tree.Marshal()
if err != nil {
	fmt.Println("marshalling error:", err)
}

Expected behavior
There is no error when loading TOML data, so marshalling should work.

Versions

• go-toml: last version of master (323fe5d)
• go: 1.14
• operating system: Linux

Additional context
Unmarshalling works:

type Button struct {
	Key string
}

type Action struct {
	Combinations [][]Button
}

var action Action
err = tree.Unmarshal(&action)
if err != nil {
	fmt.Println("unmarshalling error:", err)
}

fmt.Println(action)

Output:

{[[{Control} {Q}] [{Escape}]]}

[pelletier]

Thank you for the bug report! That's definitely wrong.

[x-hgg-x]

For information, the BurntSushi library also fails with this error:
toml: TOML array element cannot contain a table.

I think nested arrays of table might only be expressed with inline tables in TOML (see toml-lang/toml#353 (comment)).

[pelletier]

The latest spec calls out

points = [ { x = 1, y = 2, z = 3 },
           { x = 7, y = 8, z = 9 },
           { x = 2, y = 4, z = 8 } ]

And that arrays can contain any other type including arrays. I don’t see why your example shouldn’t work.

[pelletier]

I wonder if your changes at f1ba638 fix this 🤔

[x-hgg-x]

My changes don't fix this. Since nested arrays of tables can only be represented as inline tables, we should add an inline tree representation in the tomlValueStringRepresentation method. I will try to make a PR for this.