fktn-k / fkYAML / 31943414714

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Complete explicit mapping keys which are not followed by a value (#554)

* Add a mapping entry for an explicit key whose value is omitted

An omitted mapping value is a null value, but the deserializer only handled
that for implicit keys. With an explicit key the pending key node was popped
off the context stack and discarded, so the entry disappeared from the result
without any error:

```yaml
? foo
:
bar: baz
# -> {bar: baz}, expected {foo: null, bar: baz}
```

The omitted value branch of the key separator handling asserted that the
current context is MAPPING_VALUE, which does not hold for an explicit key
(the context is BLOCK_MAPPING_EXPLICIT_KEY), so a debug build tripped the
assertion and an NDEBUG build silently dropped the entry. Add the entry with
a null value in that case instead.

* Complete explicit mapping key which isn't followed by value on its line

An explicit key is held in its own parse context until its value is found, and
only a `: value` on the same line completed the entry. Every other way of
writing the entry dropped it without an error: a key with no key separator at
all, and a key whose value begins on a following line.

```yaml
? foo
? bar
baz: 123

? qux
:
  corge
```

Add two helpers for the two ways an explicit key entry gets completed, adding
it either with a null value or with an empty value node to parse the value
into, and use them where a pending explicit key was previously discarded: at
the end of a document, before another explicit key, before a mapping entry
with an implicit key, and where the value of the explicit key begins on a
following line.

Note that a document can now report a duplicate mapping key which used to be
lost with the dropped entry, for example `? foo` followed by `foo: 123`.

* Emit a parse error for a key separator which follows no mapping key

The omitted value branch of the key separator handling asserted that the
current context is a mapping value context. That does not hold for malformed
in... (continued)

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