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Previously the Token enum contained the offsets using the O generic
type parameter, which could be a usize if you're tracking offsets or
a zero-sized type if you didn't care about offsets. This commit moves
all the byte offset and syntax information to a new Trace enum,
which has several advantages:
* Traces can now easily be stored separately, while the tokens are
fed to the tree builder. (The tree builder only has to keep track
of which tree nodes originate from which tokens.)
* No needless generics for functions that take a token but don't
care about offsets (a tree construction implementation is bound
to have many of such functions).
* The FromIterator<(String, String)> impl for AttributeMap no longer
has to specify arbitrary values for the spans and the value_syntax).
* The PartialEq implementation of Token is now much more useful
(since it no longer includes all the offsets).
* The Debug formatting of Token is now more readable
(since it no longer includes all the offsets).
* Function pointers to functions accepting tokens are possible.
(Since function pointer types may not have generic parameters.)
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While the end-of-file token can also be represented by None,
this is less clear than having an explicit variant. Especially when
it comes to tree construction, the spec explicitly has conditions
named "An end-of-file token", and it's nice if the code for tree
construction can match the spec text closely.
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The HTML spec specifies that the tokenizer emits character tokens.
That html5gum always emitted strings instead was probably just done
to make the token consumption more convenient. When it comes to tree
construction character tokens are however actually more convenient
than string tokens since the spec defines that specific character
tokens should be ignored in specific states (and character tokens
let us avoid string manipulation for these conditions).
This should also make the DefaultEmitter more performant for cases
where you don't actually need the strings at all (or only a few)
since it avoids string allocations. Though I haven't benchmarked it.
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An error isn't a token (in general and also according to the spec).
You shouldn't have to filter out errors when you're just interested
in tokens but most importantly having errors in the Token enum is
annoying when implementing tree construction (since the spec conditions
exhaustively cover all Token variants except Token::Error).
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