This simplifies the logic for determining whether a token is reusable
and makes it more conservative. It should fix some incremental parsing
bugs that are being caught by the randomized tests on CI.
They were a vestige of when Tree-sitter did sentential form-based
incremental parsing (as opposed to simply state matching). This was
elegant but not compatible with GLR as far as I could tell.
The previous approach to error recovery relied on special error-recovery
states in the parse table. For each token T, there was an error recovery
state in which the parser looked for *any* token that could follow T.
Unfortunately, sometimes the set of tokens that could follow T contained
conflicts. For example, in JS, the token '}' can be followed by the
open-ended 'template_chars' token, but also by ordinary tokens like
'identifier'. So with the old algorithm, when recovering from an
unexpected '}' token, the lexer had no way to distinguish identifiers
from template_chars.
This commit drops the error recovery states. Instead, when we encounter
an unexpected token T, we recover from the error by finding a previous
state S in the stack in which T would be valid, popping all of the nodes
after S, and wrapping them in an error.
This way, the lexer is always invoked in a normal parse state, in which
it is looking for a non-conflicting set of tokens. Eliminating the error
recovery states also shrinks the lex state machine significantly.
Signed-off-by: Rick Winfrey <rewinfrey@github.com>
For some reason, there was previously some extra logic that prevented
the external scanner from being invoked if the only valid external
token also had an internal definition.
It's surprising to not call the external scanner if an external
token is valid.
The current pretty conservative approach is to avoid merging parse states which
would cause a pair tokens to co-exist for the first time in any parse state,
where the two tokens can start with the same character and at least one of the
tokens can contain a character which is part of the grammar's separators.
They used to be called e.g. `ts_language_python`. Now that there
are APIs that deal with the `TSLanguage` objects themselves, such
as `ts_language_symbol_count`, the old names were a little confusing.
Tokens that are defined in the grammar's rules may now be included in the
externals list also, so that external scanners can check if they are valid
lookaheads or not, and if so, can return them to the parser if needed.
