Make `Lexer` lazy #11244

dhruvmanila · 2024-05-02T09:12:52Z

Summary

This PR updates the Lexer to make it lazy in the sense that the tokens are emitted only when it's requested.

Lexer

Remove Iterator implementation
Remove SoftkeywordTransformer
Collect all LexicalError in the lexer and return it on finish call
Store the current token and provide methods to query it
Implement a new TokenValue struct which stores the owned value for certain tokens ¹
Update all lex_* methods to return the token instead of Result
Update Lexer::new to take a start_offset parameter
- This will be used to start lexing from a specific offset
Add checkpoint - rewind logic for the lexer, f-strings and indentation stack

Token Source

Remove Iterator implementation
Provide lookahead via checkpoint - rewind logic on the Lexer
Store all the lexed tokens

Parser

Update the soft keywords to identifier when parsing as an identifier
Add bump_value to bump the given token kind and return the corresponding owned value
Add bump_any to bump any token except for end of file

At the end, we'll remove the Tok completely ↩

crates/ruff_python_parser/src/lexer.rs

crates/ruff_python_parser/src/lexer/indentation.rs

crates/ruff_python_parser/src/lexer/cursor.rs

crates/ruff_python_parser/src/parser/expression.rs

MichaReiser

I like this a lot! Nice work.

I think we can improve the lexer performance by change how we handle errors in next_token (see inline comment).

We also need to think about how parse_expression_at should work. I think we need to pass the offset through to the lexer so that it can offset all token ranges and start lexing after offset in the source.

crates/ruff_python_parser/src/parser/expression.rs

crates/ruff_python_parser/src/parser/mod.rs

MichaReiser · 2024-05-09T08:01:57Z

crates/ruff_python_parser/src/soft_keywords.rs

It almost makes me sad to say goodby to @charliermarsh's genius implementation for supporting soft keywords with lalrpop.

crates/ruff_python_parser/src/lexer.rs

dhruvmanila · 2024-05-17T10:06:05Z

crates/ruff_python_parser/src/lexer.rs

+        if start_offset > TextSize::new(0) {
+            lexer.cursor.skip_bytes(start_offset.to_usize());
+        }


This is how the lexer will start lexing from the given offset.

dhruvmanila · 2024-05-17T11:33:29Z

I'm going to merge this into another PR to keep going. I'm finding it difficult to refactor with changes separated in different branches.

This PR updates the `Lexer` to make it lazy in the sense that the tokens are emitted only when it's requested. * Remove `Iterator` implementation * Remove `SoftkeywordTransformer` * Collect all `LexicalError` in the lexer and return it on `finish` call * Store the `current` token and provide methods to query it * Implement a new `TokenValue` struct which stores the owned value for certain tokens [^1] * Update all `lex_*` methods to return the token instead of `Result` * Update `Lexer::new` to take a `start_offset` parameter * This will be used to start lexing from a specific offset * Add checkpoint - rewind logic for the lexer, f-strings and indentation stack * Remove `Iterator` implementation * Provide lookahead via checkpoint - rewind logic on the `Lexer` * Store all the lexed tokens * Update the soft keywords to identifier when parsing as an identifier * Add `bump_value` to bump the given token kind and return the corresponding owned value * Add `bump_any` to bump any token except for end of file [^1]: At the end, we'll remove the `Tok` completely

## Summary This PR updates the entire parser stack in multiple ways: ### Make the lexer lazy * #11244 * #11473 Previously, Ruff's lexer would act as an iterator. The parser would collect all the tokens in a vector first and then process the tokens to create the syntax tree. The first task in this project is to update the entire parsing flow to make the lexer lazy. This includes the `Lexer`, `TokenSource`, and `Parser`. For context, the `TokenSource` is a wrapper around the `Lexer` to filter out the trivia tokens[^1]. Now, the parser will ask the token source to get the next token and only then the lexer will continue and emit the token. This means that the lexer needs to be aware of the "current" token. When the `next_token` is called, the current token will be updated with the newly lexed token. The main motivation to make the lexer lazy is to allow re-lexing a token in a different context. This is going to be really useful to make the parser error resilience. For example, currently the emitted tokens remains the same even if the parser can recover from an unclosed parenthesis. This is important because the lexer emits a `NonLogicalNewline` in parenthesized context while a normal `Newline` in non-parenthesized context. This different kinds of newline is also used to emit the indentation tokens which is important for the parser as it's used to determine the start and end of a block. Additionally, this allows us to implement the following functionalities: 1. Checkpoint - rewind infrastructure: The idea here is to create a checkpoint and continue lexing. At a later point, this checkpoint can be used to rewind the lexer back to the provided checkpoint. 2. Remove the `SoftKeywordTransformer` and instead use lookahead or speculative parsing to determine whether a soft keyword is a keyword or an identifier 3. Remove the `Tok` enum. The `Tok` enum represents the tokens emitted by the lexer but it contains owned data which makes it expensive to clone. The new `TokenKind` enum just represents the type of token which is very cheap. This brings up a question as to how will the parser get the owned value which was stored on `Tok`. This will be solved by introducing a new `TokenValue` enum which only contains a subset of token kinds which has the owned value. This is stored on the lexer and is requested by the parser when it wants to process the data. For example: https://github.com/astral-sh/ruff/blob/8196720f809380d8f1fc7651679ff3fc2cb58cd7/crates/ruff_python_parser/src/parser/expression.rs#L1260-L1262 [^1]: Trivia tokens are `NonLogicalNewline` and `Comment` ### Remove `SoftKeywordTransformer` * #11441 * #11459 * #11442 * #11443 * #11474 For context, https://github.com/RustPython/RustPython/pull/4519/files#diff-5de40045e78e794aa5ab0b8aacf531aa477daf826d31ca129467703855408220 added support for soft keywords in the parser which uses infinite lookahead to classify a soft keyword as a keyword or an identifier. This is a brilliant idea as it basically wraps the existing Lexer and works on top of it which means that the logic for lexing and re-lexing a soft keyword remains separate. The change here is to remove `SoftKeywordTransformer` and let the parser determine this based on context, lookahead and speculative parsing. * **Context:** The transformer needs to know the position of the lexer between it being at a statement position or a simple statement position. This is because a `match` token starts a compound statement while a `type` token starts a simple statement. **The parser already knows this.** * **Lookahead:** Now that the parser knows the context it can perform lookahead of up to two tokens to classify the soft keyword. The logic for this is mentioned in the PR implementing it for `type` and `match soft keyword. * **Speculative parsing:** This is where the checkpoint - rewind infrastructure helps. For `match` soft keyword, there are certain cases for which we can't classify based on lookahead. The idea here is to create a checkpoint and keep parsing. Based on whether the parsing was successful and what tokens are ahead we can classify the remaining cases. Refer to #11443 for more details. If the soft keyword is being parsed in an identifier context, it'll be converted to an identifier and the emitted token will be updated as well. Refer https://github.com/astral-sh/ruff/blob/8196720f809380d8f1fc7651679ff3fc2cb58cd7/crates/ruff_python_parser/src/parser/expression.rs#L487-L491. The `case` soft keyword doesn't require any special handling because it'll be a keyword only in the context of a match statement. ### Update the parser API * #11494 * #11505 Now that the lexer is in sync with the parser, and the parser helps to determine whether a soft keyword is a keyword or an identifier, the lexer cannot be used on its own. The reason being that it's not sensitive to the context (which is correct). This means that the parser API needs to be updated to not allow any access to the lexer. Previously, there were multiple ways to parse the source code: 1. Passing the source code itself 2. Or, passing the tokens Now that the lexer and parser are working together, the API corresponding to (2) cannot exists. The final API is mentioned in this PR description: #11494. ### Refactor the downstream tools (linter and formatter) * #11511 * #11515 * #11529 * #11562 * #11592 And, the final set of changes involves updating all references of the lexer and `Tok` enum. This was done in two-parts: 1. Update all the references in a way that doesn't require any changes from this PR i.e., it can be done independently * #11402 * #11406 * #11418 * #11419 * #11420 * #11424 2. Update all the remaining references to use the changes made in this PR For (2), there were various strategies used: 1. Introduce a new `Tokens` struct which wraps the token vector and add methods to query a certain subset of tokens. These includes: 1. `up_to_first_unknown` which replaces the `tokenize` function 2. `in_range` and `after` which replaces the `lex_starts_at` function where the former returns the tokens within the given range while the latter returns all the tokens after the given offset 2. Introduce a new `TokenFlags` which is a set of flags to query certain information from a token. Currently, this information is only limited to any string type token but can be expanded to include other information in the future as needed. #11578 3. Move the `CommentRanges` to the parsed output because this information is common to both the linter and the formatter. This removes the need for `tokens_and_ranges` function. ## Test Plan - [x] Update and verify the test snapshots - [x] Make sure the entire test suite is passing - [x] Make sure there are no changes in the ecosystem checks - [x] Run the fuzzer on the parser - [x] Run this change on dozens of open-source projects ### Running this change on dozens of open-source projects Refer to the PR description to get the list of open source projects used for testing. Now, the following tests were done between `main` and this branch: 1. Compare the output of `--select=E999` (syntax errors) 2. Compare the output of default rule selection 3. Compare the output of `--select=ALL` **Conclusion: all output were same** ## What's next? The next step is to introduce re-lexing logic and update the parser to feed the recovery information to the lexer so that it can emit the correct token. This moves us one step closer to having error resilience in the parser and provides Ruff the possibility to lint even if the source code contains syntax errors.

MichaReiser reviewed May 2, 2024

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MichaReiser reviewed May 2, 2024

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dhruvmanila force-pushed the dhruv/lazy-lexer branch from fe0fa75 to 8c77d9d Compare May 8, 2024 14:39

MichaReiser reviewed May 9, 2024

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MichaReiser reviewed May 9, 2024

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dhruvmanila force-pushed the dhruv/lazy-lexer branch 3 times, most recently from 3ca2ba3 to 2f35044 Compare May 17, 2024 05:57

Make the lexer lazy

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dhruvmanila force-pushed the dhruv/lazy-lexer branch from 2f35044 to 6dad12f Compare May 17, 2024 09:40

dhruvmanila changed the base branch from main to dhruv/parser-phase-2 May 17, 2024 09:40

dhruvmanila force-pushed the dhruv/lazy-lexer branch from efc7138 to 47ec6fe Compare May 17, 2024 10:04

dhruvmanila commented May 17, 2024

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Clone the entire stack for lexer checkpoint

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dhruvmanila force-pushed the dhruv/lazy-lexer branch from 47ec6fe to dcab2ef Compare May 17, 2024 10:07

dhruvmanila added the parser Related to the parser label May 17, 2024

dhruvmanila changed the title ~~WIP: Make Lexer lazy~~ Make Lexer lazy May 17, 2024

dhruvmanila marked this pull request as ready for review May 17, 2024 11:33

dhruvmanila merged commit bfc513e into dhruv/parser-phase-2 May 17, 2024
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dhruvmanila deleted the dhruv/lazy-lexer branch May 17, 2024 11:34

dhruvmanila mentioned this pull request May 17, 2024

Maintain synchronicity between the lexer and the parser #11457

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Make `Lexer` lazy #11244

Make `Lexer` lazy #11244

dhruvmanila commented May 2, 2024 •

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MichaReiser May 9, 2024

dhruvmanila May 17, 2024

dhruvmanila commented May 17, 2024

Make Lexer lazy #11244

Make Lexer lazy #11244

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dhruvmanila commented May 2, 2024 • edited Loading

Summary

Lexer

Token Source

Parser

Footnotes

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dhruvmanila commented May 17, 2024

Make `Lexer` lazy #11244

Make `Lexer` lazy #11244

dhruvmanila commented May 2, 2024 •

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