## Summary
Part of #970.
This adds Pylint's [R0244
empty_comment](https://pylint.pycqa.org/en/latest/user_guide/messages/refactor/empty-comment.html)
lint as well as an always-safe fix.
## Test Plan
The included snapshot verifies the following:
- A line containing only a non-empty comment is not changed
- A line containing leading whitespace before a non-empty comment is not
changed
- A line containing only an empty comment has its content deleted
- A line containing only leading whitespace before an empty comment has
its content deleted
- A line containing only leading and trailing whitespace on an empty
comment has its content deleted
- A line containing trailing whitespace after a non-empty comment is not
changed
- A line containing only a single newline character (i.e. a blank line)
is not changed
- A line containing code followed by a non-empty comment is not changed
- A line containing code followed by an empty comment has its content
deleted after the last non-whitespace character
- Lines containing code and no comments are not changed
- Empty comment lines within block comments are ignored
- Empty comments within triple-quoted sections are ignored
## Comparison to Pylint
Running Ruff and Pylint 3.0.3 with Python 3.12.0 against the
`empty_comment.py` file added in this PR, we see the following:
* Identical behavior:
* empty_comment.py:3:0: R2044: Line with empty comment (empty-comment)
* empty_comment.py:4:0: R2044: Line with empty comment (empty-comment)
* empty_comment.py:5:0: R2044: Line with empty comment (empty-comment)
* empty_comment.py:18:0: R2044: Line with empty comment (empty-comment)
* Differing behavior:
* Pylint doesn't ignore empty comments in block comments commonly used
for visual spacing; I decided these were fine in this implementation
since many projects use these and likely do not want them removed.
* empty_comment.py:28:0: R2044: Line with empty comment (empty-comment)
* Pylint detects "empty comments" within the triple-quoted section at
the bottom of the file, which is arguably a bug in the Pylint
implementation since these are not truly comments. These are ignored by
this implementation.
* empty_comment.py:37:0: R2044: Line with empty comment (empty-comment)
* empty_comment.py:38:0: R2044: Line with empty comment (empty-comment)
* empty_comment.py:39:0: R2044: Line with empty comment (empty-comment)
## Summary
This PR updates the string nodes (`ExprStringLiteral`,
`ExprBytesLiteral`, and `ExprFString`) to account for implicit string
concatenation.
### Motivation
In Python, implicit string concatenation are joined while parsing
because the interpreter doesn't require the information for each part.
While that's feasible for an interpreter, it falls short for a static
analysis tool where having such information is more useful. Currently,
various parts of the code uses the lexer to get the individual string
parts.
One of the main challenge this solves is that of string formatting.
Currently, the formatter relies on the lexer to get the individual
string parts, and formats them including the comments accordingly. But,
with PEP 701, f-string can also contain comments. Without this change,
it becomes very difficult to add support for f-string formatting.
### Implementation
The initial proposal was made in this discussion:
https://github.com/astral-sh/ruff/discussions/6183#discussioncomment-6591993.
There were various AST designs which were explored for this task which
are available in the linked internal document[^1].
The selected variant was the one where the nodes were kept as it is
except that the `implicit_concatenated` field was removed and instead a
new struct was added to the `Expr*` struct. This would be a private
struct would contain the actual implementation of how the AST is
designed for both single and implicitly concatenated strings.
This implementation is achieved through an enum with two variants:
`Single` and `Concatenated` to avoid allocating a vector even for single
strings. There are various public methods available on the value struct
to query certain information regarding the node.
The nodes are structured in the following way:
```
ExprStringLiteral - "foo" "bar"
|- StringLiteral - "foo"
|- StringLiteral - "bar"
ExprBytesLiteral - b"foo" b"bar"
|- BytesLiteral - b"foo"
|- BytesLiteral - b"bar"
ExprFString - "foo" f"bar {x}"
|- FStringPart::Literal - "foo"
|- FStringPart::FString - f"bar {x}"
|- StringLiteral - "bar "
|- FormattedValue - "x"
```
[^1]: Internal document:
https://www.notion.so/astral-sh/Implicit-String-Concatenation-e036345dc48943f89e416c087bf6f6d9?pvs=4
#### Visitor
The way the nodes are structured is that the entire string, including
all the parts that are implicitly concatenation, is a single node
containing individual nodes for the parts. The previous section has a
representation of that tree for all the string nodes. This means that
new visitor methods are added to visit the individual parts of string,
bytes, and f-strings for `Visitor`, `PreorderVisitor`, and
`Transformer`.
## Test Plan
- `cargo insta test --workspace --all-features --unreferenced reject`
- Verify that the ecosystem results are unchanged
## Summary
It seems like the range of an `ExprStringLiteral` can be somewhat
unreliable when the string is part of an implicit concatenation with an
f-string. Using the tokens themselves is more reliable.
Closes#8680.
Closes https://github.com/astral-sh/ruff/issues/7784.
Mike Bernard
Dhruv Manilawala
Charlie Marsh
Kar Petrosyan