Add initial formatter implementation (#2883)

# Summary

This PR contains the code for the autoformatter proof-of-concept.

## Crate structure

The primary formatting hook is the `fmt` function in `crates/ruff_python_formatter/src/lib.rs`.

The current formatter approach is outlined in `crates/ruff_python_formatter/src/lib.rs`, and is structured as follows:

- Tokenize the code using the RustPython lexer.
- In `crates/ruff_python_formatter/src/trivia.rs`, extract a variety of trivia tokens from the token stream. These include comments, trailing commas, and empty lines.
- Generate the AST via the RustPython parser.
- In `crates/ruff_python_formatter/src/cst.rs`, convert the AST to a CST structure. As of now, the CST is nearly identical to the AST, except that every node gets a `trivia` vector. But we might want to modify it further.
- In `crates/ruff_python_formatter/src/attachment.rs`, attach each trivia token to the corresponding CST node. The logic for this is mostly in `decorate_trivia` and is ported almost directly from Prettier (given each token, find its preceding, following, and enclosing nodes, then attach the token to the appropriate node in a second pass).
- In `crates/ruff_python_formatter/src/newlines.rs`, normalize newlines to match Black’s preferences. This involves traversing the CST and inserting or removing `TriviaToken` values as we go.
- Call `format!` on the CST, which delegates to type-specific formatter implementations (e.g., `crates/ruff_python_formatter/src/format/stmt.rs` for `Stmt` nodes, and similar for `Expr` nodes; the others are trivial). Those type-specific implementations delegate to kind-specific functions (e.g., `format_func_def`).

## Testing and iteration

The formatter is being developed against the Black test suite, which was copied over in-full to `crates/ruff_python_formatter/resources/test/fixtures/black`.

The Black fixtures had to be modified to create `[insta](/~https://github.com/mitsuhiko/insta)`-compatible snapshots, which now exist in the repo.

My approach thus far has been to try and improve coverage by tackling fixtures one-by-one.

## What works, and what doesn’t

- *Most* nodes are supported at a basic level (though there are a few stragglers at time of writing, like `StmtKind::Try`).
- Newlines are properly preserved in most cases.
- Magic trailing commas are properly preserved in some (but not all) cases.
- Trivial leading and trailing standalone comments mostly work (although maybe not at the end of a file).
- Inline comments, and comments within expressions, often don’t work -- they work in a few cases, but it’s one-off right now. (We’re probably associating them with the “right” nodes more often than we are actually rendering them in the right place.)
- We don’t properly normalize string quotes. (At present, we just repeat any constants verbatim.)
- We’re mishandling a bunch of wrapping cases (if we treat Black as the reference implementation). Here are a few examples (demonstrating Black's stable behavior):

```py
# In some cases, if the end expression is "self-closing" (functions,
# lists, dictionaries, sets, subscript accesses, and any length-two
# boolean operations that end in these elments), Black
# will wrap like this...
if some_expression and f(
    b,
    c,
    d,
):
    pass

# ...whereas we do this:
if (
    some_expression
    and f(
        b,
        c,
        d,
    )
):
    pass

# If function arguments can fit on a single line, then Black will
# format them like this, rather than exploding them vertically.
if f(
    a, b, c, d, e, f, g, ...
):
    pass
```

- We don’t properly preserve parentheses in all cases. Black preserves parentheses in some but not all cases.

_typos.toml

@@ -1,5 +1,5 @@
 [files]
-extend-exclude = ["snapshots"]
+extend-exclude = ["snapshots", "black"]
 
 [default.extend-words]
 trivias = "trivias"

crates/ruff/Cargo.toml

@@ -21,7 +21,7 @@ bisection = { version = "0.1.0" }
 bitflags = { version = "1.3.2" }
 cfg-if = { version = "1.0.0" }
 chrono = { version = "0.4.21", default-features = false, features = ["clock"] }
-clap = { version = "4.0.1", features = ["derive", "env"] }
+clap = { workspace = true, features = ["derive", "env"] }
 colored = { version = "2.0.0" }
 dirs = { version = "4.0.0" }
 fern = { version = "0.6.1" }

crates/ruff/src/ast/helpers.rs

@@ -723,7 +723,7 @@ where
             StmtKind::FunctionDef { .. } | StmtKind::AsyncFunctionDef { .. } => {
                 // Don't recurse.
             }
-            StmtKind::Return { value } => self.returns.push(value.as_ref().map(|expr| &**expr)),
+            StmtKind::Return { value } => self.returns.push(value.as_deref()),
             _ => visitor::walk_stmt(self, stmt),
         }
     }

crates/ruff/src/checkers/ast.rs

@@ -1560,12 +1560,7 @@ where
                     pyflakes::rules::assert_tuple(self, stmt, test);
                 }
                 if self.settings.rules.enabled(&Rule::AssertFalse) {
-                    flake8_bugbear::rules::assert_false(
-                        self,
-                        stmt,
-                        test,
-                        msg.as_ref().map(|expr| &**expr),
-                    );
+                    flake8_bugbear::rules::assert_false(self, stmt, test, msg.as_deref());
                 }
                 if self.settings.rules.enabled(&Rule::Assert) {
                     self.diagnostics

crates/ruff_cli/Cargo.toml

@@ -33,7 +33,7 @@ bincode = { version = "1.3.3" }
 bitflags = { version = "1.3.2" }
 cachedir = { version = "0.3.0" }
 chrono = { version = "0.4.21", default-features = false, features = ["clock"] }
-clap = { version = "4.0.1", features = ["derive", "env"] }
+clap = { workspace = true, features = ["derive", "env"] }
 clap_complete_command = { version = "0.4.0" }
 clearscreen = { version = "2.0.0" }
 colored = { version = "2.0.0" }

crates/ruff_python_formatter/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "ruff_python_formatter"
+version = "0.0.0"
+publish = false
+edition = "2021"
+
+[dependencies]
+anyhow = { workspace = true }
+clap = { workspace = true }
+once_cell = { workspace = true }
+ruff_formatter = { path = "../ruff_formatter" }
+ruff_text_size = { path = "../ruff_text_size" }
+rustc-hash = { workspace = true }
+rustpython-common = { workspace = true }
+rustpython-parser = { workspace = true }
+
+[dev-dependencies]
+insta = { version = "1.19.0", features = [] }
+test-case = { version = "2.2.2" }

crates/ruff_python_formatter/resources/test/fixtures/black/simple_cases/attribute_access_on_number_literals.py

@@ -0,0 +1,22 @@
+ax = 123456789 .bit_count()
+x = (123456).__abs__()
+x = .1.is_integer()
+x = 1. .imag
+x = 1E+1.imag
+x = 1E-1.real
+x = 123456789.123456789.hex()
+x = 123456789.123456789E123456789 .real
+x = 123456789E123456789 .conjugate()
+x = 123456789J.real
+x = 123456789.123456789J.__add__(0b1011.bit_length())
+x = 0XB1ACC.conjugate()
+x = 0B1011 .conjugate()
+x = 0O777 .real
+x = 0.000000006  .hex()
+x = -100.0000J
+
+if 10 .real:
+    ...
+
+y = 100[no]
+y = 100(no)

crates/ruff_python_formatter/resources/test/fixtures/black/simple_cases/beginning_backslash.py

@@ -0,0 +1,7 @@
+\
+
+
+
+
+
+print("hello, world")

crates/ruff_python_formatter/resources/test/fixtures/black/simple_cases/bracketmatch.py

@@ -0,0 +1,6 @@
+for ((x in {}) or {})['a'] in x:
+    pass
+pem_spam = lambda l, spam = {
+    "x": 3
+}: not spam.get(l.strip())
+lambda x=lambda y={1: 3}: y['x':lambda y: {1: 2}]: x

crates/ruff_python_formatter/resources/test/fixtures/black/simple_cases/class_blank_parentheses.py

@@ -0,0 +1,23 @@
+class SimpleClassWithBlankParentheses():
+    pass
+class ClassWithSpaceParentheses ( ):
+    first_test_data = 90
+    second_test_data = 100
+    def test_func(self):
+        return None
+class ClassWithEmptyFunc(object):
+
+    def func_with_blank_parentheses():
+        return 5
+
+
+def public_func_with_blank_parentheses():
+    return None
+def class_under_the_func_with_blank_parentheses():
+    class InsideFunc():
+        pass
+class NormalClass (
+):
+    def func_for_testing(self, first, second):
+        sum = first + second
+        return sum

crates/ruff_python_formatter/resources/test/fixtures/black/simple_cases/class_methods_new_line.py

@@ -0,0 +1,100 @@
+class ClassSimplest:
+    pass
+class ClassWithSingleField:
+    a = 1
+class ClassWithJustTheDocstring:
+    """Just a docstring."""
+class ClassWithInit:
+    def __init__(self):
+        pass
+class ClassWithTheDocstringAndInit:
+    """Just a docstring."""
+    def __init__(self):
+        pass
+class ClassWithInitAndVars:
+    cls_var = 100
+    def __init__(self):
+        pass
+class ClassWithInitAndVarsAndDocstring:
+    """Test class"""
+    cls_var = 100
+    def __init__(self):
+        pass
+class ClassWithDecoInit:
+    @deco
+    def __init__(self):
+        pass
+class ClassWithDecoInitAndVars:
+    cls_var = 100
+    @deco
+    def __init__(self):
+        pass
+class ClassWithDecoInitAndVarsAndDocstring:
+    """Test class"""
+    cls_var = 100
+    @deco
+    def __init__(self):
+        pass
+class ClassSimplestWithInner:
+    class Inner:
+        pass
+class ClassSimplestWithInnerWithDocstring:
+    class Inner:
+        """Just a docstring."""
+        def __init__(self):
+            pass
+class ClassWithSingleFieldWithInner:
+    a = 1
+    class Inner:
+        pass
+class ClassWithJustTheDocstringWithInner:
+    """Just a docstring."""
+    class Inner:
+        pass
+class ClassWithInitWithInner:
+    class Inner:
+        pass
+    def __init__(self):
+        pass
+class ClassWithInitAndVarsWithInner:
+    cls_var = 100
+    class Inner:
+        pass
+    def __init__(self):
+        pass
+class ClassWithInitAndVarsAndDocstringWithInner:
+    """Test class"""
+    cls_var = 100
+    class Inner:
+        pass
+    def __init__(self):
+        pass
+class ClassWithDecoInitWithInner:
+    class Inner:
+        pass
+    @deco
+    def __init__(self):
+        pass
+class ClassWithDecoInitAndVarsWithInner:
+    cls_var = 100
+    class Inner:
+        pass
+    @deco
+    def __init__(self):
+        pass
+class ClassWithDecoInitAndVarsAndDocstringWithInner:
+    """Test class"""
+    cls_var = 100
+    class Inner:
+        pass
+    @deco
+    def __init__(self):
+        pass
+class ClassWithDecoInitAndVarsAndDocstringWithInner2:
+    """Test class"""
+    class Inner:
+        pass
+    cls_var = 100
+    @deco
+    def __init__(self):
+        pass

crates/ruff_python_formatter/resources/test/fixtures/black/simple_cases/collections.py

@@ -0,0 +1,71 @@
+import core, time, a
+
+from . import A, B, C
+
+# keeps existing trailing comma
+from foo import (
+    bar,
+)
+
+# also keeps existing structure
+from foo import (
+    baz,
+    qux,
+)
+
+# `as` works as well
+from foo import (
+    xyzzy as magic,
+)
+
+a = {1,2,3,}
+b = {
+1,2,
+     3}
+c = {
+    1,
+    2,
+    3,
+}
+x = 1,
+y = narf(),
+nested = {(1,2,3),(4,5,6),}
+nested_no_trailing_comma = {(1,2,3),(4,5,6)}
+nested_long_lines = ["aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb", "cccccccccccccccccccccccccccccccccccccccc", (1, 2, 3), "dddddddddddddddddddddddddddddddddddddddd"]
+{"oneple": (1,),}
+{"oneple": (1,)}
+['ls', 'lsoneple/%s' % (foo,)]
+x = {"oneple": (1,)}
+y = {"oneple": (1,),}
+assert False, ("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa wraps %s" % bar)
+
+# looping over a 1-tuple should also not get wrapped
+for x in (1,):
+    pass
+for (x,) in (1,), (2,), (3,):
+    pass
+
+[1, 2, 3,]
+
+division_result_tuple = (6/2,)
+print("foo %r", (foo.bar,))
+
+if True:
+    IGNORED_TYPES_FOR_ATTRIBUTE_CHECKING = (
+        Config.IGNORED_TYPES_FOR_ATTRIBUTE_CHECKING
+        | {pylons.controllers.WSGIController}
+    )
+
+if True:
+    ec2client.get_waiter('instance_stopped').wait(
+        InstanceIds=[instance.id],
+        WaiterConfig={
+            'Delay': 5,
+        })
+    ec2client.get_waiter("instance_stopped").wait(
+        InstanceIds=[instance.id],
+        WaiterConfig={"Delay": 5,},
+    )
+    ec2client.get_waiter("instance_stopped").wait(
+        InstanceIds=[instance.id], WaiterConfig={"Delay": 5,},
+    )

crates/ruff_python_formatter/resources/test/fixtures/black/simple_cases/comment_after_escaped_newline.py

@@ -0,0 +1,9 @@
+def bob(): \
+         # pylint: disable=W9016
+    pass
+
+
+def bobtwo(): \
+    \
+  # some comment here
+    pass