Preview minimal f-string formatting (#9642)

## Summary

_This is preview only feature and is available using the `--preview`
command-line flag._

With the implementation of [PEP 701] in Python 3.12, f-strings can now
be broken into multiple lines, can contain comments, and can re-use the
same quote character. Currently, no other Python formatter formats the
f-strings so there's some discussion which needs to happen in defining
the style used for f-string formatting. Relevant discussion:
https://github.com/astral-sh/ruff/discussions/9785

The goal for this PR is to add minimal support for f-string formatting.
This would be to format expression within the replacement field without
introducing any major style changes.

### Newlines

The heuristics for adding newline is similar to that of
[Prettier](https://prettier.io/docs/en/next/rationale.html#template-literals)
where the formatter would only split an expression in the replacement
field across multiple lines if there was already a line break within the
replacement field.

In other words, the formatter would not add any newlines unless they
were already present i.e., they were added by the user. This makes
breaking any expression inside an f-string optional and in control of
the user. For example,

```python
# We wouldn't break this
aaaaaaaaaaa = f"asaaaaaaaaaaaaaaaa { aaaaaaaaaaaa + bbbbbbbbbbbb + ccccccccccccccc } cccccccccc"

# But, we would break the following as there's already a newline
aaaaaaaaaaa = f"asaaaaaaaaaaaaaaaa {
	aaaaaaaaaaaa + bbbbbbbbbbbb + ccccccccccccccc } cccccccccc"
```


If there are comments in any of the replacement field of the f-string,
then it will always be a multi-line f-string in which case the formatter
would prefer to break expressions i.e., introduce newlines. For example,

```python
x = f"{ # comment
    a }"
```

### Quotes

The logic for formatting quotes remains unchanged. The existing logic is
used to determine the necessary quote char and is used accordingly.

Now, if the expression inside an f-string is itself a string like, then
we need to make sure to preserve the existing quote and not change it to
the preferred quote unless it's 3.12. For example,

```python
f"outer {'inner'} outer"

# For pre 3.12, preserve the single quote
f"outer {'inner'} outer"

# While for 3.12 and later, the quotes can be changed
f"outer {"inner"} outer"
```

But, for triple-quoted strings, we can re-use the same quote char unless
the inner string is itself a triple-quoted string.

```python
f"""outer {"inner"} outer"""  # valid
f"""outer {'''inner'''} outer"""  # preserve the single quote char for the inner string
```

### Debug expressions

If debug expressions are present in the replacement field of a f-string,
then the whitespace needs to be preserved as they will be rendered as it
is (for example, `f"{ x = }"`. If there are any nested f-strings, then
the whitespace in them needs to be preserved as well which means that
we'll stop formatting the f-string as soon as we encounter a debug
expression.

```python
f"outer {   x =  !s  :.3f}"
#                  ^^
#                  We can remove these whitespaces
```

Now, the whitespace doesn't need to be preserved around conversion spec
and format specifiers, so we'll format them as usual but we won't be
formatting any nested f-string within the format specifier.

### Miscellaneous

- The
[`hug_parens_with_braces_and_square_brackets`](https://github.com/astral-sh/ruff/issues/8279)
preview style isn't implemented w.r.t. the f-string curly braces.
- The
[indentation](https://github.com/astral-sh/ruff/discussions/9785#discussioncomment-8470590)
is always relative to the f-string containing statement

## Test Plan

* Add new test cases
* Review existing snapshot changes
* Review the ecosystem changes

[PEP 701]: https://peps.python.org/pep-0701/

crates/ruff_python_formatter/tests/snapshots/black_compatibility@cases__preview_long_strings.py.snap

@@ -902,7 +902,7 @@ log.info(f"""Skipping: {'a' == 'b'} {desc['ms_name']} {money=} {dte=} {pos_share
  )
  
  dict_with_lambda_values = {
-@@ -524,61 +383,54 @@
+@@ -524,65 +383,58 @@
  
  # Complex string concatenations with a method call in the middle.
  code = (
@@ -941,7 +941,7 @@ log.info(f"""Skipping: {'a' == 'b'} {desc['ms_name']} {money=} {dte=} {pos_share
  log.info(
 -    "Skipping:"
 -    f" {desc['db_id']} {foo('bar',x=123)} {'foo' != 'bar'} {(x := 'abc=')} {pos_share=} {desc['status']} {desc['exposure_max']}"
-+    f'Skipping: {desc["db_id"]} {foo("bar",x=123)} {"foo" != "bar"} {(x := "abc=")} {pos_share=} {desc["status"]} {desc["exposure_max"]}'
++    f'Skipping: {desc["db_id"]} {foo("bar", x=123)} {"foo" != "bar"} {(x := "abc=")} {pos_share=} {desc["status"]} {desc["exposure_max"]}'
  )
  
  log.info(
@@ -981,6 +981,18 @@ log.info(f"""Skipping: {'a' == 'b'} {desc['ms_name']} {money=} {dte=} {pos_share
  )
  
  log.info(
+-    f"""Skipping: {"a" == 'b'} {desc["ms_name"]} {money=} {dte=} {pos_share=} {desc["status"]} {desc["exposure_max"]}"""
++    f"""Skipping: {"a" == "b"} {desc["ms_name"]} {money=} {dte=} {pos_share=} {desc["status"]} {desc["exposure_max"]}"""
+ )
+ 
+ log.info(
+@@ -590,5 +442,5 @@
+ )
+ 
+ log.info(
+-    f"""Skipping: {'a' == 'b'} {desc['ms_name']} {money=} {dte=} {pos_share=} {desc['status']} {desc['exposure_max']}"""
++    f"""Skipping: {"a" == "b"} {desc["ms_name"]} {money=} {dte=} {pos_share=} {desc["status"]} {desc["exposure_max"]}"""
+ )
 ```
 
 ## Ruff Output
@@ -1394,7 +1406,7 @@ log.info(
 )
 
 log.info(
-    f'Skipping: {desc["db_id"]} {foo("bar",x=123)} {"foo" != "bar"} {(x := "abc=")} {pos_share=} {desc["status"]} {desc["exposure_max"]}'
+    f'Skipping: {desc["db_id"]} {foo("bar", x=123)} {"foo" != "bar"} {(x := "abc=")} {pos_share=} {desc["status"]} {desc["exposure_max"]}'
 )
 
 log.info(
@@ -1422,7 +1434,7 @@ log.info(
 )
 
 log.info(
-    f"""Skipping: {"a" == 'b'} {desc["ms_name"]} {money=} {dte=} {pos_share=} {desc["status"]} {desc["exposure_max"]}"""
+    f"""Skipping: {"a" == "b"} {desc["ms_name"]} {money=} {dte=} {pos_share=} {desc["status"]} {desc["exposure_max"]}"""
 )
 
 log.info(
@@ -1430,7 +1442,7 @@ log.info(
 )
 
 log.info(
-    f"""Skipping: {'a' == 'b'} {desc['ms_name']} {money=} {dte=} {pos_share=} {desc['status']} {desc['exposure_max']}"""
+    f"""Skipping: {"a" == "b"} {desc["ms_name"]} {money=} {dte=} {pos_share=} {desc["status"]} {desc["exposure_max"]}"""
 )
 ```
 

crates/ruff_python_formatter/tests/snapshots/black_compatibility@cases__preview_long_strings__regression.py.snap

@@ -832,7 +832,7 @@ s = f'Lorem Ipsum is simply dummy text of the printing and typesetting industry:
  
  some_commented_string = (  # This comment stays at the top.
      "This string is long but not so long that it needs hahahah toooooo be so greatttt"
-@@ -279,36 +280,25 @@
+@@ -279,37 +280,26 @@
  )
  
  lpar_and_rpar_have_comments = func_call(  # LPAR Comment
@@ -852,31 +852,32 @@ s = f'Lorem Ipsum is simply dummy text of the printing and typesetting industry:
 -    f" {'' if ID is None else ID} | perl -nE 'print if /^{field}:/'"
 -)
 +cmd_fstring = f"sudo -E deluge-console info --detailed --sort-reverse=time_added {'' if ID is None else ID} | perl -nE 'print if /^{field}:/'"
++
++cmd_fstring = f"sudo -E deluge-console info --detailed --sort-reverse=time_added {'{{}}' if ID is None else ID} | perl -nE 'print if /^{field}:/'"
  
 -cmd_fstring = (
 -    "sudo -E deluge-console info --detailed --sort-reverse=time_added"
 -    f" {'{{}}' if ID is None else ID} | perl -nE 'print if /^{field}:/'"
 -)
-+cmd_fstring = f"sudo -E deluge-console info --detailed --sort-reverse=time_added {'{{}}' if ID is None else ID} | perl -nE 'print if /^{field}:/'"
++cmd_fstring = f"sudo -E deluge-console info --detailed --sort-reverse=time_added {{'' if ID is None else ID}} | perl -nE 'print if /^{field}:/'"
  
 -cmd_fstring = (
 -    "sudo -E deluge-console info --detailed --sort-reverse=time_added {'' if ID is"
 -    f" None else ID}} | perl -nE 'print if /^{field}:/'"
 -)
-+cmd_fstring = f"sudo -E deluge-console info --detailed --sort-reverse=time_added {{'' if ID is None else ID}} | perl -nE 'print if /^{field}:/'"
- 
 +fstring = f"This string really doesn't need to be an {{{{fstring}}}}, but this one most certainly, absolutely {does}."
-+
+ 
  fstring = (
 -    "This string really doesn't need to be an {{fstring}}, but this one most"
 -    f" certainly, absolutely {does}."
 +    f"We have to remember to escape {braces}." " Like {these}." f" But not {this}."
  )
--
--fstring = f"We have to remember to escape {braces}. Like {{these}}. But not {this}."
  
+-fstring = f"We have to remember to escape {braces}. Like {{these}}. But not {this}."
+-
  
  class A:
+     class B:
 @@ -364,10 +354,7 @@
          def foo():
              if not hasattr(module, name):
@@ -979,7 +980,13 @@ s = f'Lorem Ipsum is simply dummy text of the printing and typesetting industry:
  )
  
  # The parens should NOT be removed in this case.
-@@ -518,88 +494,78 @@
+@@ -513,93 +489,83 @@
+ 
+ 
+ temp_msg = (
+-    f"{f'{humanize_number(pos)}.': <{pound_len+2}} "
++    f"{f'{humanize_number(pos)}.': <{pound_len + 2}} "
+     f"{balance: <{bal_len + 5}} "
      f"<<{author.display_name}>>\n"
  )
  
@@ -1103,7 +1110,13 @@ s = f'Lorem Ipsum is simply dummy text of the printing and typesetting industry:
      "6. Click on Create Credential at the top."
      '7. At the top click the link for "API key".'
      "8. No application restrictions are needed. Click Create at the bottom."
-@@ -613,55 +579,40 @@
+@@ -608,60 +574,45 @@
+ 
+ # It shouldn't matter if the string prefixes are capitalized.
+ temp_msg = (
+-    f"{F'{humanize_number(pos)}.': <{pound_len+2}} "
++    f"{f'{humanize_number(pos)}.': <{pound_len + 2}} "
+     f"{balance: <{bal_len + 5}} "
      f"<<{author.display_name}>>\n"
  )
  
@@ -1688,7 +1701,7 @@ class X:
 
 
 temp_msg = (
-    f"{f'{humanize_number(pos)}.': <{pound_len+2}} "
+    f"{f'{humanize_number(pos)}.': <{pound_len + 2}} "
     f"{balance: <{bal_len + 5}} "
     f"<<{author.display_name}>>\n"
 )
@@ -1773,7 +1786,7 @@ message = (
 
 # It shouldn't matter if the string prefixes are capitalized.
 temp_msg = (
-    f"{F'{humanize_number(pos)}.': <{pound_len+2}} "
+    f"{f'{humanize_number(pos)}.': <{pound_len + 2}} "
     f"{balance: <{bal_len + 5}} "
     f"<<{author.display_name}>>\n"
 )

crates/ruff_python_formatter/tests/snapshots/format@expression__fstring_py312.py.snap

@@ -0,0 +1,54 @@
+---
+source: crates/ruff_python_formatter/tests/fixtures.rs
+input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/fstring_py312.py
+---
+## Input
+```python
+# This file contains test cases only for cases where the logic tests for whether
+# the target version is 3.12 or later. A user can have 3.12 syntax even if the target
+# version isn't set.
+
+# Quotes re-use
+f"{'a'}"
+```
+
+## Outputs
+### Output 1
+```
+indent-style               = space
+line-width                 = 88
+indent-width               = 4
+quote-style                = Double
+line-ending                = LineFeed
+magic-trailing-comma       = Respect
+docstring-code             = Disabled
+docstring-code-line-width  = "dynamic"
+preview                    = Disabled
+target_version             = Py312
+source_type                = Python
+```
+
+```python
+# This file contains test cases only for cases where the logic tests for whether
+# the target version is 3.12 or later. A user can have 3.12 syntax even if the target
+# version isn't set.
+
+# Quotes re-use
+f"{'a'}"
+```
+
+
+#### Preview changes
+```diff
+--- Stable
++++ Preview
+@@ -3,4 +3,4 @@
+ # version isn't set.
+ 
+ # Quotes re-use
+-f"{'a'}"
++f"{"a"}"
+```
+
+
+