Preserve quotes in generated f-strings (#15794)

## Summary

This is another follow-up to #15726 and #15778, extending the
quote-preserving behavior to f-strings and deleting the now-unused
`Generator::quote` field.

## Details
I also made one unrelated change to `rules/flynt/helpers.rs` to remove a
`to_string` call for making a `Box<str>` and tweaked some arguments to
some of the `Generator::unparse_f_string` methods to make the code
easier to follow, in my opinion. Happy to revert especially the latter
of these if needed.

Unfortunately this still does not fix the issue in #9660, which appears
to be more of an escaping issue than a quote-preservation issue. After
#15726, the result is now `a = f'# {"".join([])}' if 1 else ""` instead
of `a = f"# {''.join([])}" if 1 else ""` (single quotes on the outside
now), but we still don't have the desired behavior of double quotes
everywhere on Python 3.12+. I added a test for this but split it off
into another branch since it ended up being unaddressed here, but my
`dbg!` statements showed the correct preferred quotes going into
[`UnicodeEscape::with_preferred_quote`](https://github.com/astral-sh/ruff/blob/main/crates/ruff_python_literal/src/escape.rs#L54).

## Test Plan

Existing rule and `Generator` tests.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ruff_linter/resources/test/fixtures/flake8_simplify/SIM108.py

@@ -194,3 +194,17 @@ def f():
     z = not foo()
 else:
     z = other
+
+
+# These two cases double as tests for f-string quote preservation. The first
+# f-string should preserve its double quotes, and the second should preserve
+# single quotes
+if cond:
+    var = "str"
+else:
+    var = f"{first}-{second}"
+
+if cond:
+    var = "str"
+else:
+    var = f'{first}-{second}'

crates/ruff_linter/src/checkers/ast/mod.rs

@@ -294,11 +294,7 @@ impl<'a> Checker<'a> {
 
     /// Create a [`Generator`] to generate source code based on the current AST state.
     pub(crate) fn generator(&self) -> Generator {
-        Generator::new(
-            self.stylist.indentation(),
-            self.preferred_quote(),
-            self.stylist.line_ending(),
-        )
+        Generator::new(self.stylist.indentation(), self.stylist.line_ending())
     }
 
     /// Return the preferred quote for a generated `StringLiteral` node, given where we are in the
@@ -319,6 +315,12 @@ impl<'a> Checker<'a> {
         ast::BytesLiteralFlags::empty().with_quote_style(self.preferred_quote())
     }
 
+    /// Return the default f-string flags a generated `FString` node should use, given where we are
+    /// in the AST.
+    pub(crate) fn default_fstring_flags(&self) -> ast::FStringFlags {
+        ast::FStringFlags::empty().with_quote_style(self.preferred_quote())
+    }
+
     /// Returns the appropriate quoting for f-string by reversing the one used outside of
     /// the f-string.
     ///

crates/ruff_linter/src/rules/flake8_simplify/snapshots/ruff_linter__rules__flake8_simplify__tests__SIM108_SIM108.py.snap

@@ -301,3 +301,55 @@ SIM108.py:193:1: SIM108 [*] Use ternary operator `z = not foo() if foo() else ot
 195     |-else:
 196     |-    z = other
     193 |+z = not foo() if foo() else other
+197 194 | 
+198 195 | 
+199 196 | # These two cases double as tests for f-string quote preservation. The first
+
+SIM108.py:202:1: SIM108 [*] Use ternary operator `var = "str" if cond else f"{first}-{second}"` instead of `if`-`else`-block
+    |
+200 |   # f-string should preserve its double quotes, and the second should preserve
+201 |   # single quotes
+202 | / if cond:
+203 | |     var = "str"
+204 | | else:
+205 | |     var = f"{first}-{second}"
+    | |_____________________________^ SIM108
+206 |
+207 |   if cond:
+    |
+    = help: Replace `if`-`else`-block with `var = "str" if cond else f"{first}-{second}"`
+
+ℹ Unsafe fix
+199 199 | # These two cases double as tests for f-string quote preservation. The first
+200 200 | # f-string should preserve its double quotes, and the second should preserve
+201 201 | # single quotes
+202     |-if cond:
+203     |-    var = "str"
+204     |-else:
+205     |-    var = f"{first}-{second}"
+    202 |+var = "str" if cond else f"{first}-{second}"
+206 203 | 
+207 204 | if cond:
+208 205 |     var = "str"
+
+SIM108.py:207:1: SIM108 [*] Use ternary operator `var = "str" if cond else f'{first}-{second}'` instead of `if`-`else`-block
+    |
+205 |       var = f"{first}-{second}"
+206 |
+207 | / if cond:
+208 | |     var = "str"
+209 | | else:
+210 | |     var = f'{first}-{second}'
+    | |_____________________________^ SIM108
+    |
+    = help: Replace `if`-`else`-block with `var = "str" if cond else f'{first}-{second}'`
+
+ℹ Unsafe fix
+204 204 | else:
+205 205 |     var = f"{first}-{second}"
+206 206 | 
+207     |-if cond:
+208     |-    var = "str"
+209     |-else:
+210     |-    var = f'{first}-{second}'
+    207 |+var = "str" if cond else f'{first}-{second}'

crates/ruff_linter/src/rules/flake8_simplify/snapshots/ruff_linter__rules__flake8_simplify__tests__preview__SIM108_SIM108.py.snap

@@ -301,3 +301,55 @@ SIM108.py:193:1: SIM108 [*] Use ternary operator `z = not foo() if foo() else ot
 195     |-else:
 196     |-    z = other
     193 |+z = not foo() if foo() else other
+197 194 | 
+198 195 | 
+199 196 | # These two cases double as tests for f-string quote preservation. The first
+
+SIM108.py:202:1: SIM108 [*] Use ternary operator `var = "str" if cond else f"{first}-{second}"` instead of `if`-`else`-block
+    |
+200 |   # f-string should preserve its double quotes, and the second should preserve
+201 |   # single quotes
+202 | / if cond:
+203 | |     var = "str"
+204 | | else:
+205 | |     var = f"{first}-{second}"
+    | |_____________________________^ SIM108
+206 |
+207 |   if cond:
+    |
+    = help: Replace `if`-`else`-block with `var = "str" if cond else f"{first}-{second}"`
+
+ℹ Unsafe fix
+199 199 | # These two cases double as tests for f-string quote preservation. The first
+200 200 | # f-string should preserve its double quotes, and the second should preserve
+201 201 | # single quotes
+202     |-if cond:
+203     |-    var = "str"
+204     |-else:
+205     |-    var = f"{first}-{second}"
+    202 |+var = "str" if cond else f"{first}-{second}"
+206 203 | 
+207 204 | if cond:
+208 205 |     var = "str"
+
+SIM108.py:207:1: SIM108 [*] Use ternary operator `var = "str" if cond else f'{first}-{second}'` instead of `if`-`else`-block
+    |
+205 |       var = f"{first}-{second}"
+206 |
+207 | / if cond:
+208 | |     var = "str"
+209 | | else:
+210 | |     var = f'{first}-{second}'
+    | |_____________________________^ SIM108
+    |
+    = help: Replace `if`-`else`-block with `var = "str" if cond else f'{first}-{second}'`
+
+ℹ Unsafe fix
+204 204 | else:
+205 205 |     var = f"{first}-{second}"
+206 206 | 
+207     |-if cond:
+208     |-    var = "str"
+209     |-else:
+210     |-    var = f'{first}-{second}'
+    207 |+var = "str" if cond else f'{first}-{second}'

crates/ruff_linter/src/rules/flake8_type_checking/helpers.rs

@@ -363,11 +363,7 @@ impl<'a> QuoteAnnotator<'a> {
         let generator = Generator::from(self.stylist);
         // we first generate the annotation with the inverse quote, so we can
         // generate the string literal with the preferred quote
-        let subgenerator = Generator::new(
-            self.stylist.indentation(),
-            self.stylist.quote().opposite(),
-            self.stylist.line_ending(),
-        );
+        let subgenerator = Generator::new(self.stylist.indentation(), self.stylist.line_ending());
         let annotation = subgenerator.expr(&expr_without_forward_references);
         generator.expr(&Expr::from(ast::StringLiteral {
             range: TextRange::default(),

crates/ruff_linter/src/rules/flynt/helpers.rs

@@ -15,7 +15,7 @@ fn to_f_string_expression_element(inner: &Expr) -> ast::FStringElement {
 /// Convert a string to a [`ast::FStringElement::Literal`].
 pub(super) fn to_f_string_literal_element(s: &str) -> ast::FStringElement {
     ast::FStringElement::Literal(ast::FStringLiteralElement {
-        value: s.to_string().into_boxed_str(),
+        value: Box::from(s),
         range: TextRange::default(),
     })
 }

crates/ruff_linter/src/rules/flynt/rules/static_join_to_fstring.rs

@@ -60,7 +60,8 @@ fn is_static_length(elts: &[Expr]) -> bool {
     elts.iter().all(|e| !e.is_starred_expr())
 }
 
-fn build_fstring(joiner: &str, joinees: &[Expr]) -> Option<Expr> {
+/// Build an f-string consisting of `joinees` joined by `joiner` with `flags`.
+fn build_fstring(joiner: &str, joinees: &[Expr], flags: FStringFlags) -> Option<Expr> {
     // If all elements are string constants, join them into a single string.
     if joinees.iter().all(Expr::is_string_literal_expr) {
         let mut flags = None;
@@ -104,7 +105,7 @@ fn build_fstring(joiner: &str, joinees: &[Expr]) -> Option<Expr> {
     let node = ast::FString {
         elements: f_string_elements.into(),
         range: TextRange::default(),
-        flags: FStringFlags::default(),
+        flags,
     };
     Some(node.into())
 }
@@ -137,7 +138,7 @@ pub(crate) fn static_join_to_fstring(checker: &mut Checker, expr: &Expr, joiner:
 
     // Try to build the fstring (internally checks whether e.g. the elements are
     // convertible to f-string elements).
-    let Some(new_expr) = build_fstring(joiner, joinees) else {
+    let Some(new_expr) = build_fstring(joiner, joinees, checker.default_fstring_flags()) else {
         return;
     };
 

crates/ruff_linter/src/rules/ruff/rules/assert_with_print_message.rs

@@ -66,8 +66,7 @@ pub(crate) fn assert_with_print_message(checker: &mut Checker, stmt: &ast::StmtA
             diagnostic.set_fix(Fix::unsafe_edit(Edit::range_replacement(
                 checker.generator().stmt(&Stmt::Assert(ast::StmtAssert {
                     test: stmt.test.clone(),
-                    msg: print_arguments::to_expr(&call.arguments, checker.default_string_flags())
-                        .map(Box::new),
+                    msg: print_arguments::to_expr(&call.arguments, checker).map(Box::new),
                     range: TextRange::default(),
                 })),
                 // We have to replace the entire statement,
@@ -96,6 +95,8 @@ mod print_arguments {
     };
     use ruff_text_size::TextRange;
 
+    use crate::checkers::ast::Checker;
+
     /// Converts an expression to a list of `FStringElement`s.
     ///
     /// Three cases are handled:
@@ -222,6 +223,7 @@ mod print_arguments {
     fn args_to_fstring_expr(
         mut args: impl ExactSizeIterator<Item = Vec<FStringElement>>,
         sep: impl ExactSizeIterator<Item = FStringElement>,
+        flags: FStringFlags,
     ) -> Option<Expr> {
         // If there are no arguments, short-circuit and return `None`
         let first_arg = args.next()?;
@@ -236,7 +238,7 @@ mod print_arguments {
         Some(Expr::FString(ExprFString {
             value: FStringValue::single(FString {
                 elements: FStringElements::from(fstring_elements),
-                flags: FStringFlags::default(),
+                flags,
                 range: TextRange::default(),
             }),
             range: TextRange::default(),
@@ -256,7 +258,7 @@ mod print_arguments {
     /// - [`Some`]<[`Expr::StringLiteral`]> if all arguments including `sep` are string literals.
     /// - [`Some`]<[`Expr::FString`]> if any of the arguments are not string literals.
     /// - [`None`] if the `print` contains no positional arguments at all.
-    pub(super) fn to_expr(arguments: &Arguments, flags: StringLiteralFlags) -> Option<Expr> {
+    pub(super) fn to_expr(arguments: &Arguments, checker: &Checker) -> Option<Expr> {
         // Convert the `sep` argument into `FStringElement`s
         let sep = arguments
             .find_keyword("sep")
@@ -286,7 +288,13 @@ mod print_arguments {
 
         // Attempt to convert the `sep` and `args` arguments to a string literal,
         // falling back to an f-string if the arguments are not all string literals.
-        args_to_string_literal_expr(args.iter(), sep.iter(), flags)
-            .or_else(|| args_to_fstring_expr(args.into_iter(), sep.into_iter()))
+        args_to_string_literal_expr(args.iter(), sep.iter(), checker.default_string_flags())
+            .or_else(|| {
+                args_to_fstring_expr(
+                    args.into_iter(),
+                    sep.into_iter(),
+                    checker.default_fstring_flags(),
+                )
+            })
     }
 }

crates/ruff_python_ast/src/nodes.rs

@@ -1063,10 +1063,32 @@ bitflags! {
 
 /// Flags that can be queried to obtain information
 /// regarding the prefixes and quotes used for an f-string.
-#[derive(Default, Copy, Clone, Eq, PartialEq, Hash)]
+///
+/// ## Notes on usage
+///
+/// If you're using a `Generator` from the `ruff_python_codegen` crate to generate a lint-rule fix
+/// from an existing f-string literal, consider passing along the [`FString::flags`] field. If you
+/// don't have an existing literal but have a `Checker` from the `ruff_linter` crate available,
+/// consider using `Checker::default_fstring_flags` to create instances of this struct; this method
+/// will properly handle nested f-strings. For usage that doesn't fit into one of these categories,
+/// the public constructor [`FStringFlags::empty`] can be used.
+#[derive(Copy, Clone, Eq, PartialEq, Hash)]
 pub struct FStringFlags(FStringFlagsInner);
 
 impl FStringFlags {
+    /// Construct a new [`FStringFlags`] with **no flags set**.
+    ///
+    /// See [`FStringFlags::with_quote_style`], [`FStringFlags::with_triple_quotes`], and
+    /// [`FStringFlags::with_prefix`] for ways of setting the quote style (single or double),
+    /// enabling triple quotes, and adding prefixes (such as `r`), respectively.
+    ///
+    /// See the documentation for [`FStringFlags`] for additional caveats on this constructor, and
+    /// situations in which alternative ways to construct this struct should be used, especially
+    /// when writing lint rules.
+    pub fn empty() -> Self {
+        Self(FStringFlagsInner::empty())
+    }
+
     #[must_use]
     pub fn with_quote_style(mut self, quote_style: Quote) -> Self {
         self.0
@@ -2229,7 +2251,7 @@ impl From<AnyStringFlags> for FStringFlags {
                 value.prefix()
             )
         };
-        let new = FStringFlags::default()
+        let new = FStringFlags::empty()
             .with_quote_style(value.quote_style())
             .with_prefix(fstring_prefix);
         if value.is_triple_quoted() {