[flake8-quotes] Fix Autofix Error (Q000, Q002) (#10199)

## Summary
In issue #6785 it is reported
that a docstring in the form of `''"assert" ' SAM macro definitions '''`
is autocorrected to `"""assert" ' SAM macro definitions '''` (note the
triple quotes one only one side), which breaks the python program due
`undetermined string lateral`.

* `Q002`: Not only would docstrings in the form of `''"assert" ' SAM
macro definitions '''` (single quotes) be autofixed wrongly, but also
e.g. `""'assert' ' SAM macro definitions '''` (double quotes). The bug
is present for docstrings in all scopes (e.g. module docstrings, class
docstrings, function docstrings)

* `Q000`: The autofix error is not only present for `Q002` (docstrings),
but also for inline strings (`Q000`). Therefore `s = ''"assert" ' SAM
macro definitions '''` will also be wrongly autofixed.

Note that situation in which the first string is non-empty can be fixed,
e.g. `'123'"assert" ' SAM macro definitions '''` -> `"123""assert" ' SAM
macro definitions '''` is valid.

## What
* Change FixAvailability of `Q000` `Q002` to `Sometimes`
* Changed both rules such that docstrings/inline strings that cannot be
fixed are still reported as bad quotes via diagnostics, but no fix is
provided

## Test Plan
* For `Q000`: Add docstrings in different scopes that (partially) would
have been autofixed wrongly
* For `Q002`: Add inline strings that (partially) would have been
autofixed wrongly

Closes #6785

crates/ruff_linter/resources/test/fixtures/flake8_quotes/docstring_doubles_mixed_quotes_class_var_1.py

@@ -0,0 +1,9 @@
+class SingleLineDocstrings():
+    ""'Start with empty string' ' and lint docstring safely'
+    """ Not a docstring """
+
+    def foo(self, bar="""not a docstring"""):
+        ""'Start with empty string' ' and lint docstring safely'
+        pass
+
+    class Nested(foo()[:]): ""'Start with empty string' ' and lint docstring safely'; pass

crates/ruff_linter/resources/test/fixtures/flake8_quotes/docstring_doubles_mixed_quotes_class_var_2.py

@@ -0,0 +1,9 @@
+class SingleLineDocstrings():
+    "Do not"' start with empty string' ' and lint docstring safely'
+    """ Not a docstring """
+
+    def foo(self, bar="""not a docstring"""):
+        "Do not"' start with empty string' ' and lint docstring safely'
+        pass
+
+    class Nested(foo()[:]): "Do not"' start with empty string' ' and lint docstring safely'; pass

crates/ruff_linter/resources/test/fixtures/flake8_quotes/docstring_doubles_mixed_quotes_module_singleline_var_1.py

@@ -0,0 +1,5 @@
+""'Start with empty string' ' and lint docstring safely'
+
+def foo():
+    pass
+""" this is not a docstring """

crates/ruff_linter/resources/test/fixtures/flake8_quotes/docstring_doubles_mixed_quotes_module_singleline_var_2.py

@@ -0,0 +1,5 @@
+"Do not"' start with empty string' ' and lint docstring safely'
+
+def foo():
+    pass
+""" this is not a docstring """

crates/ruff_linter/resources/test/fixtures/flake8_quotes/docstring_singles_mixed_quotes_class_var_1.py

@@ -0,0 +1,9 @@
+class SingleLineDocstrings():
+    ''"Start with empty string" ' and lint docstring safely'
+    ''' Not a docstring '''
+
+    def foo(self, bar='''not a docstring'''):
+        ''"Start with empty string" ' and lint docstring safely'
+        pass
+
+    class Nested(foo()[:]): ''"Start with empty string" ' and lint docstring safely'; pass

crates/ruff_linter/resources/test/fixtures/flake8_quotes/docstring_singles_mixed_quotes_class_var_2.py

@@ -0,0 +1,9 @@
+class SingleLineDocstrings():
+    'Do not'" start with empty string" ' and lint docstring safely'
+    ''' Not a docstring '''
+
+    def foo(self, bar='''not a docstring'''):
+        'Do not'" start with empty string" ' and lint docstring safely'
+        pass
+
+    class Nested(foo()[:]): 'Do not'" start with empty string" ' and lint docstring safely'; pass

crates/ruff_linter/resources/test/fixtures/flake8_quotes/docstring_singles_mixed_quotes_module_singleline_var_1.py

@@ -0,0 +1,5 @@
+''"Start with empty string" ' and lint docstring safely'
+
+def foo():
+    pass
+""" this is not a docstring """

crates/ruff_linter/resources/test/fixtures/flake8_quotes/docstring_singles_mixed_quotes_module_singleline_var_2.py

@@ -0,0 +1,5 @@
+'Do not'" start with empty string" ' and lint docstring safely'
+
+def foo():
+    pass
+""" this is not a docstring """

crates/ruff_linter/resources/test/fixtures/flake8_quotes/doubles_would_be_triple_quotes.py

@@ -0,0 +1,2 @@
+s = ""'Start with empty string' ' and lint docstring safely'
+s = "Do not"' start with empty string' ' and lint docstring safely'

crates/ruff_linter/resources/test/fixtures/flake8_quotes/singles_would_be_triple_quotes.py

@@ -0,0 +1,2 @@
+s = ''"Start with empty string" ' and lint docstring safely'
+s = 'Do not'" start with empty string" ' and lint docstring safely'

crates/ruff_linter/src/rules/flake8_quotes/mod.rs

@@ -24,6 +24,7 @@ mod tests {
     #[test_case(Path::new("doubles_multiline_string.py"))]
     #[test_case(Path::new("doubles_noqa.py"))]
     #[test_case(Path::new("doubles_wrapped.py"))]
+    #[test_case(Path::new("doubles_would_be_triple_quotes.py"))]
     fn require_singles(path: &Path) -> Result<()> {
         let snapshot = format!("require_singles_over_{}", path.to_string_lossy());
         let diagnostics = test_path(
@@ -93,6 +94,7 @@ mod tests {
     #[test_case(Path::new("singles_multiline_string.py"))]
     #[test_case(Path::new("singles_noqa.py"))]
     #[test_case(Path::new("singles_wrapped.py"))]
+    #[test_case(Path::new("singles_would_be_triple_quotes.py"))]
     fn require_doubles(path: &Path) -> Result<()> {
         let snapshot = format!("require_doubles_over_{}", path.to_string_lossy());
         let diagnostics = test_path(
@@ -127,6 +129,10 @@ mod tests {
     #[test_case(Path::new("docstring_singles_module_singleline.py"))]
     #[test_case(Path::new("docstring_singles_class.py"))]
     #[test_case(Path::new("docstring_singles_function.py"))]
+    #[test_case(Path::new("docstring_singles_mixed_quotes_module_singleline_var_1.py"))]
+    #[test_case(Path::new("docstring_singles_mixed_quotes_module_singleline_var_2.py"))]
+    #[test_case(Path::new("docstring_singles_mixed_quotes_class_var_1.py"))]
+    #[test_case(Path::new("docstring_singles_mixed_quotes_class_var_2.py"))]
     fn require_docstring_doubles(path: &Path) -> Result<()> {
         let snapshot = format!("require_docstring_doubles_over_{}", path.to_string_lossy());
         let diagnostics = test_path(
@@ -161,6 +167,10 @@ mod tests {
     #[test_case(Path::new("docstring_singles_module_singleline.py"))]
     #[test_case(Path::new("docstring_singles_class.py"))]
     #[test_case(Path::new("docstring_singles_function.py"))]
+    #[test_case(Path::new("docstring_doubles_mixed_quotes_module_singleline_var_1.py"))]
+    #[test_case(Path::new("docstring_doubles_mixed_quotes_module_singleline_var_2.py"))]
+    #[test_case(Path::new("docstring_doubles_mixed_quotes_class_var_1.py"))]
+    #[test_case(Path::new("docstring_doubles_mixed_quotes_class_var_2.py"))]
     fn require_docstring_singles(path: &Path) -> Result<()> {
         let snapshot = format!("require_docstring_singles_over_{}", path.to_string_lossy());
         let diagnostics = test_path(

crates/ruff_linter/src/rules/flake8_quotes/rules/check_string_quotes.rs

@@ -2,7 +2,7 @@ use ruff_python_parser::lexer::LexResult;
 use ruff_python_parser::Tok;
 use ruff_text_size::{TextRange, TextSize};
 
-use ruff_diagnostics::{AlwaysFixableViolation, Diagnostic, Edit, Fix};
+use ruff_diagnostics::{AlwaysFixableViolation, Diagnostic, Edit, Fix, FixAvailability, Violation};
 use ruff_macros::{derive_message_formats, violation};
 use ruff_source_file::Locator;
 
@@ -44,7 +44,9 @@ pub struct BadQuotesInlineString {
     preferred_quote: Quote,
 }
 
-impl AlwaysFixableViolation for BadQuotesInlineString {
+impl Violation for BadQuotesInlineString {
+    const FIX_AVAILABILITY: FixAvailability = FixAvailability::Sometimes;
+
     #[derive_message_formats]
     fn message(&self) -> String {
         let BadQuotesInlineString { preferred_quote } = self;
@@ -54,11 +56,11 @@ impl AlwaysFixableViolation for BadQuotesInlineString {
         }
     }
 
-    fn fix_title(&self) -> String {
+    fn fix_title(&self) -> Option<String> {
         let BadQuotesInlineString { preferred_quote } = self;
         match preferred_quote {
-            Quote::Double => "Replace single quotes with double quotes".to_string(),
-            Quote::Single => "Replace double quotes with single quotes".to_string(),
+            Quote::Double => Some("Replace single quotes with double quotes".to_string()),
+            Quote::Single => Some("Replace double quotes with single quotes".to_string()),
         }
     }
 }
@@ -155,7 +157,9 @@ pub struct BadQuotesDocstring {
     preferred_quote: Quote,
 }
 
-impl AlwaysFixableViolation for BadQuotesDocstring {
+impl Violation for BadQuotesDocstring {
+    const FIX_AVAILABILITY: FixAvailability = FixAvailability::Sometimes;
+
     #[derive_message_formats]
     fn message(&self) -> String {
         let BadQuotesDocstring { preferred_quote } = self;
@@ -165,11 +169,11 @@ impl AlwaysFixableViolation for BadQuotesDocstring {
         }
     }
 
-    fn fix_title(&self) -> String {
+    fn fix_title(&self) -> Option<String> {
         let BadQuotesDocstring { preferred_quote } = self;
         match preferred_quote {
-            Quote::Double => "Replace single quotes docstring with double quotes".to_string(),
-            Quote::Single => "Replace double quotes docstring with single quotes".to_string(),
+            Quote::Double => Some("Replace single quotes docstring with double quotes".to_string()),
+            Quote::Single => Some("Replace double quotes docstring with single quotes".to_string()),
         }
     }
 }
@@ -188,10 +192,10 @@ const fn good_multiline_ending(quote: Quote) -> &'static str {
     }
 }
 
-const fn good_docstring(quote: Quote) -> &'static str {
+const fn good_docstring(quote: Quote) -> char {
     match quote {
-        Quote::Double => "\"",
-        Quote::Single => "'",
+        Quote::Double => '"',
+        Quote::Single => '\'',
     }
 }
 
@@ -203,6 +207,12 @@ struct Trivia<'a> {
     is_multiline: bool,
 }
 
+impl Trivia<'_> {
+    fn has_empty_text(&self) -> bool {
+        self.raw_text == "\"\"" || self.raw_text == "''"
+    }
+}
+
 impl<'a> From<&'a str> for Trivia<'a> {
     fn from(value: &'a str) -> Self {
         // Remove any prefixes (e.g., remove `u` from `u"foo"`).
@@ -231,12 +241,38 @@ impl<'a> From<&'a str> for Trivia<'a> {
     }
 }
 
+/// Returns `true` if the [`TextRange`] is preceded by two consecutive quotes.
+fn text_starts_at_consecutive_quote(locator: &Locator, range: TextRange, quote: Quote) -> bool {
+    let mut previous_two_chars = locator.up_to(range.start()).chars().rev();
+    previous_two_chars.next() == Some(good_docstring(quote))
+        && previous_two_chars.next() == Some(good_docstring(quote))
+}
+
+/// Returns `true` if the [`TextRange`] ends at a quote character.
+fn text_ends_at_quote(locator: &Locator, range: TextRange, quote: Quote) -> bool {
+    locator
+        .after(range.end())
+        .starts_with(good_docstring(quote))
+}
+
 /// Q002
 fn docstring(locator: &Locator, range: TextRange, settings: &LinterSettings) -> Option<Diagnostic> {
     let quotes_settings = &settings.flake8_quotes;
 
     let text = locator.slice(range);
     let trivia: Trivia = text.into();
+    if trivia.has_empty_text()
+        && text_ends_at_quote(locator, range, settings.flake8_quotes.docstring_quotes)
+    {
+        // Fixing this would result in a one-sided multi-line docstring, which would
+        // introduce a syntax error.
+        return Some(Diagnostic::new(
+            BadQuotesDocstring {
+                preferred_quote: quotes_settings.docstring_quotes,
+            },
+            range,
+        ));
+    }
 
     if trivia
         .raw_text
@@ -253,7 +289,9 @@ fn docstring(locator: &Locator, range: TextRange, settings: &LinterSettings) ->
     );
     let quote_count = if trivia.is_multiline { 3 } else { 1 };
     let string_contents = &trivia.raw_text[quote_count..trivia.raw_text.len() - quote_count];
-    let quote = good_docstring(quotes_settings.docstring_quotes).repeat(quote_count);
+    let quote = good_docstring(quotes_settings.docstring_quotes)
+        .to_string()
+        .repeat(quote_count);
     let mut fixed_contents =
         String::with_capacity(trivia.prefix.len() + string_contents.len() + quote.len() * 2);
     fixed_contents.push_str(trivia.prefix);
@@ -344,6 +382,42 @@ fn strings(
             // If we're not using the preferred type, only allow use to avoid escapes.
             && !relax_quote
         {
+            if trivia.has_empty_text()
+                && text_ends_at_quote(locator, *range, settings.flake8_quotes.inline_quotes)
+            {
+                // Fixing this would introduce a syntax error. For example, changing the initial
+                // single quotes to double quotes would result in a syntax error:
+                // ```python
+                // ''"assert" ' SAM macro definitions '''
+                // ```
+                diagnostics.push(Diagnostic::new(
+                    BadQuotesInlineString {
+                        preferred_quote: quotes_settings.inline_quotes,
+                    },
+                    *range,
+                ));
+                continue;
+            }
+
+            if text_starts_at_consecutive_quote(
+                locator,
+                *range,
+                settings.flake8_quotes.inline_quotes,
+            ) {
+                // Fixing this would introduce a syntax error. For example, changing the double
+                // doubles to single quotes would result in a syntax error:
+                // ```python
+                // ''"assert" ' SAM macro definitions '''
+                // ```
+                diagnostics.push(Diagnostic::new(
+                    BadQuotesInlineString {
+                        preferred_quote: quotes_settings.inline_quotes,
+                    },
+                    *range,
+                ));
+                continue;
+            }
+
             let mut diagnostic = Diagnostic::new(
                 BadQuotesInlineString {
                     preferred_quote: quotes_settings.inline_quotes,

crates/ruff_linter/src/rules/flake8_quotes/snapshots/ruff_linter__rules__flake8_quotes__tests__require_docstring_doubles_over_docstring_singles_mixed_quotes_class_var_1.py.snap

@@ -0,0 +1,56 @@
+---
+source: crates/ruff_linter/src/rules/flake8_quotes/mod.rs
+---
+docstring_singles_mixed_quotes_class_var_1.py:2:5: Q002 Single quote docstring found but double quotes preferred
+  |
+1 | class SingleLineDocstrings():
+2 |     ''"Start with empty string" ' and lint docstring safely'
+  |     ^^ Q002
+3 |     ''' Not a docstring '''
+  |
+  = help: Replace single quotes docstring with double quotes
+
+docstring_singles_mixed_quotes_class_var_1.py:2:7: Q000 Double quotes found but single quotes preferred
+  |
+1 | class SingleLineDocstrings():
+2 |     ''"Start with empty string" ' and lint docstring safely'
+  |       ^^^^^^^^^^^^^^^^^^^^^^^^^ Q000
+3 |     ''' Not a docstring '''
+  |
+  = help: Replace double quotes with single quotes
+
+docstring_singles_mixed_quotes_class_var_1.py:6:9: Q002 Single quote docstring found but double quotes preferred
+  |
+5 |     def foo(self, bar='''not a docstring'''):
+6 |         ''"Start with empty string" ' and lint docstring safely'
+  |         ^^ Q002
+7 |         pass
+  |
+  = help: Replace single quotes docstring with double quotes
+
+docstring_singles_mixed_quotes_class_var_1.py:6:11: Q000 Double quotes found but single quotes preferred
+  |
+5 |     def foo(self, bar='''not a docstring'''):
+6 |         ''"Start with empty string" ' and lint docstring safely'
+  |           ^^^^^^^^^^^^^^^^^^^^^^^^^ Q000
+7 |         pass
+  |
+  = help: Replace double quotes with single quotes
+
+docstring_singles_mixed_quotes_class_var_1.py:9:29: Q002 Single quote docstring found but double quotes preferred
+  |
+7 |         pass
+8 | 
+9 |     class Nested(foo()[:]): ''"Start with empty string" ' and lint docstring safely'; pass
+  |                             ^^ Q002
+  |
+  = help: Replace single quotes docstring with double quotes
+
+docstring_singles_mixed_quotes_class_var_1.py:9:31: Q000 Double quotes found but single quotes preferred
+  |
+7 |         pass
+8 | 
+9 |     class Nested(foo()[:]): ''"Start with empty string" ' and lint docstring safely'; pass
+  |                               ^^^^^^^^^^^^^^^^^^^^^^^^^ Q000
+  |
+  = help: Replace double quotes with single quotes