Fix panic on RUF027 (astral-sh#9990)

## Summary

Fixes astral-sh#9895 

The cause for this panic came from an offset error in the code. When
analyzing a hypothetical f-string, we attempt to re-parse it as an
f-string, and use the AST data to determine, among other things, whether
the format specifiers are correct. To determine the 'correctness' of a
format specifier, we actually have to re-parse the format specifier, and
this is where the issue lies. To get the source text for the specifier,
we were taking a slice from the original file source text... even though
the AST data for the specifier belongs to the standalone parsed f-string
expression, meaning that the ranges are going to be way off. In a file
with Unicode, this can cause panics if the slice is inside a char
boundary.

To fix this, we now slice from the temporary source we created earlier
to parse the literal as an f-string.

## Test Plan

The RUF027 snapshot test was amended to include a string with format
specifiers which we _should_ be calling out. This is to ensure we do
slice format specifiers from the source text correctly.

crates/ruff_linter/resources/test/fixtures/ruff/RUF027_0.py

@@ -68,3 +68,7 @@ def method_calls():
     first = "Wendy"
     last = "Appleseed"
     value.method("{first} {last}")  # RUF027
+
+def format_specifiers():
+    a = 4
+    b = "{a:b} {a:^5}"

crates/ruff_linter/resources/test/fixtures/ruff/RUF027_2.py

@@ -0,0 +1,2 @@
+# 测试eval函数,eval()函数用来执行一个字符串表达式，并返t表达式的值。另外，可以讲字符串转换成列表或元组或字典
+a = "{1: 'a', 2: 'b'}"

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

@@ -48,6 +48,7 @@ mod tests {
     #[test_case(Rule::DefaultFactoryKwarg, Path::new("RUF026.py"))]
     #[test_case(Rule::MissingFStringSyntax, Path::new("RUF027_0.py"))]
     #[test_case(Rule::MissingFStringSyntax, Path::new("RUF027_1.py"))]
+    #[test_case(Rule::MissingFStringSyntax, Path::new("RUF027_2.py"))]
     fn rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!("{}_{}", rule_code.noqa_code(), path.to_string_lossy());
         let diagnostics = test_path(

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

@@ -88,8 +88,10 @@ fn should_be_fstring(
         return false;
     }
 
-    let Ok(ast::Expr::FString(ast::ExprFString { value, .. })) =
-        parse_expression(&format!("f{}", locator.slice(literal.range())))
+    let fstring_expr = format!("f{}", locator.slice(literal));
+
+    // Note: Range offsets for `value` are based on `fstring_expr`
+    let Ok(ast::Expr::FString(ast::ExprFString { value, .. })) = parse_expression(&fstring_expr)
     else {
         return false;
     };
@@ -159,7 +161,7 @@ fn should_be_fstring(
                 has_name = true;
             }
             if let Some(spec) = &element.format_spec {
-                let spec = locator.slice(spec.range());
+                let spec = &fstring_expr[spec.range()];
                 if FormatSpec::parse(spec).is_err() {
                     return false;
                 }

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__RUF027_RUF027_0.py.snap

@@ -285,6 +285,8 @@ RUF027_0.py:70:18: RUF027 [*] Possible f-string without an `f` prefix
 69 |     last = "Appleseed"
 70 |     value.method("{first} {last}")  # RUF027
    |                  ^^^^^^^^^^^^^^^^ RUF027
+71 | 
+72 | def format_specifiers():
    |
    = help: Add `f` prefix
 
@@ -294,5 +296,24 @@ RUF027_0.py:70:18: RUF027 [*] Possible f-string without an `f` prefix
 69 69 |     last = "Appleseed"
 70    |-    value.method("{first} {last}")  # RUF027
    70 |+    value.method(f"{first} {last}")  # RUF027
+71 71 | 
+72 72 | def format_specifiers():
+73 73 |     a = 4
+
+RUF027_0.py:74:9: RUF027 [*] Possible f-string without an `f` prefix
+   |
+72 | def format_specifiers():
+73 |     a = 4
+74 |     b = "{a:b} {a:^5}"
+   |         ^^^^^^^^^^^^^^ RUF027
+   |
+   = help: Add `f` prefix
+
+ℹ Unsafe fix
+71 71 | 
+72 72 | def format_specifiers():
+73 73 |     a = 4
+74    |-    b = "{a:b} {a:^5}"
+   74 |+    b = f"{a:b} {a:^5}"
 
 

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__RUF027_RUF027_2.py.snap

@@ -0,0 +1,4 @@
+---
+source: crates/ruff_linter/src/rules/ruff/mod.rs
+---
+