Add support for nested replacements inside format specifications

crates/ruff/resources/test/fixtures/pylint/bad_string_format_character.py

@@ -14,7 +14,10 @@
 
 "{:s} {:y}".format("hello", "world")  # [bad-format-character]
 
-"{:*^30s}".format("centered")
+"{:*^30s}".format("centered") # OK
+"{:{s}}".format("hello", s="s")  # OK (nested replacement value not checked)
+
+"{:{s:y}}".format("hello", s="s")  # [bad-format-character] (nested replacement format spec checked)
 
 ## f-strings
 

crates/ruff/src/rules/pyflakes/format.rs

@@ -11,7 +11,9 @@ pub(crate) fn error_to_string(err: &FormatParseError) -> String {
         FormatParseError::InvalidCharacterAfterRightBracket => {
             "Only '.' or '[' may follow ']' in format field specifier"
         }
-        FormatParseError::InvalidFormatSpecifier => "Max string recursion exceeded",
+        FormatParseError::PlaceholderRecursionExceeded => {
+            "Max format placeholder recursion exceeded"
+        }
         FormatParseError::MissingStartBracket => "Single '}' encountered in format string",
         FormatParseError::MissingRightBracket => "Expected '}' before end of string",
         FormatParseError::UnmatchedBracket => "Single '{' encountered in format string",

crates/ruff/src/rules/pyflakes/snapshots/ruff__rules__pyflakes__tests__F521_F521.py.snap

@@ -28,7 +28,7 @@ F521.py:3:1: F521 `.format` call has invalid format string: Expected '}' before
 5 | "{:{:{}}}".format(1, 2, 3)
   |
 
-F521.py:5:1: F521 `.format` call has invalid format string: Max string recursion exceeded
+F521.py:5:1: F521 `.format` call has invalid format string: Max format placeholder recursion exceeded
   |
 3 | "{foo[}".format(foo=1)
 4 | # too much string recursion (placeholder-in-placeholder)

crates/ruff/src/rules/pylint/rules/bad_string_format_character.rs

@@ -49,16 +49,39 @@ pub(crate) fn call(checker: &mut Checker, string: &str, range: TextRange) {
                 continue;
             };
 
-            if let Err(FormatSpecError::InvalidFormatType) = FormatSpec::parse(format_spec) {
-                checker.diagnostics.push(Diagnostic::new(
-                    BadStringFormatCharacter {
-                        // The format type character is always the last one.
-                        // More info in the official spec:
-                        // https://docs.python.org/3/library/string.html#format-specification-mini-language
-                        format_char: format_spec.chars().last().unwrap(),
-                    },
-                    range,
-                ));
+            match FormatSpec::parse(format_spec) {
+                Err(FormatSpecError::InvalidFormatType) => {
+                    checker.diagnostics.push(Diagnostic::new(
+                        BadStringFormatCharacter {
+                            // The format type character is always the last one.
+                            // More info in the official spec:
+                            // https://docs.python.org/3/library/string.html#format-specification-mini-language
+                            format_char: format_spec.chars().last().unwrap(),
+                        },
+                        range,
+                    ));
+                }
+                Err(_) => {}
+                Ok(format_spec) => {
+                    for replacement in format_spec.replacements() {
+                        let FormatPart::Field { format_spec, .. } = replacement else {
+                            continue;
+                        };
+                        if let Err(FormatSpecError::InvalidFormatType) =
+                            FormatSpec::parse(format_spec)
+                        {
+                            checker.diagnostics.push(Diagnostic::new(
+                                BadStringFormatCharacter {
+                                    // The format type character is always the last one.
+                                    // More info in the official spec:
+                                    // https://docs.python.org/3/library/string.html#format-specification-mini-language
+                                    format_char: format_spec.chars().last().unwrap(),
+                                },
+                                range,
+                            ));
+                        }
+                    }
+                }
             }
         }
     }

crates/ruff/src/rules/pylint/snapshots/ruff__rules__pylint__tests__PLE1300_bad_string_format_character.py.snap

@@ -48,7 +48,17 @@ bad_string_format_character.py:15:1: PLE1300 Unsupported format character 'y'
 15 | "{:s} {:y}".format("hello", "world")  # [bad-format-character]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ PLE1300
 16 | 
-17 | "{:*^30s}".format("centered")
+17 | "{:*^30s}".format("centered") # OK
+   |
+
+bad_string_format_character.py:20:1: PLE1300 Unsupported format character 'y'
+   |
+18 | "{:{s}}".format("hello", s="s")  # OK (nested replacement value not checked)
+19 | 
+20 | "{:{s:y}}".format("hello", s="s")  # [bad-format-character] (nested replacement format spec checked)
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ PLE1300
+21 | 
+22 | ## f-strings
    |