Fix bug where PLE1307 was raised when formatting %c with characte…

…rs (#8407)

Closes #8406

---------

Co-authored-by: Dhruv Manilawala <dhruvmanila@gmail.com>

crates/ruff_linter/resources/test/fixtures/pylint/bad_string_format_type.py

@@ -57,3 +57,9 @@
 '(%r, %r, %r, %r)' % (hostname, address, username, '$PASSWORD')
 '%r' % ({'server_school_roles': server_school_roles, 'is_school_multiserver_domain': is_school_multiserver_domain}, )
 "%d" % (1 if x > 0 else 2)
+
+# Special cases for %c allowing single character strings
+# https://github.com/astral-sh/ruff/issues/8406
+"%c" % ("x",)
+"%c" % "x"
+"%c" % "œ"

crates/ruff_linter/src/rules/pylint/rules/bad_string_format_type.rs

@@ -119,12 +119,23 @@ fn collect_specs(formats: &[CFormatStrOrBytes<String>]) -> Vec<&CFormatSpec> {
 
 /// Return `true` if the format string is equivalent to the constant type
 fn equivalent(format: &CFormatSpec, value: &Expr) -> bool {
-    let format = FormatType::from(format.format_char);
+    let format_type = FormatType::from(format.format_char);
     match ResolvedPythonType::from(value) {
-        ResolvedPythonType::Atom(atom) => format.is_compatible_with(atom),
-        ResolvedPythonType::Union(atoms) => {
-            atoms.iter().all(|atom| format.is_compatible_with(*atom))
+        ResolvedPythonType::Atom(atom) => {
+            // Special case where `%c` allows single character strings to be formatted
+            if format.format_char == 'c' {
+                if let Expr::StringLiteral(string) = value {
+                    let mut chars = string.chars();
+                    if chars.next().is_some() && chars.next().is_none() {
+                        return true;
+                    }
+                }
+            }
+            format_type.is_compatible_with(atom)
         }
+        ResolvedPythonType::Union(atoms) => atoms
+            .iter()
+            .all(|atom| format_type.is_compatible_with(*atom)),
         ResolvedPythonType::Unknown => true,
         ResolvedPythonType::TypeError => true,
     }