Avoid allocations for isort module names (#11251)

## Summary

Random refactor I noticed when investigating the F401 changes. We don't
need to allocate in most cases here.

crates/ruff_linter/src/checkers/ast/analyze/definitions.rs

@@ -109,7 +109,7 @@ pub(crate) fn definitions(checker: &mut Checker) {
     };
 
     let definitions = std::mem::take(&mut checker.semantic.definitions);
-    let mut overloaded_name: Option<String> = None;
+    let mut overloaded_name: Option<&str> = None;
     for ContextualizedDefinition {
         definition,
         visibility,
@@ -127,7 +127,7 @@ pub(crate) fn definitions(checker: &mut Checker) {
             if !overloaded_name.is_some_and(|overloaded_name| {
                 flake8_annotations::helpers::is_overload_impl(
                     definition,
-                    &overloaded_name,
+                    overloaded_name,
                     &checker.semantic,
                 )
             }) {

crates/ruff_linter/src/checkers/ast/analyze/statement.rs

@@ -877,7 +877,7 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
                         if !matches!(checker.semantic.current_scope().kind, ScopeKind::Module) {
                             checker.diagnostics.push(Diagnostic::new(
                                 pyflakes::rules::UndefinedLocalWithNestedImportStarUsage {
-                                    name: helpers::format_import_from(level, module),
+                                    name: helpers::format_import_from(level, module).to_string(),
                                 },
                                 stmt.range(),
                             ));
@@ -886,7 +886,7 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
                     if checker.enabled(Rule::UndefinedLocalWithImportStar) {
                         checker.diagnostics.push(Diagnostic::new(
                             pyflakes::rules::UndefinedLocalWithImportStar {
-                                name: helpers::format_import_from(level, module),
+                                name: helpers::format_import_from(level, module).to_string(),
                             },
                             stmt.range(),
                         ));

crates/ruff_linter/src/rules/flake8_annotations/helpers.rs

@@ -17,10 +17,13 @@ use crate::importer::{ImportRequest, Importer};
 use crate::settings::types::PythonVersion;
 
 /// Return the name of the function, if it's overloaded.
-pub(crate) fn overloaded_name(definition: &Definition, semantic: &SemanticModel) -> Option<String> {
+pub(crate) fn overloaded_name<'a>(
+    definition: &'a Definition,
+    semantic: &SemanticModel,
+) -> Option<&'a str> {
     let function = definition.as_function_def()?;
     if visibility::is_overload(&function.decorator_list, semantic) {
-        Some(function.name.to_string())
+        Some(function.name.as_str())
     } else {
         None
     }

crates/ruff_linter/src/rules/isort/types.rs

@@ -29,29 +29,38 @@ pub(crate) struct CommentSet<'a> {
     pub(crate) inline: Vec<Cow<'a, str>>,
 }
 
-pub(crate) trait Importable {
+pub(crate) trait Importable<'a> {
-    fn module_name(&self) -> String;
+    fn module_name(&self) -> Cow<'a, str>;
-    fn module_base(&self) -> String;
-}
 
-impl Importable for AliasData<'_> {
+    fn module_base(&self) -> Cow<'a, str> {
-    fn module_name(&self) -> String {
+        match self.module_name() {
-        self.name.to_string()
+            Cow::Borrowed(module_name) => Cow::Borrowed(
+                module_name
+                    .split('.')
+                    .next()
+                    .expect("module to include at least one segment"),
+            ),
+            Cow::Owned(module_name) => Cow::Owned(
+                module_name
+                    .split('.')
+                    .next()
+                    .expect("module to include at least one segment")
+                    .to_owned(),
+            ),
         }
-
-    fn module_base(&self) -> String {
-        self.module_name().split('.').next().unwrap().to_string()
     }
 }
 
-impl Importable for ImportFromData<'_> {
+impl<'a> Importable<'a> for AliasData<'a> {
-    fn module_name(&self) -> String {
+    fn module_name(&self) -> Cow<'a, str> {
+        Cow::Borrowed(self.name)
+    }
+}
+
+impl<'a> Importable<'a> for ImportFromData<'a> {
+    fn module_name(&self) -> Cow<'a, str> {
         format_import_from(self.level, self.module)
     }
-
-    fn module_base(&self) -> String {
-        self.module_name().split('.').next().unwrap().to_string()
-    }
 }
 
 #[derive(Debug, Default)]

crates/ruff_python_ast/src/helpers.rs

@@ -715,17 +715,21 @@ where
 /// assert_eq!(format_import_from(1, None), ".".to_string());
 /// assert_eq!(format_import_from(1, Some("foo")), ".foo".to_string());
 /// ```
-pub fn format_import_from(level: u32, module: Option<&str>) -> String {
+pub fn format_import_from(level: u32, module: Option<&str>) -> Cow<str> {
-    let mut module_name = String::with_capacity(16);
+    match (level, module) {
-    if level > 0 {
+        (0, Some(module)) => Cow::Borrowed(module),
+        (level, module) => {
+            let mut module_name =
+                String::with_capacity((level as usize) + module.map_or(0, str::len));
             for _ in 0..level {
                 module_name.push('.');
             }
-    }
             if let Some(module) = module {
                 module_name.push_str(module);
             }
-    module_name
+            Cow::Owned(module_name)
+        }
+    }
 }
 
 /// Format the member reference name for a relative import.
@@ -740,9 +744,8 @@ pub fn format_import_from(level: u32, module: Option<&str>) -> String {
 /// assert_eq!(format_import_from_member(1, Some("foo"), "bar"), ".foo.bar".to_string());
 /// ```
 pub fn format_import_from_member(level: u32, module: Option<&str>, member: &str) -> String {
-    let mut qualified_name = String::with_capacity(
+    let mut qualified_name =
-        (level as usize) + module.as_ref().map_or(0, |module| module.len()) + 1 + member.len(),
+        String::with_capacity((level as usize) + module.map_or(0, str::len) + 1 + member.len());
-    );
     if level > 0 {
         for _ in 0..level {
             qualified_name.push('.');