Avoid allocations for isort module names

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,

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 {
-    fn module_name(&self) -> String;
-    fn module_base(&self) -> String;
-}
-
-impl Importable for AliasData<'_> {
-    fn module_name(&self) -> String {
-        self.name.to_string()
+pub(crate) trait Importable<'a> {
+    fn module_name(&self) -> Cow<'a, str>;
+
+    fn module_base(&self) -> Cow<'a, str> {
+        match self.module_name() {
+            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<'a> Importable<'a> for AliasData<'a> {
+    fn module_name(&self) -> Cow<'a, str> {
+        Cow::Borrowed(self.name)
     }
 }
 
-impl Importable for ImportFromData<'_> {
-    fn module_name(&self) -> String {
+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 +715,22 @@ 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 {
-    let mut module_name = String::with_capacity(16);
-    if level > 0 {
-        for _ in 0..level {
-            module_name.push('.');
+pub fn format_import_from(level: u32, module: Option<&str>) -> Cow<str> {
+    match (level, module) {
+        (0, Some(module)) => Cow::Borrowed(module),
+        (level, module) => {
+            let mut module_name = String::with_capacity(
+                (level as usize) + module.as_ref().map_or(0, |module| module.len()),
+            );
+            for _ in 0..level {
+                module_name.push('.');
+            }
+            if let Some(module) = module {
+                module_name.push_str(module);
+            }
+            Cow::Owned(module_name)
         }
     }
-    if let Some(module) = module {
-        module_name.push_str(module);
-    }
-    module_name
 }
 
 /// Format the member reference name for a relative import.