Explicit `as_str` (no deref), add no allocation methods (#8826)

## Summary

This PR is a follow-up to the AST refactor which does the following:
- Remove `Deref` implementation on `StringLiteralValue` and use explicit
`as_str` calls instead. The `Deref` implementation would implicitly
perform allocations in case of implicitly concatenated strings. This is
to make sure the allocation is explicit.
- Now, certain methods can be implemented to do zero allocations which
have been implemented in this PR. They are:
    - `is_empty`
    - `len`
    - `chars`
    - Custom `PartialEq` implementation to compare each character

## Test Plan

Run the linter test suite and make sure all tests pass.

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

@@ -369,18 +369,24 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
             ]) {
                 if let Expr::Attribute(ast::ExprAttribute { value, attr, .. }) = func.as_ref() {
                     let attr = attr.as_str();
-                    if let Expr::StringLiteral(ast::ExprStringLiteral { value: string, .. }) =
-                        value.as_ref()
+                    if let Expr::StringLiteral(ast::ExprStringLiteral {
+                        value: string_value,
+                        ..
+                    }) = value.as_ref()
                     {
                         if attr == "join" {
                             // "...".join(...) call
                             if checker.enabled(Rule::StaticJoinToFString) {
-                                flynt::rules::static_join_to_fstring(checker, expr, string);
+                                flynt::rules::static_join_to_fstring(
+                                    checker,
+                                    expr,
+                                    string_value.as_str(),
+                                );
                             }
                         } else if attr == "format" {
                             // "...".format(...) call
                             let location = expr.range();
-                            match pyflakes::format::FormatSummary::try_from(string.as_ref()) {
+                            match pyflakes::format::FormatSummary::try_from(string_value.as_str()) {
                                 Err(e) => {
                                     if checker.enabled(Rule::StringDotFormatInvalidFormat) {
                                         checker.diagnostics.push(Diagnostic::new(
@@ -425,7 +431,9 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
 
                             if checker.enabled(Rule::BadStringFormatCharacter) {
                                 pylint::rules::bad_string_format_character::call(
-                                    checker, string, location,
+                                    checker,
+                                    string_value.as_str(),
+                                    location,
                                 );
                             }
                         }

crates/ruff_linter/src/checkers/ast/mod.rs

@@ -799,7 +799,7 @@ where
             if let Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) = expr {
                 self.deferred.string_type_definitions.push((
                     expr.range(),
-                    value,
+                    value.as_str(),
                     self.semantic.snapshot(),
                 ));
             } else {
@@ -1219,7 +1219,7 @@ where
                 {
                     self.deferred.string_type_definitions.push((
                         expr.range(),
-                        value,
+                        value.as_str(),
                         self.semantic.snapshot(),
                     ));
                 }

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

@@ -505,14 +505,10 @@ fn check_dynamically_typed<F>(
 ) where
     F: FnOnce() -> String,
 {
-    if let Expr::StringLiteral(ast::ExprStringLiteral {
-        range,
-        value: string,
-    }) = annotation
-    {
+    if let Expr::StringLiteral(ast::ExprStringLiteral { range, value }) = annotation {
         // Quoted annotations
         if let Ok((parsed_annotation, _)) =
-            parse_type_annotation(string, *range, checker.locator().contents())
+            parse_type_annotation(value.as_str(), *range, checker.locator().contents())
         {
             if type_hint_resolves_to_any(
                 &parsed_annotation,

crates/ruff_linter/src/rules/flake8_bandit/rules/hardcoded_password_string.rs

@@ -55,7 +55,7 @@ fn password_target(target: &Expr) -> Option<&str> {
         Expr::Name(ast::ExprName { id, .. }) => id.as_str(),
         // d["password"] = "s3cr3t"
         Expr::Subscript(ast::ExprSubscript { slice, .. }) => match slice.as_ref() {
-            Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) => value,
+            Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) => value.as_str(),
             _ => return None,
         },
         // obj.password = "s3cr3t"

crates/ruff_linter/src/rules/flake8_bandit/rules/hardcoded_sql_expression.rs

@@ -93,7 +93,7 @@ pub(crate) fn hardcoded_sql_expression(checker: &mut Checker, expr: &Expr) {
             let Some(string) = left.as_string_literal_expr() else {
                 return;
             };
-            string.value.escape_default().to_string()
+            string.value.as_str().escape_default().to_string()
         }
         Expr::Call(ast::ExprCall { func, .. }) => {
             let Expr::Attribute(ast::ExprAttribute { attr, value, .. }) = func.as_ref() else {
@@ -106,7 +106,7 @@ pub(crate) fn hardcoded_sql_expression(checker: &mut Checker, expr: &Expr) {
             let Some(string) = value.as_string_literal_expr() else {
                 return;
             };
-            string.value.escape_default().to_string()
+            string.value.as_str().escape_default().to_string()
         }
         // f"select * from table where val = {val}"
         Expr::FString(f_string) => concatenated_f_string(f_string, checker.locator()),

crates/ruff_linter/src/rules/flake8_bandit/rules/hardcoded_tmp_directory.rs

@@ -57,7 +57,7 @@ pub(crate) fn hardcoded_tmp_directory(checker: &mut Checker, string: &ast::ExprS
         .flake8_bandit
         .hardcoded_tmp_directory
         .iter()
-        .any(|prefix| string.value.starts_with(prefix))
+        .any(|prefix| string.value.as_str().starts_with(prefix))
     {
         return;
     }

crates/ruff_linter/src/rules/flake8_bandit/rules/suspicious_function_call.rs

@@ -855,7 +855,7 @@ pub(crate) fn suspicious_function_call(checker: &mut Checker, call: &ExprCall) {
                 // If the `url` argument is a string literal, allow `http` and `https` schemes.
                 if call.arguments.args.iter().all(|arg| !arg.is_starred_expr()) && call.arguments.keywords.iter().all(|keyword| keyword.arg.is_some()) {
                     if let Some(Expr::StringLiteral(ast::ExprStringLiteral { value, .. })) = &call.arguments.find_argument("url", 0) {
-                        let url = value.trim_start();
+                        let url = value.as_str().trim_start();
                         if url.starts_with("http://") || url.starts_with("https://") {
                             return None;
                         }

crates/ruff_linter/src/rules/flake8_bugbear/rules/getattr_with_constant.rs

@@ -69,10 +69,10 @@ pub(crate) fn getattr_with_constant(
     let Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) = arg else {
         return;
     };
-    if !is_identifier(value) {
+    if !is_identifier(value.as_str()) {
         return;
     }
-    if is_mangled_private(value) {
+    if is_mangled_private(value.as_str()) {
         return;
     }
     if !checker.semantic().is_builtin("getattr") {

crates/ruff_linter/src/rules/flake8_bugbear/rules/setattr_with_constant.rs

@@ -83,10 +83,10 @@ pub(crate) fn setattr_with_constant(
     let Expr::StringLiteral(ast::ExprStringLiteral { value: name, .. }) = name else {
         return;
     };
-    if !is_identifier(name) {
+    if !is_identifier(name.as_str()) {
         return;
     }
-    if is_mangled_private(name) {
+    if is_mangled_private(name.as_str()) {
         return;
     }
     if !checker.semantic().is_builtin("setattr") {
@@ -104,7 +104,7 @@ pub(crate) fn setattr_with_constant(
         if expr == child.as_ref() {
             let mut diagnostic = Diagnostic::new(SetAttrWithConstant, expr.range());
             diagnostic.set_fix(Fix::safe_edit(Edit::range_replacement(
-                assignment(obj, name, value, checker.generator()),
+                assignment(obj, name.as_str(), value, checker.generator()),
                 expr.range(),
             )));
             checker.diagnostics.push(diagnostic);

crates/ruff_linter/src/rules/flake8_datetimez/rules/call_datetime_strptime_without_zone.rs

@@ -78,7 +78,7 @@ pub(crate) fn call_datetime_strptime_without_zone(checker: &mut Checker, call: &
     if let Some(Expr::StringLiteral(ast::ExprStringLiteral { value: format, .. })) =
         call.arguments.args.get(1).as_ref()
     {
-        if format.contains("%z") {
+        if format.as_str().contains("%z") {
             return;
         }
     };

crates/ruff_linter/src/rules/flake8_logging_format/rules/logging_call.rs

@@ -93,7 +93,7 @@ fn check_log_record_attr_clash(checker: &mut Checker, extra: &Keyword) {
             for key in keys {
                 if let Some(key) = &key {
                     if let Expr::StringLiteral(ast::ExprStringLiteral { value: attr, .. }) = key {
-                        if is_reserved_attr(attr) {
+                        if is_reserved_attr(attr.as_str()) {
                             checker.diagnostics.push(Diagnostic::new(
                                 LoggingExtraAttrClash(attr.to_string()),
                                 key.range(),

crates/ruff_linter/src/rules/flake8_pie/rules/unnecessary_dict_kwargs.rs

@@ -110,7 +110,7 @@ pub(crate) fn unnecessary_dict_kwargs(checker: &mut Checker, expr: &Expr, kwargs
 /// Return `Some` if a key is a valid keyword argument name, or `None` otherwise.
 fn as_kwarg(key: &Expr) -> Option<&str> {
     if let Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) = key {
-        if is_identifier(value) {
+        if is_identifier(value.as_str()) {
             return Some(value.as_str());
         }
     }

crates/ruff_linter/src/rules/flake8_pytest_style/rules/parametrize.rs

@@ -300,8 +300,8 @@ fn check_names(checker: &mut Checker, decorator: &Decorator, expr: &Expr) {
     let names_type = checker.settings.flake8_pytest_style.parametrize_names_type;
 
     match expr {
-        Expr::StringLiteral(ast::ExprStringLiteral { value: string, .. }) => {
-            let names = split_names(string);
+        Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) => {
+            let names = split_names(value.as_str());
             if names.len() > 1 {
                 match names_type {
                     types::ParametrizeNameType::Tuple => {
@@ -475,12 +475,11 @@ fn check_values(checker: &mut Checker, names: &Expr, values: &Expr) {
         .flake8_pytest_style
         .parametrize_values_row_type;
 
-    let is_multi_named =
-        if let Expr::StringLiteral(ast::ExprStringLiteral { value: string, .. }) = &names {
-            split_names(string).len() > 1
-        } else {
-            true
-        };
+    let is_multi_named = if let Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) = &names {
+        split_names(value.as_str()).len() > 1
+    } else {
+        true
+    };
 
     match values {
         Expr::List(ast::ExprList { elts, .. }) => {

crates/ruff_linter/src/rules/flake8_simplify/rules/ast_expr.rs

@@ -161,11 +161,11 @@ pub(crate) fn use_capital_environment_variables(checker: &mut Checker, expr: &Ex
         return;
     }
 
-    if is_lowercase_allowed(env_var) {
+    if is_lowercase_allowed(env_var.as_str()) {
         return;
     }
 
-    let capital_env_var = env_var.to_ascii_uppercase();
+    let capital_env_var = env_var.as_str().to_ascii_uppercase();
     if capital_env_var == env_var.as_str() {
         return;
     }
@@ -201,11 +201,11 @@ fn check_os_environ_subscript(checker: &mut Checker, expr: &Expr) {
         return;
     };
 
-    if is_lowercase_allowed(env_var) {
+    if is_lowercase_allowed(env_var.as_str()) {
         return;
     }
 
-    let capital_env_var = env_var.to_ascii_uppercase();
+    let capital_env_var = env_var.as_str().to_ascii_uppercase();
     if capital_env_var == env_var.as_str() {
         return;
     }

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

@@ -12,7 +12,7 @@ pub(super) fn type_param_name(arguments: &Arguments) -> Option<&str> {
     // Handle both `TypeVar("T")` and `TypeVar(name="T")`.
     let name_param = arguments.find_argument("name", 0)?;
     if let Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) = &name_param {
-        Some(value)
+        Some(value.as_str())
     } else {
         None
     }

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

@@ -154,7 +154,7 @@ impl TryFrom<char> for OpenMode {
 }
 
 /// Returns `true` if the open mode is valid.
-fn is_valid_mode(mode: &str) -> bool {
+fn is_valid_mode(mode: &ast::StringLiteralValue) -> bool {
     // Flag duplicates and invalid characters.
     let mut flags = OpenMode::empty();
     for char in mode.chars() {

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

@@ -122,7 +122,7 @@ impl fmt::Display for RemovalKind {
 
 /// Return `true` if a string contains duplicate characters, taking into account
 /// escapes.
-fn has_duplicates(s: &str) -> bool {
+fn has_duplicates(s: &ast::StringLiteralValue) -> bool {
     let mut escaped = false;
     let mut seen = FxHashSet::default();
     for ch in s.chars() {

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

@@ -60,7 +60,7 @@ pub(crate) fn repeated_keyword_argument(checker: &mut Checker, call: &ExprCall)
             // Ex) `func(**{"a": 1, "a": 2})`
             for key in keys.iter().flatten() {
                 if let Expr::StringLiteral(ExprStringLiteral { value, .. }) = key {
-                    if !seen.insert(value) {
+                    if !seen.insert(value.as_str()) {
                         checker.diagnostics.push(Diagnostic::new(
                             RepeatedKeywordArgument {
                                 duplicate_keyword: value.to_string(),

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

@@ -81,7 +81,12 @@ pub(crate) fn unspecified_encoding(checker: &mut Checker, call: &ast::ExprCall)
 
 /// Returns `true` if the given expression is a string literal containing a `b` character.
 fn is_binary_mode(expr: &ast::Expr) -> Option<bool> {
-    Some(expr.as_string_literal_expr()?.value.contains('b'))
+    Some(
+        expr.as_string_literal_expr()?
+            .value
+            .chars()
+            .any(|c| c == 'b'),
+    )
 }
 
 /// Returns `true` if the given call lacks an explicit `encoding`.

crates/ruff_linter/src/rules/pyupgrade/rules/convert_named_tuple_functional_to_class.rs

@@ -185,13 +185,13 @@ fn create_fields_from_fields_arg(fields: &Expr) -> Option<Vec<Stmt>> {
                     return None;
                 }
                 let ast::ExprStringLiteral { value: field, .. } = field.as_string_literal_expr()?;
-                if !is_identifier(field) {
+                if !is_identifier(field.as_str()) {
                     return None;
                 }
-                if is_dunder(field) {
+                if is_dunder(field.as_str()) {
                     return None;
                 }
-                Some(create_field_assignment_stmt(field, annotation))
+                Some(create_field_assignment_stmt(field.as_str(), annotation))
             })
             .collect()
     }

crates/ruff_linter/src/rules/pyupgrade/rules/convert_typed_dict_functional_to_class.rs

@@ -174,13 +174,13 @@ fn fields_from_dict_literal(keys: &[Option<Expr>], values: &[Expr]) -> Option<Ve
             .zip(values.iter())
             .map(|(key, value)| match key {
                 Some(Expr::StringLiteral(ast::ExprStringLiteral { value: field, .. })) => {
-                    if !is_identifier(field) {
+                    if !is_identifier(field.as_str()) {
                         return None;
                     }
-                    if is_dunder(field) {
+                    if is_dunder(field.as_str()) {
                         return None;
                     }
-                    Some(create_field_assignment_stmt(field, value))
+                    Some(create_field_assignment_stmt(field.as_str(), value))
                 }
                 _ => None,
             })

crates/ruff_linter/src/rules/pyupgrade/rules/printf_string_formatting.rs

@@ -228,14 +228,14 @@ fn clean_params_dictionary(right: &Expr, locator: &Locator, stylist: &Stylist) -
                     }) = key
                     {
                         // If the dictionary key is not a valid variable name, abort.
-                        if !is_identifier(key_string) {
+                        if !is_identifier(key_string.as_str()) {
                             return None;
                         }
                         // If there are multiple entries of the same key, abort.
                         if seen.contains(&key_string.as_str()) {
                             return None;
                         }
-                        seen.push(key_string);
+                        seen.push(key_string.as_str());
                         if is_multi_line {
                             if indent.is_none() {
                                 indent = indentation(locator, key);

crates/ruff_linter/src/rules/pyupgrade/rules/redundant_open_modes.rs

@@ -76,7 +76,7 @@ pub(crate) fn redundant_open_modes(checker: &mut Checker, call: &ast::ExprCall)
                         ..
                     }) = &keyword.value
                     {
-                        if let Ok(mode) = OpenMode::from_str(mode_param_value) {
+                        if let Ok(mode) = OpenMode::from_str(mode_param_value.as_str()) {
                             checker.diagnostics.push(create_check(
                                 call,
                                 &keyword.value,
@@ -91,7 +91,7 @@ pub(crate) fn redundant_open_modes(checker: &mut Checker, call: &ast::ExprCall)
         }
         Some(mode_param) => {
             if let Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) = &mode_param {
-                if let Ok(mode) = OpenMode::from_str(value) {
+                if let Ok(mode) = OpenMode::from_str(value.as_str()) {
                     checker.diagnostics.push(create_check(
                         call,
                         mode_param,

crates/ruff_linter/src/rules/pyupgrade/rules/unnecessary_encode_utf8.rs

@@ -74,7 +74,7 @@ fn match_encoded_variable(func: &Expr) -> Option<&Expr> {
 
 fn is_utf8_encoding_arg(arg: &Expr) -> bool {
     if let Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) = &arg {
-        UTF8_LITERALS.contains(&value.to_lowercase().as_str())
+        UTF8_LITERALS.contains(&value.as_str().to_lowercase().as_str())
     } else {
         false
     }
@@ -161,7 +161,7 @@ pub(crate) fn unnecessary_encode_utf8(checker: &mut Checker, call: &ast::ExprCal
         Expr::StringLiteral(ast::ExprStringLiteral { value: literal, .. }) => {
             // Ex) `"str".encode()`, `"str".encode("utf-8")`
             if let Some(encoding_arg) = match_encoding_arg(&call.arguments) {
-                if literal.is_ascii() {
+                if literal.as_str().is_ascii() {
                     // Ex) Convert `"foo".encode()` to `b"foo"`.
                     let mut diagnostic = Diagnostic::new(
                         UnnecessaryEncodeUTF8 {

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

@@ -181,14 +181,10 @@ pub(crate) fn implicit_optional(checker: &mut Checker, parameters: &Parameters)
             continue;
         };
 
-        if let Expr::StringLiteral(ast::ExprStringLiteral {
-            range,
-            value: string,
-        }) = annotation.as_ref()
-        {
+        if let Expr::StringLiteral(ast::ExprStringLiteral { range, value }) = annotation.as_ref() {
             // Quoted annotation.
             if let Ok((annotation, kind)) =
-                parse_type_annotation(string, *range, checker.locator().contents())
+                parse_type_annotation(value.as_str(), *range, checker.locator().contents())
             {
                 let Some(expr) = type_hint_explicitly_allows_none(
                     &annotation,

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

@@ -108,11 +108,10 @@ impl<'a> TypingTarget<'a> {
                 ..
             }) => Some(TypingTarget::PEP604Union(left, right)),
             Expr::NoneLiteral(_) => Some(TypingTarget::None),
-            Expr::StringLiteral(ast::ExprStringLiteral {
-                value: string,
-                range,
-            }) => parse_type_annotation(string, *range, locator.contents())
-                .map_or(None, |(expr, _)| Some(TypingTarget::ForwardReference(expr))),
+            Expr::StringLiteral(ast::ExprStringLiteral { value, range }) => {
+                parse_type_annotation(value.as_str(), *range, locator.contents())
+                    .map_or(None, |(expr, _)| Some(TypingTarget::ForwardReference(expr)))
+            }
             _ => semantic.resolve_call_path(expr).map_or(
                 // If we can't resolve the call path, it must be defined in the
                 // same file, so we assume it's `Any` as it could be a type alias.

crates/ruff_python_ast/src/all.rs

@@ -24,7 +24,7 @@ where
     fn add_to_names<'a>(elts: &'a [Expr], names: &mut Vec<&'a str>, flags: &mut DunderAllFlags) {
         for elt in elts {
             if let Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) = elt {
-                names.push(value);
+                names.push(value.as_str());
             } else {
                 *flags |= DunderAllFlags::INVALID_OBJECT;
             }

crates/ruff_python_ast/src/nodes.rs

@@ -1213,7 +1213,26 @@ impl StringLiteralValue {
         }
     }
 
+    /// Returns `true` if the string literal value is empty.
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// Returns the total length of the string literal value, in bytes, not
+    /// [`char`]s or graphemes.
+    pub fn len(&self) -> usize {
+        self.parts().fold(0, |acc, part| acc + part.value.len())
+    }
+
+    /// Returns an iterator over the [`char`]s of each string literal part.
+    pub fn chars(&self) -> impl Iterator<Item = char> + '_ {
+        self.parts().flat_map(|part| part.value.chars())
+    }
+
     /// Returns the concatenated string value as a [`str`].
+    ///
+    /// Note that this will perform an allocation on the first invocation if the
+    /// string value is implicitly concatenated.
     pub fn as_str(&self) -> &str {
         match &self.inner {
             StringLiteralValueInner::Single(value) => value.as_str(),
@@ -1224,21 +1243,17 @@ impl StringLiteralValue {
 
 impl PartialEq<str> for StringLiteralValue {
     fn eq(&self, other: &str) -> bool {
-        self.as_str() == other
+        if self.len() != other.len() {
+            return false;
+        }
+        // The `zip` here is safe because we have checked the length of both parts.
+        self.chars().zip(other.chars()).all(|(c1, c2)| c1 == c2)
     }
 }
 
 impl PartialEq<String> for StringLiteralValue {
     fn eq(&self, other: &String) -> bool {
-        self.as_str() == other
-    }
-}
-
-impl Deref for StringLiteralValue {
-    type Target = str;
-
-    fn deref(&self) -> &Self::Target {
-        self.as_str()
+        self == other.as_str()
     }
 }