[pyupgrade] Implement unnecessary-default-type-args (UP043) (#1…

…2371)

## Summary

Add new rule and implement for `unnecessary default type arguments`
under the `UP` category (`UP043`).

```py
// < py313
Generator[int, None, None] 

// >= py313
Generator[int]
```

I think that as Python 3.13 develops, there might be more default type
arguments added besides `Generator` and `AsyncGenerator`. So, I made
this more flexible to accommodate future changes.

related issue: #12286

## Test Plan

snapshot included..!

crates/ruff_linter/resources/test/fixtures/pyupgrade/UP043.py

@@ -0,0 +1,41 @@
+from typing import Generator, AsyncGenerator
+
+
+def func() -> Generator[int, None, None]:
+    yield 42
+
+
+def func() -> Generator[int, None]:
+    yield 42
+
+
+def func() -> Generator[int]:
+    yield 42
+
+
+def func() -> Generator[int, int, int]:
+    foo = yield 42
+    return foo
+
+
+def func() -> Generator[int, int, None]:
+    _ = yield 42
+    return None
+
+
+def func() -> Generator[int, None, int]:
+    yield 42
+    return 42
+
+
+async def func() -> AsyncGenerator[int, None]:
+    yield 42
+
+
+async def func() -> AsyncGenerator[int]:
+    yield 42
+
+
+async def func() -> AsyncGenerator[int, int]:
+    foo = yield 42
+    return foo

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

@@ -110,6 +110,12 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
                 ruff::rules::never_union(checker, expr);
             }
 
+            if checker.enabled(Rule::UnnecessaryDefaultTypeArgs) {
+                if checker.settings.target_version >= PythonVersion::Py313 {
+                    pyupgrade::rules::unnecessary_default_type_args(checker, expr);
+                }
+            }
+
             if checker.any_enabled(&[
                 Rule::SysVersionSlice3,
                 Rule::SysVersion2,

crates/ruff_linter/src/codes.rs

@@ -521,6 +521,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Pyupgrade, "040") => (RuleGroup::Stable, rules::pyupgrade::rules::NonPEP695TypeAlias),
         (Pyupgrade, "041") => (RuleGroup::Stable, rules::pyupgrade::rules::TimeoutErrorAlias),
         (Pyupgrade, "042") => (RuleGroup::Preview, rules::pyupgrade::rules::ReplaceStrEnum),
+        (Pyupgrade, "043") => (RuleGroup::Preview, rules::pyupgrade::rules::UnnecessaryDefaultTypeArgs),
 
         // pydocstyle
         (Pydocstyle, "100") => (RuleGroup::Stable, rules::pydocstyle::rules::UndocumentedPublicModule),

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

@@ -80,6 +80,7 @@ mod tests {
     #[test_case(Rule::UnicodeKindPrefix, Path::new("UP025.py"))]
     #[test_case(Rule::UnnecessaryBuiltinImport, Path::new("UP029.py"))]
     #[test_case(Rule::UnnecessaryClassParentheses, Path::new("UP039.py"))]
+    #[test_case(Rule::UnnecessaryDefaultTypeArgs, Path::new("UP043.py"))]
     #[test_case(Rule::UnnecessaryEncodeUTF8, Path::new("UP012.py"))]
     #[test_case(Rule::UnnecessaryFutureImport, Path::new("UP010.py"))]
     #[test_case(Rule::UnpackedListComprehension, Path::new("UP027.py"))]

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

@@ -28,6 +28,7 @@ pub(crate) use unicode_kind_prefix::*;
 pub(crate) use unnecessary_builtin_import::*;
 pub(crate) use unnecessary_class_parentheses::*;
 pub(crate) use unnecessary_coding_comment::*;
+pub(crate) use unnecessary_default_type_args::*;
 pub(crate) use unnecessary_encode_utf8::*;
 pub(crate) use unnecessary_future_import::*;
 pub(crate) use unpacked_list_comprehension::*;
@@ -69,6 +70,7 @@ mod unicode_kind_prefix;
 mod unnecessary_builtin_import;
 mod unnecessary_class_parentheses;
 mod unnecessary_coding_comment;
+mod unnecessary_default_type_args;
 mod unnecessary_encode_utf8;
 mod unnecessary_future_import;
 mod unpacked_list_comprehension;

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

@@ -0,0 +1,179 @@
+use ruff_diagnostics::{AlwaysFixableViolation, Diagnostic, Edit, Fix};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast::{self as ast, Expr};
+use ruff_text_size::{Ranged, TextRange};
+
+use crate::checkers::ast::Checker;
+
+/// ## What it does
+/// Checks for unnecessary default type arguments.
+///
+/// ## Why is this bad?
+/// Python 3.13 introduced the ability for type parameters to specify default
+/// values. As such, the default type arguments for some types in the standard
+/// library (e.g., Generator, AsyncGenerator) are now optional.
+///
+/// Omitting type parameters that match the default values can make the code
+/// more concise and easier to read.
+///
+/// ## Examples
+///
+/// ```python
+/// from typing import Generator, AsyncGenerator
+///
+///
+/// def sync_gen() -> Generator[int, None, None]:
+///     yield 42
+///
+///
+/// async def async_gen() -> AsyncGenerator[int, None]:
+///     yield 42
+/// ```
+///
+/// Use instead:
+///
+/// ```python
+/// from typing import Generator, AsyncGenerator
+///
+///
+/// def sync_gen() -> Generator[int]:
+///     yield 42
+///
+///
+/// async def async_gen() -> AsyncGenerator[int]:
+///     yield 42
+/// ```
+///
+/// ## References
+///
+/// - [PEP 696 – Type Defaults for Type Parameters](https://peps.python.org/pep-0696/)
+/// - [typing.Generator](https://docs.python.org/3.13/library/typing.html#typing.Generator)
+/// - [typing.AsyncGenerator](https://docs.python.org/3.13/library/typing.html#typing.AsyncGenerator)
+#[violation]
+pub struct UnnecessaryDefaultTypeArgs;
+
+impl AlwaysFixableViolation for UnnecessaryDefaultTypeArgs {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        format!("Unnecessary default type arguments")
+    }
+
+    fn fix_title(&self) -> String {
+        format!("Remove default type arguments")
+    }
+}
+
+/// UP043
+pub(crate) fn unnecessary_default_type_args(checker: &mut Checker, expr: &Expr) {
+    let Expr::Subscript(ast::ExprSubscript { value, slice, .. }) = expr else {
+        return;
+    };
+
+    let Expr::Tuple(ast::ExprTuple {
+        elts,
+        ctx: _,
+        range: _,
+        parenthesized: _,
+    }) = slice.as_ref()
+    else {
+        return;
+    };
+
+    // The type annotation must be `Generator` or `AsyncGenerator`.
+    let Some(type_annotation) = DefaultedTypeAnnotation::from_expr(value, checker.semantic())
+    else {
+        return;
+    };
+
+    let valid_elts = type_annotation.trim_unnecessary_defaults(elts);
+
+    // If we didn't trim any elements, then the default type arguments are necessary.
+    if *elts == valid_elts {
+        return;
+    }
+
+    let mut diagnostic = Diagnostic::new(UnnecessaryDefaultTypeArgs, expr.range());
+    diagnostic.set_fix(Fix::safe_edit(Edit::range_replacement(
+        checker
+            .generator()
+            .expr(&Expr::Subscript(ast::ExprSubscript {
+                value: value.clone(),
+                slice: Box::new(if let [elt] = valid_elts.as_slice() {
+                    elt.clone()
+                } else {
+                    Expr::Tuple(ast::ExprTuple {
+                        elts: valid_elts,
+                        ctx: ast::ExprContext::Load,
+                        range: TextRange::default(),
+                        parenthesized: true,
+                    })
+                }),
+                ctx: ast::ExprContext::Load,
+                range: TextRange::default(),
+            })),
+        expr.range(),
+    )));
+    checker.diagnostics.push(diagnostic);
+}
+
+/// Trim trailing `None` literals from the given elements.
+///
+/// For example, given `[int, None, None]`, return `[int]`.
+fn trim_trailing_none(elts: &[Expr]) -> &[Expr] {
+    match elts.iter().rposition(|elt| !elt.is_none_literal_expr()) {
+        Some(trimmed_last_index) => elts[..=trimmed_last_index].as_ref(),
+        None => &[],
+    }
+}
+
+/// Type annotations that include default type arguments as of Python 3.13.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum DefaultedTypeAnnotation {
+    /// `typing.Generator[YieldType, SendType = None, ReturnType = None]`
+    Generator,
+    /// `typing.AsyncGenerator[YieldType, SendType = None]`
+    AsyncGenerator,
+}
+
+impl DefaultedTypeAnnotation {
+    /// Returns the [`DefaultedTypeAnnotation`], if the given expression is a type annotation that
+    /// includes default type arguments.
+    fn from_expr(expr: &Expr, semantic: &ruff_python_semantic::SemanticModel) -> Option<Self> {
+        let qualified_name = semantic.resolve_qualified_name(expr)?;
+        if semantic.match_typing_qualified_name(&qualified_name, "Generator") {
+            Some(Self::Generator)
+        } else if semantic.match_typing_qualified_name(&qualified_name, "AsyncGenerator") {
+            Some(Self::AsyncGenerator)
+        } else {
+            None
+        }
+    }
+
+    /// Trim any unnecessary default type arguments from the given elements.
+    fn trim_unnecessary_defaults(self, elts: &[Expr]) -> Vec<Expr> {
+        match self {
+            Self::Generator => {
+                // Check only if the number of elements is 2 or 3 (e.g., `Generator[int, None]` or `Generator[int, None, None]`).
+                // Otherwise, ignore (e.g., `Generator[]`, `Generator[int]`, `Generator[int, None, None, None]`)
+                if elts.len() != 2 && elts.len() != 3 {
+                    return elts.to_vec();
+                }
+
+                std::iter::once(elts[0].clone())
+                    .chain(trim_trailing_none(&elts[1..]).iter().cloned())
+                    .collect::<Vec<_>>()
+            }
+            Self::AsyncGenerator => {
+                // Check only if the number of elements is 2 (e.g., `AsyncGenerator[int, None]`).
+                // Otherwise, ignore (e.g., `AsyncGenerator[]`, `AsyncGenerator[int]`, `AsyncGenerator[int, None, None]`)
+                if elts.len() != 2 {
+                    return elts.to_vec();
+                }
+
+                std::iter::once(elts[0].clone())
+                    .chain(trim_trailing_none(&elts[1..]).iter().cloned())
+                    .collect::<Vec<_>>()
+            }
+        }
+    }
+}

crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP043.py.snap

@@ -0,0 +1,75 @@
+---
+source: crates/ruff_linter/src/rules/pyupgrade/mod.rs
+---
+UP043.py:4:15: UP043 [*] Unnecessary default type arguments
+  |
+4 | def func() -> Generator[int, None, None]:
+  |               ^^^^^^^^^^^^^^^^^^^^^^^^^^ UP043
+5 |     yield 42
+  |
+  = help: Remove default type arguments
+
+ℹ Safe fix
+1 1 | from typing import Generator, AsyncGenerator
+2 2 | 
+3 3 | 
+4   |-def func() -> Generator[int, None, None]:
+  4 |+def func() -> Generator[int]:
+5 5 |     yield 42
+6 6 | 
+7 7 | 
+
+UP043.py:8:15: UP043 [*] Unnecessary default type arguments
+  |
+8 | def func() -> Generator[int, None]:
+  |               ^^^^^^^^^^^^^^^^^^^^ UP043
+9 |     yield 42
+  |
+  = help: Remove default type arguments
+
+ℹ Safe fix
+5 5 |     yield 42
+6 6 | 
+7 7 | 
+8   |-def func() -> Generator[int, None]:
+  8 |+def func() -> Generator[int]:
+9 9 |     yield 42
+10 10 | 
+11 11 | 
+
+UP043.py:21:15: UP043 [*] Unnecessary default type arguments
+   |
+21 | def func() -> Generator[int, int, None]:
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^ UP043
+22 |     _ = yield 42
+23 |     return None
+   |
+   = help: Remove default type arguments
+
+ℹ Safe fix
+18 18 |     return foo
+19 19 | 
+20 20 | 
+21    |-def func() -> Generator[int, int, None]:
+   21 |+def func() -> Generator[int, int]:
+22 22 |     _ = yield 42
+23 23 |     return None
+24 24 | 
+
+UP043.py:31:21: UP043 [*] Unnecessary default type arguments
+   |
+31 | async def func() -> AsyncGenerator[int, None]:
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^ UP043
+32 |     yield 42
+   |
+   = help: Remove default type arguments
+
+ℹ Safe fix
+28 28 |     return 42
+29 29 | 
+30 30 | 
+31    |-async def func() -> AsyncGenerator[int, None]:
+   31 |+async def func() -> AsyncGenerator[int]:
+32 32 |     yield 42
+33 33 | 
+34 34 |