[ruff] Unnecessary cast to int (RUF046) (#14697)

## Summary

Resolves #11412.

## Test Plan

`cargo nextest run` and `cargo insta test`.

---------

Co-authored-by: Micha Reiser <[email protected]>

crates/ruff_linter/resources/test/fixtures/ruff/RUF046.py

@@ -0,0 +1,50 @@
+import math
+
+
+### Safely fixable
+
+# Arguments are not checked
+int(id())
+int(len([]))
+int(ord(foo))
+int(hash(foo, bar))
+int(int(''))
+
+int(math.comb())
+int(math.factorial())
+int(math.gcd())
+int(math.lcm())
+int(math.isqrt())
+int(math.perm())
+
+
+### Unsafe
+
+int(math.ceil())
+int(math.floor())
+int(math.trunc())
+
+
+### `round()`
+
+## Errors
+int(round(0))
+int(round(0, 0))
+int(round(0, None))
+
+int(round(0.1))
+int(round(0.1, None))
+
+# Argument type is not checked
+foo = type("Foo", (), {"__round__": lambda self: 4.2})()
+
+int(round(foo))
+int(round(foo, 0))
+int(round(foo, None))
+
+## No errors
+int(round(0, 3.14))
+int(round(0, non_literal))
+int(round(0, 0), base)
+int(round(0, 0, extra=keyword))
+int(round(0.1, 0))

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

@@ -1093,6 +1093,9 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
             if checker.enabled(Rule::Airflow3Removal) {
                 airflow::rules::removed_in_3(checker, expr);
             }
+            if checker.enabled(Rule::UnnecessaryCastToInt) {
+                ruff::rules::unnecessary_cast_to_int(checker, call);
+            }
         }
         Expr::Dict(dict) => {
             if checker.any_enabled(&[

crates/ruff_linter/src/codes.rs

@@ -983,6 +983,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Ruff, "039") => (RuleGroup::Preview, rules::ruff::rules::UnrawRePattern),
         (Ruff, "040") => (RuleGroup::Preview, rules::ruff::rules::InvalidAssertMessageLiteralArgument),
         (Ruff, "041") => (RuleGroup::Preview, rules::ruff::rules::UnnecessaryNestedLiteral),
+        (Ruff, "046") => (RuleGroup::Preview, rules::ruff::rules::UnnecessaryCastToInt),
         (Ruff, "048") => (RuleGroup::Preview, rules::ruff::rules::MapIntVersionParsing),
         (Ruff, "052") => (RuleGroup::Preview, rules::ruff::rules::UsedDummyVariable),
         (Ruff, "055") => (RuleGroup::Preview, rules::ruff::rules::UnnecessaryRegularExpression),

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

@@ -413,6 +413,7 @@ mod tests {
     #[test_case(Rule::UnrawRePattern, Path::new("RUF039_concat.py"))]
     #[test_case(Rule::UnnecessaryRegularExpression, Path::new("RUF055_0.py"))]
     #[test_case(Rule::UnnecessaryRegularExpression, Path::new("RUF055_1.py"))]
+    #[test_case(Rule::UnnecessaryCastToInt, Path::new("RUF046.py"))]
     fn preview_rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!(
             "preview__{}_{}",

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

@@ -30,6 +30,7 @@ pub(crate) use sort_dunder_slots::*;
 pub(crate) use static_key_dict_comprehension::*;
 #[cfg(any(feature = "test-rules", test))]
 pub(crate) use test_rules::*;
+pub(crate) use unnecessary_cast_to_int::*;
 pub(crate) use unnecessary_iterable_allocation_for_first_element::*;
 pub(crate) use unnecessary_key_check::*;
 pub(crate) use unnecessary_nested_literal::*;
@@ -78,6 +79,7 @@ mod static_key_dict_comprehension;
 mod suppression_comment_visitor;
 #[cfg(any(feature = "test-rules", test))]
 pub(crate) mod test_rules;
+mod unnecessary_cast_to_int;
 mod unnecessary_iterable_allocation_for_first_element;
 mod unnecessary_key_check;
 mod unnecessary_nested_literal;

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

@@ -0,0 +1,183 @@
+use crate::checkers::ast::Checker;
+use ruff_diagnostics::{AlwaysFixableViolation, Applicability, Diagnostic, Edit, Fix};
+use ruff_macros::{derive_message_formats, ViolationMetadata};
+use ruff_python_ast::{Arguments, Expr, ExprCall, ExprName, ExprNumberLiteral, Number};
+use ruff_python_semantic::analyze::typing;
+use ruff_python_semantic::SemanticModel;
+use ruff_text_size::TextRange;
+
+/// ## What it does
+/// Checks for `int` conversions of values that are already integers.
+///
+/// ## Why is this bad?
+/// Such a conversion is unnecessary.
+///
+/// ## Known problems
+/// This rule may produce false positives for `round`, `math.ceil`, `math.floor`,
+/// and `math.trunc` calls when values override the `__round__`, `__ceil__`, `__floor__`,
+/// or `__trunc__` operators such that they don't return an integer.
+///
+/// ## Example
+///
+/// ```python
+/// int(len([]))
+/// int(round(foo, None))
+/// ```
+///
+/// Use instead:
+///
+/// ```python
+/// len([])
+/// round(foo)
+/// ```
+///
+/// ## Fix safety
+/// The fix for `round`, `math.ceil`, `math.floor`, and `math.truncate` is unsafe
+/// because removing the `int` conversion can change the semantics for values
+/// overriding the `__round__`, `__ceil__`, `__floor__`, or `__trunc__` dunder methods
+/// such that they don't return an integer.
+#[derive(ViolationMetadata)]
+pub(crate) struct UnnecessaryCastToInt;
+
+impl AlwaysFixableViolation for UnnecessaryCastToInt {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        "Value being casted is already an integer".to_string()
+    }
+
+    fn fix_title(&self) -> String {
+        "Remove unnecessary conversion to `int`".to_string()
+    }
+}
+
+/// RUF046
+pub(crate) fn unnecessary_cast_to_int(checker: &mut Checker, call: &ExprCall) {
+    let semantic = checker.semantic();
+
+    let Some(Expr::Call(inner_call)) = single_argument_to_int_call(semantic, call) else {
+        return;
+    };
+
+    let (func, arguments) = (&inner_call.func, &inner_call.arguments);
+    let (outer_range, inner_range) = (call.range, inner_call.range);
+
+    let Some(qualified_name) = checker.semantic().resolve_qualified_name(func) else {
+        return;
+    };
+
+    let fix = match qualified_name.segments() {
+        // Always returns a strict instance of `int`
+        ["" | "builtins", "len" | "id" | "hash" | "ord" | "int"]
+        | ["math", "comb" | "factorial" | "gcd" | "lcm" | "isqrt" | "perm"] => {
+            Fix::safe_edit(replace_with_inner(checker, outer_range, inner_range))
+        }
+
+        // Depends on `ndigits` and `number.__round__`
+        ["" | "builtins", "round"] => {
+            if let Some(fix) = replace_with_shortened_round_call(checker, outer_range, arguments) {
+                fix
+            } else {
+                return;
+            }
+        }
+
+        // Depends on `__ceil__`/`__floor__`/`__trunc__`
+        ["math", "ceil" | "floor" | "trunc"] => {
+            Fix::unsafe_edit(replace_with_inner(checker, outer_range, inner_range))
+        }
+
+        _ => return,
+    };
+
+    checker
+        .diagnostics
+        .push(Diagnostic::new(UnnecessaryCastToInt, call.range).with_fix(fix));
+}
+
+fn single_argument_to_int_call<'a>(
+    semantic: &SemanticModel,
+    call: &'a ExprCall,
+) -> Option<&'a Expr> {
+    let ExprCall {
+        func, arguments, ..
+    } = call;
+
+    if !semantic.match_builtin_expr(func, "int") {
+        return None;
+    }
+
+    if !arguments.keywords.is_empty() {
+        return None;
+    }
+
+    let [argument] = &*arguments.args else {
+        return None;
+    };
+
+    Some(argument)
+}
+
+/// Returns an [`Edit`] when the call is of any of the forms:
+/// * `round(integer)`, `round(integer, 0)`, `round(integer, None)`
+/// * `round(whatever)`, `round(whatever, None)`
+fn replace_with_shortened_round_call(
+    checker: &Checker,
+    outer_range: TextRange,
+    arguments: &Arguments,
+) -> Option<Fix> {
+    if arguments.len() > 2 {
+        return None;
+    }
+
+    let number = arguments.find_argument("number", 0)?;
+    let ndigits = arguments.find_argument("ndigits", 1);
+
+    let number_is_int = match number {
+        Expr::Name(name) => is_int(checker.semantic(), name),
+        Expr::NumberLiteral(ExprNumberLiteral { value, .. }) => matches!(value, Number::Int(..)),
+        _ => false,
+    };
+
+    match ndigits {
+        Some(Expr::NumberLiteral(ExprNumberLiteral { value, .. }))
+            if is_literal_zero(value) && number_is_int => {}
+        Some(Expr::NoneLiteral(_)) | None => {}
+        _ => return None,
+    };
+
+    let number_expr = checker.locator().slice(number);
+    let new_content = format!("round({number_expr})");
+
+    let applicability = if number_is_int {
+        Applicability::Safe
+    } else {
+        Applicability::Unsafe
+    };
+
+    Some(Fix::applicable_edit(
+        Edit::range_replacement(new_content, outer_range),
+        applicability,
+    ))
+}
+
+fn is_int(semantic: &SemanticModel, name: &ExprName) -> bool {
+    let Some(binding) = semantic.only_binding(name).map(|id| semantic.binding(id)) else {
+        return false;
+    };
+
+    typing::is_int(binding, semantic)
+}
+
+fn is_literal_zero(value: &Number) -> bool {
+    let Number::Int(int) = value else {
+        return false;
+    };
+
+    matches!(int.as_u8(), Some(0))
+}
+
+fn replace_with_inner(checker: &Checker, outer_range: TextRange, inner_range: TextRange) -> Edit {
+    let inner_expr = checker.locator().slice(inner_range);
+
+    Edit::range_replacement(inner_expr.to_string(), outer_range)
+}