[refurb] Implement unnecessary-enumerate (FURB148) (astral-sh#7454

)

## Summary

Implement
[`no-ignored-enumerate-items`](https://github.com/dosisod/refurb/blob/master/refurb/checks/builtin/no_ignored_enumerate.py)
as `unnecessary-enumerate` (`FURB148`).

The auto-fix considers if a `start` argument is passed to the
`enumerate()` function. If only the index is used, then the suggested
fix is to pass the `start` value to the `range()` function. So,

```python
for i, _ in enumerate(xs, 1):
    ...
```

becomes

```python
for i in range(1, len(xs)):
    ...
```

If the index is ignored and only the value is ignored, and if a start
value greater than zero is passed to `enumerate()`, the rule doesn't
produce a suggestion. I couldn't find a unanimously accepted best way to
iterate over a collection whilst skipping the first n elements. The rule
still triggers, however.

Related to astral-sh#1348.

## Test Plan

`cargo test`

---------

Co-authored-by: Dhruv Manilawala <dhruvmanila@gmail.com>
Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

crates/ruff/resources/test/fixtures/refurb/FURB148.py

@@ -0,0 +1,66 @@
+books = ["Dune", "Foundation", "Neuromancer"]
+
+# Errors
+for index, _ in enumerate(books):
+    print(index)
+
+for index, _ in enumerate(books, start=0):
+    print(index)
+
+for index, _ in enumerate(books, 0):
+    print(index)
+
+for index, _ in enumerate(books, start=1):
+    print(index)
+
+for index, _ in enumerate(books, 1):
+    print(index)
+
+for index, _ in enumerate(books, start=x):
+    print(book)
+
+for index, _ in enumerate(books, x):
+    print(book)
+
+for _, book in enumerate(books):
+    print(book)
+
+for _, book in enumerate(books, start=0):
+    print(book)
+
+for _, book in enumerate(books, 0):
+    print(book)
+
+for _, book in enumerate(books, start=1):
+    print(book)
+
+for _, book in enumerate(books, 1):
+    print(book)
+
+for _, book in enumerate(books, start=x):
+    print(book)
+
+for _, book in enumerate(books, x):
+    print(book)
+
+for index, (_, _) in enumerate(books):
+    print(index)
+
+for (_, _), book in enumerate(books):
+    print(book)
+
+for(index, _)in enumerate(books):
+    print(index)
+
+for(index), _ in enumerate(books):
+    print(index)
+
+# OK
+for index, book in enumerate(books):
+    print(index, book)
+
+for index in range(len(books)):
+    print(index)
+
+for book in books:
+    print(book)

crates/ruff/src/checkers/ast/analyze/deferred_for_loops.rs

@@ -2,7 +2,7 @@ use ruff_python_ast::Stmt;
 
 use crate::checkers::ast::Checker;
 use crate::codes::Rule;
-use crate::rules::{flake8_bugbear, perflint, pyupgrade};
+use crate::rules::{flake8_bugbear, perflint, pyupgrade, refurb};
 
 /// Run lint rules over all deferred for-loops in the [`SemanticModel`].
 pub(crate) fn deferred_for_loops(checker: &mut Checker) {
@@ -24,6 +24,9 @@ pub(crate) fn deferred_for_loops(checker: &mut Checker) {
             if checker.enabled(Rule::YieldInForLoop) {
                 pyupgrade::rules::yield_in_for_loop(checker, stmt_for);
             }
+            if checker.enabled(Rule::UnnecessaryEnumerate) {
+                refurb::rules::unnecessary_enumerate(checker, stmt_for);
+            }
         }
     }
 }

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

@@ -1190,6 +1190,7 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
             if checker.any_enabled(&[
                 Rule::UnusedLoopControlVariable,
                 Rule::IncorrectDictIterator,
+                Rule::UnnecessaryEnumerate,
                 Rule::YieldInForLoop,
             ]) {
                 checker.deferred.for_loops.push(checker.semantic.snapshot());

crates/ruff/src/codes.rs

@@ -921,6 +921,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Refurb, "132") => (RuleGroup::Nursery, rules::refurb::rules::CheckAndRemoveFromSet),
         (Refurb, "140") => (RuleGroup::Preview, rules::refurb::rules::ReimplementedStarmap),
         (Refurb, "145") => (RuleGroup::Preview, rules::refurb::rules::SliceCopy),
+        (Refurb, "148") => (RuleGroup::Preview, rules::refurb::rules::UnnecessaryEnumerate),
 
         // flake8-logging
         (Flake8Logging, "001") => (RuleGroup::Preview, rules::flake8_logging::rules::DirectLoggerInstantiation),

crates/ruff/src/rules/perflint/rules/incorrect_dict_iterator.rs

@@ -4,7 +4,6 @@ use ruff_diagnostics::{AlwaysAutofixableViolation, Diagnostic, Edit, Fix};
 use ruff_macros::{derive_message_formats, violation};
 use ruff_python_ast as ast;
 use ruff_python_ast::{Arguments, Expr};
-use ruff_python_semantic::SemanticModel;
 use ruff_text_size::Ranged;
 
 use crate::checkers::ast::Checker;
@@ -83,8 +82,8 @@ pub(crate) fn incorrect_dict_iterator(checker: &mut Checker, stmt_for: &ast::Stm
     }
 
     match (
-        is_unused(key, checker.semantic()),
-        is_unused(value, checker.semantic()),
+        checker.semantic().is_unused(key),
+        checker.semantic().is_unused(value),
     ) {
         (true, true) => {
             // Both the key and the value are unused.
@@ -145,36 +144,3 @@ impl fmt::Display for DictSubset {
         }
     }
 }
-
-/// Returns `true` if the given expression is either an unused value or a tuple of unused values.
-fn is_unused(expr: &Expr, semantic: &SemanticModel) -> bool {
-    match expr {
-        Expr::Tuple(ast::ExprTuple { elts, .. }) => {
-            elts.iter().all(|expr| is_unused(expr, semantic))
-        }
-        Expr::Name(ast::ExprName { id, .. }) => {
-            // Treat a variable as used if it has any usages, _or_ it's shadowed by another variable
-            // with usages.
-            //
-            // If we don't respect shadowing, we'll incorrectly flag `bar` as unused in:
-            // ```python
-            // from random import random
-            //
-            // for bar in range(10):
-            //     if random() > 0.5:
-            //         break
-            // else:
-            //     bar = 1
-            //
-            // print(bar)
-            // ```
-            let scope = semantic.current_scope();
-            scope
-                .get_all(id)
-                .map(|binding_id| semantic.binding(binding_id))
-                .filter(|binding| binding.start() >= expr.start())
-                .all(|binding| !binding.is_used())
-        }
-        _ => false,
-    }
-}

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

@@ -19,6 +19,7 @@ mod tests {
     #[test_case(Rule::CheckAndRemoveFromSet, Path::new("FURB132.py"))]
     #[test_case(Rule::ReimplementedStarmap, Path::new("FURB140.py"))]
     #[test_case(Rule::SliceCopy, Path::new("FURB145.py"))]
+    #[test_case(Rule::UnnecessaryEnumerate, Path::new("FURB148.py"))]
     fn rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!("{}_{}", rule_code.noqa_code(), path.to_string_lossy());
         let diagnostics = test_path(

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

@@ -3,9 +3,11 @@ pub(crate) use delete_full_slice::*;
 pub(crate) use reimplemented_starmap::*;
 pub(crate) use repeated_append::*;
 pub(crate) use slice_copy::*;
+pub(crate) use unnecessary_enumerate::*;
 
 mod check_and_remove_from_set;
 mod delete_full_slice;
 mod reimplemented_starmap;
 mod repeated_append;
 mod slice_copy;
+mod unnecessary_enumerate;