Add RUF016: Detection of invalid index types (#5602)

Detects invalid types for tuple, list, bytes, string indices.

For example, the following will raise a `TypeError` at runtime and when
imported Python will display a `SyntaxWarning`

```python
var = [1, 2, 3]["x"]
```

```
example.py:1: SyntaxWarning: list indices must be integers or slices, not str; perhaps you missed a comma?
  var = [1, 2, 3]["x"]
Traceback (most recent call last):
  File "example.py", line 1, in <module>
    var = [1, 2, 3]["x"]
          ~~~~~~~~~^^^^^
TypeError: list indices must be integers or slices, not str
```

Previously, Ruff would not report the invalid syntax but now a violation
will be reported. This does not apply to cases where a variable, call,
or complex expression is used in the index — detection is roughly
limited to static definitions, which matches Python's warnings.

```
❯ ./target/debug/ruff example.py --select RUF015 --show-source --no-cache
example.py:1:17: RUF015 Indexed access to type `list` uses type `str` instead of an integer or slice.
  |
1 | var = [1, 2, 3]["x"]
  |                 ^^^ RUF015
  |
```

Closes #5082
xref
python/cpython@ffff144

crates/ruff/resources/test/fixtures/ruff/RUF016.py

@@ -0,0 +1,115 @@
+# Should not emit for valid access with index
+var = "abc"[0]
+var = f"abc"[0]
+var = [1, 2, 3][0]
+var = (1, 2, 3)[0]
+var = b"abc"[0]
+
+# Should not emit for valid access with slice
+var = "abc"[0:2]
+var = f"abc"[0:2]
+var = b"abc"[0:2]
+var = [1, 2, 3][0:2]
+var = (1, 2, 3)[0:2]
+var = [1, 2, 3][None:2]
+var = [1, 2, 3][0:None]
+var = [1, 2, 3][:2]
+var = [1, 2, 3][0:]
+
+# Should emit for invalid access on strings
+var = "abc"["x"]
+var = f"abc"["x"]
+
+# Should emit for invalid access on bytes
+var = b"abc"["x"]
+
+# Should emit for invalid access on lists and tuples
+var = [1, 2, 3]["x"]
+var = (1, 2, 3)["x"]
+
+# Should emit for invalid access on list comprehensions
+var = [x for x in range(10)]["x"]
+
+# Should emit for invalid access using tuple
+var = "abc"[1, 2]
+
+# Should emit for invalid access using string
+var = [1, 2]["x"]
+
+# Should emit for invalid access using float
+var = [1, 2][0.25]
+
+# Should emit for invalid access using dict
+var = [1, 2][{"x": "y"}]
+
+# Should emit for invalid access using dict comp
+var = [1, 2][{x: "y" for x in range(2)}]
+
+# Should emit for invalid access using list 
+var = [1, 2][2, 3]
+
+# Should emit for invalid access using list comp
+var = [1, 2][[x for x in range(2)]]
+
+# Should emit on invalid access using set
+var = [1, 2][{"x", "y"}]
+
+# Should emit on invalid access using set comp
+var = [1, 2][{x for x in range(2)}]
+
+# Should emit on invalid access using bytes
+var = [1, 2][b"x"]
+
+# Should emit for non-integer slice start
+var = [1, 2, 3]["x":2]
+var = [1, 2, 3][f"x":2]
+var = [1, 2, 3][1.2:2]
+var = [1, 2, 3][{"x"}:2]
+var = [1, 2, 3][{x for x in range(2)}:2]
+var = [1, 2, 3][{"x": x for x in range(2)}:2]
+var = [1, 2, 3][[x for x in range(2)]:2]
+
+# Should emit for non-integer slice end
+var = [1, 2, 3][0:"x"]
+var = [1, 2, 3][0:f"x"]
+var = [1, 2, 3][0:1.2]
+var = [1, 2, 3][0:{"x"}]
+var = [1, 2, 3][0:{x for x in range(2)}]
+var = [1, 2, 3][0:{"x": x for x in range(2)}]
+var = [1, 2, 3][0:[x for x in range(2)]]
+
+# Should emit for non-integer slice step
+var = [1, 2, 3][0:1:"x"]
+var = [1, 2, 3][0:1:f"x"]
+var = [1, 2, 3][0:1:1.2]
+var = [1, 2, 3][0:1:{"x"}]
+var = [1, 2, 3][0:1:{x for x in range(2)}]
+var = [1, 2, 3][0:1:{"x": x for x in range(2)}]
+var = [1, 2, 3][0:1:[x for x in range(2)]]
+
+# Should emit for non-integer slice start and end; should emit twice with specific ranges
+var = [1, 2, 3]["x":"y"]
+
+# Should emit once for repeated invalid access
+var = [1, 2, 3]["x"]["y"]["z"]
+
+# Cannot emit on invalid access using variable in index
+x = "x"
+var = "abc"[x]
+
+# Cannot emit on invalid access using call
+def func():
+    return 1
+var = "abc"[func()]
+
+# Cannot emit on invalid access using a variable in parent
+x = [1, 2, 3]
+var = x["y"]
+
+# Cannot emit for invalid access on byte array
+var = bytearray(b"abc")["x"]
+
+# Cannot emit for slice bound using variable
+x = "x"
+var = [1, 2, 3][0:x]
+var = [1, 2, 3][x:1]

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

@@ -2142,7 +2142,7 @@ where
 
         // Pre-visit.
         match expr {
-            subscript @ Expr::Subscript(ast::ExprSubscript { value, slice, .. }) => {
+            Expr::Subscript(subscript @ ast::ExprSubscript { value, slice, .. }) => {
                 // Ex) Optional[...], Union[...]
                 if self.any_enabled(&[
                     Rule::FutureRewritableTypeAnnotation,
@@ -2235,6 +2235,10 @@ where
                     ruff::rules::unnecessary_iterable_allocation_for_first_element(self, subscript);
                 }
 
+                if self.enabled(Rule::InvalidIndexType) {
+                    ruff::rules::invalid_index_type(self, subscript);
+                }
+
                 pandas_vet::rules::subscript(self, value, expr);
             }
             Expr::Tuple(ast::ExprTuple {

crates/ruff/src/codes.rs

@@ -781,6 +781,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         #[cfg(feature = "unreachable-code")]
         (Ruff, "014") => (RuleGroup::Nursery, rules::ruff::rules::UnreachableCode),
         (Ruff, "015") => (RuleGroup::Unspecified, rules::ruff::rules::UnnecessaryIterableAllocationForFirstElement),
+        (Ruff, "016") => (RuleGroup::Unspecified, rules::ruff::rules::InvalidIndexType),
         (Ruff, "100") => (RuleGroup::Unspecified, rules::ruff::rules::UnusedNOQA),
         (Ruff, "200") => (RuleGroup::Unspecified, rules::ruff::rules::InvalidPyprojectToml),
 

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

@@ -34,6 +34,7 @@ mod tests {
         Rule::UnnecessaryIterableAllocationForFirstElement,
         Path::new("RUF015.py")
     )]
+    #[test_case(Rule::InvalidIndexType, Path::new("RUF016.py"))]
     #[cfg_attr(
         feature = "unreachable-code",
         test_case(Rule::UnreachableCode, Path::new("RUF014.py"))

crates/ruff/src/rules/ruff/rules/invalid_index_type.rs

@@ -0,0 +1,214 @@
+use rustpython_parser::ast::{Constant, Expr, ExprConstant, ExprSlice, ExprSubscript, Ranged};
+
+use ruff_diagnostics::{Diagnostic, Violation};
+use ruff_macros::{derive_message_formats, violation};
+use std::fmt;
+
+use crate::checkers::ast::Checker;
+
+/// ## What it does
+/// Checks for indexed access to lists, strings, tuples, bytes, and comprehensions
+/// using a type other than an integer or slice.
+///
+/// ## Why is this bad?
+/// Only integers or slices can be used as indices to these types. Using
+/// other types will result in a `TypeError` at runtime and a `SyntaxWarning` at
+/// import time.
+///
+/// ## Example
+/// ```python
+/// var = [1, 2, 3]["x"]
+/// ```
+///
+/// Use instead:
+/// ```python
+/// var = [1, 2, 3][0]
+/// ```
+#[violation]
+pub struct InvalidIndexType {
+    value_type: String,
+    index_type: String,
+    is_slice: bool,
+}
+
+impl Violation for InvalidIndexType {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        let InvalidIndexType {
+            value_type,
+            index_type,
+            is_slice,
+        } = self;
+        if *is_slice {
+            format!("Slice in indexed access to type `{value_type}` uses type `{index_type}` instead of an integer.")
+        } else {
+            format!(
+                "Indexed access to type `{value_type}` uses type `{index_type}` instead of an integer or slice."
+            )
+        }
+    }
+}
+
+/// RUF015
+pub(crate) fn invalid_index_type(checker: &mut Checker, expr: &ExprSubscript) {
+    let ExprSubscript {
+        value,
+        slice: index,
+        ..
+    } = expr;
+
+    // Check the value being indexed is a list, tuple, string, f-string, bytes, or comprehension
+    if !matches!(
+        value.as_ref(),
+        Expr::List(_)
+            | Expr::ListComp(_)
+            | Expr::Tuple(_)
+            | Expr::JoinedStr(_)
+            | Expr::Constant(ExprConstant {
+                value: Constant::Str(_) | Constant::Bytes(_),
+                ..
+            })
+    ) {
+        return;
+    }
+
+    // The value types supported by this rule should always be checkable
+    let Some(value_type) = CheckableExprType::try_from(value) else {
+        debug_assert!(false, "Index value must be a checkable type to generate a violation message.");
+        return;
+    };
+
+    // If the index is not a checkable type then we can't easily determine if there is a violation
+    let Some(index_type) = CheckableExprType::try_from(index) else {
+        return;
+    };
+
+    // Then check the contents of the index
+    match index.as_ref() {
+        Expr::Constant(ExprConstant {
+            value: index_value, ..
+        }) => {
+            // If the index is a constant, require an integer
+            if !index_value.is_int() {
+                checker.diagnostics.push(Diagnostic::new(
+                    InvalidIndexType {
+                        value_type: value_type.to_string(),
+                        index_type: constant_type_name(index_value).to_string(),
+                        is_slice: false,
+                    },
+                    index.range(),
+                ));
+            }
+        }
+        Expr::Slice(ExprSlice {
+            lower, upper, step, ..
+        }) => {
+            // If the index is a slice, require integer or null bounds
+            for is_slice in [lower, upper, step].into_iter().flatten() {
+                if let Expr::Constant(ExprConstant {
+                    value: index_value, ..
+                }) = is_slice.as_ref()
+                {
+                    if !(index_value.is_int() || index_value.is_none()) {
+                        checker.diagnostics.push(Diagnostic::new(
+                            InvalidIndexType {
+                                value_type: value_type.to_string(),
+                                index_type: constant_type_name(index_value).to_string(),
+                                is_slice: true,
+                            },
+                            is_slice.range(),
+                        ));
+                    }
+                } else if let Some(is_slice_type) = CheckableExprType::try_from(is_slice.as_ref()) {
+                    checker.diagnostics.push(Diagnostic::new(
+                        InvalidIndexType {
+                            value_type: value_type.to_string(),
+                            index_type: is_slice_type.to_string(),
+                            is_slice: true,
+                        },
+                        is_slice.range(),
+                    ));
+                }
+            }
+        }
+        _ => {
+            // If it's some other checkable data type, it's a violation
+            checker.diagnostics.push(Diagnostic::new(
+                InvalidIndexType {
+                    value_type: value_type.to_string(),
+                    index_type: index_type.to_string(),
+                    is_slice: false,
+                },
+                index.range(),
+            ));
+        }
+    }
+}
+
+/// An expression that can be checked for type compatibility.
+///
+/// These are generally "literal" type expressions in that we know their concrete type
+/// without additional analysis; opposed to expressions like a function call where we
+/// cannot determine what type it may return.
+#[derive(Debug)]
+enum CheckableExprType<'a> {
+    Constant(&'a Constant),
+    JoinedStr,
+    List,
+    ListComp,
+    SetComp,
+    DictComp,
+    Set,
+    Dict,
+    Tuple,
+    Slice,
+}
+
+impl fmt::Display for CheckableExprType<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            Self::Constant(constant) => f.write_str(constant_type_name(constant)),
+            Self::JoinedStr => f.write_str("str"),
+            Self::List => f.write_str("list"),
+            Self::SetComp => f.write_str("set comprehension"),
+            Self::ListComp => f.write_str("list comprehension"),
+            Self::DictComp => f.write_str("dict comprehension"),
+            Self::Set => f.write_str("set"),
+            Self::Slice => f.write_str("slice"),
+            Self::Dict => f.write_str("dict"),
+            Self::Tuple => f.write_str("tuple"),
+        }
+    }
+}
+
+impl<'a> CheckableExprType<'a> {
+    fn try_from(expr: &'a Expr) -> Option<Self> {
+        match expr {
+            Expr::Constant(ExprConstant { value, .. }) => Some(Self::Constant(value)),
+            Expr::JoinedStr(_) => Some(Self::JoinedStr),
+            Expr::List(_) => Some(Self::List),
+            Expr::ListComp(_) => Some(Self::ListComp),
+            Expr::SetComp(_) => Some(Self::SetComp),
+            Expr::DictComp(_) => Some(Self::DictComp),
+            Expr::Set(_) => Some(Self::Set),
+            Expr::Dict(_) => Some(Self::Dict),
+            Expr::Tuple(_) => Some(Self::Tuple),
+            Expr::Slice(_) => Some(Self::Slice),
+            _ => None,
+        }
+    }
+}
+
+fn constant_type_name(constant: &Constant) -> &'static str {
+    match constant {
+        Constant::None => "None",
+        Constant::Bool(_) => "bool",
+        Constant::Str(_) => "str",
+        Constant::Bytes(_) => "bytes",
+        Constant::Int(_) => "int",
+        Constant::Tuple(_) => "tuple",
+        Constant::Float(_) => "float",
+        Constant::Complex { .. } => "complex",
+        Constant::Ellipsis => "ellipsis",
+    }
+}

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

@@ -4,6 +4,7 @@ pub(crate) use collection_literal_concatenation::*;
 pub(crate) use explicit_f_string_type_conversion::*;
 pub(crate) use function_call_in_dataclass_default::*;
 pub(crate) use implicit_optional::*;
+pub(crate) use invalid_index_type::*;
 pub(crate) use invalid_pyproject_toml::*;
 pub(crate) use mutable_class_default::*;
 pub(crate) use mutable_dataclass_default::*;
@@ -22,6 +23,7 @@ mod explicit_f_string_type_conversion;
 mod function_call_in_dataclass_default;
 mod helpers;
 mod implicit_optional;
+mod invalid_index_type;
 mod invalid_pyproject_toml;
 mod mutable_class_default;
 mod mutable_dataclass_default;