Fix sign-handling bugs and false negatives in cast_sign_loss

clippy_lints/src/casts/cast_sign_loss.rs

@@ -1,15 +1,47 @@
+use std::convert::Infallible;
+use std::ops::ControlFlow;
+
 use clippy_utils::consts::{constant, Constant};
 use clippy_utils::diagnostics::span_lint;
-use clippy_utils::{clip, method_chain_args, sext};
+use clippy_utils::visitors::{for_each_expr, Descend};
+use clippy_utils::{method_chain_args, sext};
 use rustc_hir::{BinOpKind, Expr, ExprKind};
 use rustc_lint::LateContext;
-use rustc_middle::ty::{self, Ty, UintTy};
+use rustc_middle::ty::{self, Ty};
 
 use super::CAST_SIGN_LOSS;
 
-const METHODS_RET_POSITIVE: &[&str] = &["abs", "checked_abs", "rem_euclid", "checked_rem_euclid"];
+/// A list of methods that can never return a negative value.
+/// Includes methods that panic rather than returning a negative value.
+///
+/// Methods that can overflow and return a negative value must not be included in this list,
+/// because casting their return values can still result in sign loss.
+const METHODS_RET_POSITIVE: &[&str] = &[
+    "checked_abs",
+    "saturating_abs",
+    "isqrt",
+    "checked_isqrt",
+    "rem_euclid",
+    "checked_rem_euclid",
+    "wrapping_rem_euclid",
+];
+
+/// A list of methods that act like `pow()`. See `pow_call_result_sign()` for details.
+///
+/// Methods that can overflow and return a negative value must not be included in this list,
+/// because casting their return values can still result in sign loss.
+const METHODS_POW: &[&str] = &["pow", "saturating_pow", "checked_pow"];
 
-pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_op: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+/// A list of methods that act like `unwrap()`, and don't change the sign of the inner value.
+const METHODS_UNWRAP: &[&str] = &["unwrap", "unwrap_unchecked", "expect", "into_ok"];
+
+pub(super) fn check<'cx>(
+    cx: &LateContext<'cx>,
+    expr: &Expr<'_>,
+    cast_op: &Expr<'_>,
+    cast_from: Ty<'cx>,
+    cast_to: Ty<'_>,
+) {
     if should_lint(cx, cast_op, cast_from, cast_to) {
         span_lint(
             cx,
@@ -20,35 +52,27 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_op: &Expr<'_>, c
     }
 }
 
-fn should_lint(cx: &LateContext<'_>, cast_op: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) -> bool {
+fn should_lint<'cx>(cx: &LateContext<'cx>, cast_op: &Expr<'_>, cast_from: Ty<'cx>, cast_to: Ty<'_>) -> bool {
     match (cast_from.is_integral(), cast_to.is_integral()) {
         (true, true) => {
             if !cast_from.is_signed() || cast_to.is_signed() {
                 return false;
             }
 
-            // Don't lint if `cast_op` is known to be positive.
+            // Don't lint if `cast_op` is known to be positive, ignoring overflow.
             if let Sign::ZeroOrPositive = expr_sign(cx, cast_op, cast_from) {
                 return false;
             }
 
-            let (mut uncertain_count, mut negative_count) = (0, 0);
-            // Peel off possible binary expressions, e.g. x * x * y => [x, x, y]
-            let Some(exprs) = exprs_with_selected_binop_peeled(cast_op) else {
-                // Assume cast sign lose if we cannot determine the sign of `cast_op`
-                return true;
-            };
-            for expr in exprs {
-                let ty = cx.typeck_results().expr_ty(expr);
-                match expr_sign(cx, expr, ty) {
-                    Sign::Negative => negative_count += 1,
-                    Sign::Uncertain => uncertain_count += 1,
-                    Sign::ZeroOrPositive => (),
-                };
+            if let Sign::ZeroOrPositive = expr_muldiv_sign(cx, cast_op) {
+                return false;
+            }
+
+            if let Sign::ZeroOrPositive = expr_add_sign(cx, cast_op) {
+                return false;
             }
 
-            // Lint if there are odd number of uncertain or negative results
-            uncertain_count % 2 == 1 || negative_count % 2 == 1
+            true
         },
 
         (false, true) => !cast_to.is_signed(),
@@ -57,7 +81,13 @@ fn should_lint(cx: &LateContext<'_>, cast_op: &Expr<'_>, cast_from: Ty<'_>, cast
     }
 }
 
-fn get_const_int_eval(cx: &LateContext<'_>, expr: &Expr<'_>, ty: Ty<'_>) -> Option<i128> {
+fn get_const_signed_int_eval<'cx>(
+    cx: &LateContext<'cx>,
+    expr: &Expr<'_>,
+    ty: impl Into<Option<Ty<'cx>>>,
+) -> Option<i128> {
+    let ty = ty.into().unwrap_or_else(|| cx.typeck_results().expr_ty(expr));
+
     if let Constant::Int(n) = constant(cx, cx.typeck_results(), expr)?
         && let ty::Int(ity) = *ty.kind()
     {
@@ -66,29 +96,52 @@ fn get_const_int_eval(cx: &LateContext<'_>, expr: &Expr<'_>, ty: Ty<'_>) -> Opti
     None
 }
 
+fn get_const_unsigned_int_eval<'cx>(
+    cx: &LateContext<'cx>,
+    expr: &Expr<'_>,
+    ty: impl Into<Option<Ty<'cx>>>,
+) -> Option<u128> {
+    let ty = ty.into().unwrap_or_else(|| cx.typeck_results().expr_ty(expr));
+
+    if let Constant::Int(n) = constant(cx, cx.typeck_results(), expr)?
+        && let ty::Uint(_ity) = *ty.kind()
+    {
+        return Some(n);
+    }
+    None
+}
+
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
 enum Sign {
     ZeroOrPositive,
     Negative,
     Uncertain,
 }
 
-fn expr_sign(cx: &LateContext<'_>, expr: &Expr<'_>, ty: Ty<'_>) -> Sign {
+fn expr_sign<'cx>(cx: &LateContext<'cx>, expr: &Expr<'_>, ty: impl Into<Option<Ty<'cx>>>) -> Sign {
     // Try evaluate this expr first to see if it's positive
-    if let Some(val) = get_const_int_eval(cx, expr, ty) {
+    if let Some(val) = get_const_signed_int_eval(cx, expr, ty) {
         return if val >= 0 { Sign::ZeroOrPositive } else { Sign::Negative };
     }
+    if let Some(_val) = get_const_unsigned_int_eval(cx, expr, None) {
+        return Sign::ZeroOrPositive;
+    }
+
     // Calling on methods that always return non-negative values.
     if let ExprKind::MethodCall(path, caller, args, ..) = expr.kind {
         let mut method_name = path.ident.name.as_str();
 
-        if method_name == "unwrap"
-            && let Some(arglist) = method_chain_args(expr, &["unwrap"])
+        // Peel unwrap(), expect(), etc.
+        while let Some(&found_name) = METHODS_UNWRAP.iter().find(|&name| &method_name == name)
+            && let Some(arglist) = method_chain_args(expr, &[found_name])
             && let ExprKind::MethodCall(inner_path, ..) = &arglist[0].0.kind
         {
+            // The original type has changed, but we can't use `ty` here anyway, because it has been
+            // moved.
             method_name = inner_path.ident.name.as_str();
         }
 
-        if method_name == "pow"
+        if METHODS_POW.iter().any(|&name| method_name == name)
             && let [arg] = args
         {
             return pow_call_result_sign(cx, caller, arg);
@@ -100,53 +153,182 @@ fn expr_sign(cx: &LateContext<'_>, expr: &Expr<'_>, ty: Ty<'_>) -> Sign {
     Sign::Uncertain
 }
 
-/// Return the sign of the `pow` call's result.
+/// Return the sign of the `pow` call's result, ignoring overflow.
 ///
-/// If the caller is a positive number, the result is always positive,
-/// If the `power_of` is a even number, the result is always positive as well,
-/// Otherwise a [`Sign::Uncertain`] will be returned.
-fn pow_call_result_sign(cx: &LateContext<'_>, caller: &Expr<'_>, power_of: &Expr<'_>) -> Sign {
-    let caller_ty = cx.typeck_results().expr_ty(caller);
-    if let Some(caller_val) = get_const_int_eval(cx, caller, caller_ty)
-        && caller_val >= 0
-    {
-        return Sign::ZeroOrPositive;
+/// If the base is positive, the result is always positive.
+/// If the exponent is a even number, the result is always positive,
+/// Otherwise, if the base is negative, and the exponent is an odd number, the result is always
+/// negative.
+///
+/// Otherwise, returns [`Sign::Uncertain`].
+fn pow_call_result_sign(cx: &LateContext<'_>, base: &Expr<'_>, exponent: &Expr<'_>) -> Sign {
+    let base_sign = expr_sign(cx, base, None);
+
+    // Rust's integer pow() functions take an unsigned exponent.
+    let exponent_val = get_const_unsigned_int_eval(cx, exponent, None);
+    let exponent_is_even = exponent_val.map(|val| val % 2 == 0);
+
+    match (base_sign, exponent_is_even) {
+        // Non-negative bases always return non-negative results, ignoring overflow.
+        (Sign::ZeroOrPositive, _) |
+        // Any base raised to an even exponent is non-negative.
+        // These both hold even if we don't know the value of the base.
+        (_, Some(true))
+            => Sign::ZeroOrPositive,
+
+        // A negative base raised to an odd exponent is non-negative.
+        (Sign::Negative, Some(false)) => Sign::Negative,
+
+        // Negative/unknown base to an unknown exponent, or unknown base to an odd exponent.
+        // Could be negative or positive depending on the actual values.
+        (Sign::Negative | Sign::Uncertain, None) |
+        (Sign::Uncertain, Some(false)) => Sign::Uncertain,
     }
+}
 
-    if let Some(Constant::Int(n)) = constant(cx, cx.typeck_results(), power_of)
-        && clip(cx.tcx, n, UintTy::U32) % 2 == 0
-    {
-        return Sign::ZeroOrPositive;
+/// Peels binary operators such as [`BinOpKind::Mul`] or [`BinOpKind::Rem`],
+/// where the result could always be positive. See [`exprs_with_muldiv_binop_peeled()`] for details.
+///
+/// Returns the sign of the list of peeled expressions.
+fn expr_muldiv_sign(cx: &LateContext<'_>, expr: &Expr<'_>) -> Sign {
+    let mut negative_count = 0;
+
+    // Peel off possible binary expressions, for example:
+    // x * x / y => [x, x, y]
+    // a % b => [a]
+    let exprs = exprs_with_muldiv_binop_peeled(expr);
+    for expr in exprs {
+        match expr_sign(cx, expr, None) {
+            Sign::Negative => negative_count += 1,
+            // A mul/div is:
+            // - uncertain if there are any uncertain values (because they could be negative or positive),
+            Sign::Uncertain => return Sign::Uncertain,
+            Sign::ZeroOrPositive => (),
+        };
     }
 
-    Sign::Uncertain
+    // A mul/div is:
+    // - negative if there are an odd number of negative values,
+    // - positive or zero otherwise.
+    if negative_count % 2 == 1 {
+        Sign::Negative
+    } else {
+        Sign::ZeroOrPositive
+    }
+}
+
+/// Peels binary operators such as [`BinOpKind::Add`], where the result could always be positive.
+/// See [`exprs_with_add_binop_peeled()`] for details.
+///
+/// Returns the sign of the list of peeled expressions.
+fn expr_add_sign(cx: &LateContext<'_>, expr: &Expr<'_>) -> Sign {
+    let mut negative_count = 0;
+    let mut positive_count = 0;
+
+    // Peel off possible binary expressions, for example:
+    // a + b + c => [a, b, c]
+    let exprs = exprs_with_add_binop_peeled(expr);
+    for expr in exprs {
+        match expr_sign(cx, expr, None) {
+            Sign::Negative => negative_count += 1,
+            // A sum is:
+            // - uncertain if there are any uncertain values (because they could be negative or positive),
+            Sign::Uncertain => return Sign::Uncertain,
+            Sign::ZeroOrPositive => positive_count += 1,
+        };
+    }
+
+    // A sum is:
+    // - positive or zero if there are only positive (or zero) values,
+    // - negative if there are only negative (or zero) values, or
+    // - uncertain if there are both.
+    // We could split Zero out into its own variant, but we don't yet.
+    if negative_count == 0 {
+        Sign::ZeroOrPositive
+    } else if positive_count == 0 {
+        Sign::Negative
+    } else {
+        Sign::Uncertain
+    }
 }
 
 /// Peels binary operators such as [`BinOpKind::Mul`], [`BinOpKind::Div`] or [`BinOpKind::Rem`],
-/// which the result could always be positive under certain condition.
+/// where the result depends on:
+/// - the number of negative values in the entire expression, or
+/// - the number of negative values on the left hand side of the expression.
+/// Ignores overflow.
 ///
-/// Other operators such as `+`/`-` causing the result's sign hard to determine, which we will
-/// return `None`
-fn exprs_with_selected_binop_peeled<'a>(expr: &'a Expr<'_>) -> Option<Vec<&'a Expr<'a>>> {
-    #[inline]
-    fn collect_operands<'a>(expr: &'a Expr<'a>, operands: &mut Vec<&'a Expr<'a>>) -> Option<()> {
-        match expr.kind {
-            ExprKind::Binary(op, lhs, rhs) => {
-                if matches!(op.node, BinOpKind::Mul | BinOpKind::Div | BinOpKind::Rem) {
-                    collect_operands(lhs, operands);
-                    operands.push(rhs);
-                } else {
-                    // Things are complicated when there are other binary ops exist,
-                    // abort checking by returning `None` for now.
-                    return None;
-                }
-            },
-            _ => operands.push(expr),
+///
+/// Expressions using other operators are preserved, so we can try to evaluate them later.
+fn exprs_with_muldiv_binop_peeled<'e>(expr: &'e Expr<'_>) -> Vec<&'e Expr<'e>> {
+    let mut res = vec![];
+
+    for_each_expr(expr, |sub_expr| -> ControlFlow<Infallible, Descend> {
+        // We don't check for mul/div/rem methods here, but we could.
+        if let ExprKind::Binary(op, lhs, _rhs) = sub_expr.kind {
+            if matches!(op.node, BinOpKind::Mul | BinOpKind::Div) {
+                // For binary operators where both sides contribute to the sign of the result,
+                // collect all their operands, recursively. This ignores overflow.
+                ControlFlow::Continue(Descend::Yes)
+            } else if matches!(op.node, BinOpKind::Rem | BinOpKind::Shr) {
+                // For binary operators where the left hand side determines the sign of the result,
+                // only collect that side, recursively. Overflow panics, so this always holds.
+                //
+                // Large left shifts turn negatives into zeroes, so we can't use it here.
+                //
+                // > Given remainder = dividend % divisor, the remainder will have the same sign as the dividend
+                // > ...
+                // > Arithmetic right shift on signed integer types
+                // https://doc.rust-lang.org/reference/expressions/operator-expr.html#arithmetic-and-logical-binary-operators
+
+                // We want to descend into the lhs, but skip the rhs.
+                // That's tricky to do using for_each_expr(), so we just keep the lhs intact.
+                res.push(lhs);
+                ControlFlow::Continue(Descend::No)
+            } else {
+                // The sign of the result of other binary operators depends on the values of the operands,
+                // so try to evaluate the expression.
+                res.push(sub_expr);
+                ControlFlow::Continue(Descend::No)
+            }
+        } else {
+            // For other expressions, including unary operators and constants, try to evaluate the expression.
+            res.push(sub_expr);
+            ControlFlow::Continue(Descend::No)
         }
-        Some(())
-    }
+    });
 
+    res
+}
+
+/// Peels binary operators such as [`BinOpKind::Add`], where the result depends on:
+/// - all the expressions being positive, or
+/// - all the expressions being negative.
+/// Ignores overflow.
+///
+/// Expressions using other operators are preserved, so we can try to evaluate them later.
+fn exprs_with_add_binop_peeled<'e>(expr: &'e Expr<'_>) -> Vec<&'e Expr<'e>> {
     let mut res = vec![];
-    collect_operands(expr, &mut res)?;
-    Some(res)
+
+    for_each_expr(expr, |sub_expr| -> ControlFlow<Infallible, Descend> {
+        // We don't check for add methods here, but we could.
+        if let ExprKind::Binary(op, _lhs, _rhs) = sub_expr.kind {
+            if matches!(op.node, BinOpKind::Add) {
+                // For binary operators where both sides contribute to the sign of the result,
+                // collect all their operands, recursively. This ignores overflow.
+                ControlFlow::Continue(Descend::Yes)
+            } else {
+                // The sign of the result of other binary operators depends on the values of the operands,
+                // so try to evaluate the expression.
+                res.push(sub_expr);
+                ControlFlow::Continue(Descend::No)
+            }
+        } else {
+            // For other expressions, including unary operators and constants, try to evaluate the expression.
+            res.push(sub_expr);
+            ControlFlow::Continue(Descend::No)
+        }
+    });
+
+    res
 }