Restrict the cases where Clippy proposes to switch range types

To limit false positives, the `range_plus_one` and `range_minus_one` lints
will restrict themselves to situations where the iterator types can be easily
switched from exclusive to inclusive or vice-versa. This includes situations
where the range is used as an iterator, or is used for indexing. Also,
when the range is used as a function or method call argument and the
parameter type is generic over traits implemented by both kind of
ranges, the lint will trigger.

On the other hand, assignments of the range to variables, including
automatically typed ones or wildcards, will no longer trigger the lint.
However, the cases where such an assignment would benefit from the lint
are probably rare.

This fix doesn't prevent false positives from happening. A more complete
fix would check if the obligations can be satisfied by the new proposed
range type.

Author: samueltardieu

clippy_lints/src/cognitive_complexity.rs

@@ -103,7 +103,6 @@ impl CognitiveComplexity {
                 FnKind::ItemFn(ident, _, _) | FnKind::Method(ident, _) => ident.span,
                 FnKind::Closure => {
                     let header_span = body_span.with_hi(decl.output.span().lo());
-                    #[expect(clippy::range_plus_one)]
                     if let Some(range) = header_span.map_range(cx, |_, src, range| {
                         let mut idxs = src.get(range.clone())?.match_indices('|');
                         Some(range.start + idxs.next()?.0..range.start + idxs.next()?.0 + 1)

clippy_lints/src/doc/mod.rs

@@ -1159,7 +1159,6 @@ fn check_doc<'a, Events: Iterator<Item = (pulldown_cmark::Event<'a>, Range<usize
     headers
 }
 
-#[expect(clippy::range_plus_one)] // inclusive ranges aren't the same type
 fn looks_like_refdef(doc: &str, range: Range<usize>) -> Option<Range<usize>> {
     if range.end < range.start {
         return None;

clippy_lints/src/methods/manual_inspect.rs

@@ -100,7 +100,6 @@ pub(crate) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, arg: &Expr<'_>, name:
             match x {
                 UseKind::Return(s) => edits.push((s.with_leading_whitespace(cx).with_ctxt(s.ctxt()), String::new())),
                 UseKind::Borrowed(s) => {
-                    #[expect(clippy::range_plus_one)]
                     let range = s.map_range(cx, |_, src, range| {
                         let src = src.get(range.clone())?;
                         let trimmed = src.trim_start_matches([' ', '\t', '\n', '\r', '(']);

clippy_lints/src/ranges.rs

@@ -4,15 +4,19 @@ use clippy_utils::diagnostics::{span_lint, span_lint_and_sugg, span_lint_and_the
 use clippy_utils::msrvs::{self, Msrv};
 use clippy_utils::source::{SpanRangeExt, snippet, snippet_with_applicability};
 use clippy_utils::sugg::Sugg;
-use clippy_utils::{get_parent_expr, higher, is_in_const_context, is_integer_const, path_to_local};
+use clippy_utils::ty::implements_trait;
+use clippy_utils::{
+    fn_def_id, get_parent_expr, higher, is_in_const_context, is_integer_const, is_path_lang_item, path_to_local,
+};
+use rustc_ast::Mutability;
 use rustc_ast::ast::RangeLimits;
 use rustc_errors::Applicability;
-use rustc_hir::{BinOpKind, Expr, ExprKind, HirId};
+use rustc_hir::{BinOpKind, Expr, ExprKind, HirId, LangItem};
 use rustc_lint::{LateContext, LateLintPass};
-use rustc_middle::ty;
+use rustc_middle::ty::{self, ClauseKind, GenericArgKind, PredicatePolarity, Ty};
 use rustc_session::impl_lint_pass;
-use rustc_span::Span;
 use rustc_span::source_map::Spanned;
+use rustc_span::{Span, sym};
 use std::cmp::Ordering;
 
 declare_clippy_lint! {
@@ -24,6 +28,12 @@ declare_clippy_lint! {
     /// The code is more readable with an inclusive range
     /// like `x..=y`.
     ///
+    /// ### Limitations
+    /// The lint is conservative and will trigger only when switching
+    /// from an exclusive to an inclusive range is provably safe from
+    /// a typing point of view. This corresponds to situations where
+    /// the range is used as an iterator, or for indexing.
+    ///
     /// ### Known problems
     /// Will add unnecessary pair of parentheses when the
     /// expression is not wrapped in a pair but starts with an opening parenthesis
@@ -34,11 +44,6 @@ declare_clippy_lint! {
     /// exclusive ranges, because they essentially add an extra branch that
     /// LLVM may fail to hoist out of the loop.
     ///
-    /// This will cause a warning that cannot be fixed if the consumer of the
-    /// range only accepts a specific range type, instead of the generic
-    /// `RangeBounds` trait
-    /// ([#3307](https://github.com/rust-lang/rust-clippy/issues/3307)).
-    ///
     /// ### Example
     /// ```no_run
     /// # let x = 0;
@@ -71,11 +76,11 @@ declare_clippy_lint! {
     /// The code is more readable with an exclusive range
     /// like `x..y`.
     ///
-    /// ### Known problems
-    /// This will cause a warning that cannot be fixed if
-    /// the consumer of the range only accepts a specific range type, instead of
-    /// the generic `RangeBounds` trait
-    /// ([#3307](https://github.com/rust-lang/rust-clippy/issues/3307)).
+    /// ### Limitations
+    /// The lint is conservative and will trigger only when switching
+    /// from an inclusive to an exclusive range is provably safe from
+    /// a typing point of view. This corresponds to situations where
+    /// the range is used as an iterator, or for indexing.
     ///
     /// ### Example
     /// ```no_run
@@ -344,6 +349,115 @@ fn check_range_bounds<'a, 'tcx>(cx: &'a LateContext<'tcx>, ex: &'a Expr<'_>) ->
     None
 }
 
+/// Check whether `expr` could switch range types without breaking the typing requirements. This is
+/// generally the case when `expr` is used as an iterator for example, or as a slice or `&str`
+/// index.
+///
+/// FIXME: Note that the current implementation may still return false positives. A proper fix would
+/// check that the obligations are still satisfied after switching the range type.
+fn can_switch_ranges<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>, original: RangeLimits, inner_ty: Ty<'tcx>) -> bool {
+    let Some(parent_expr) = get_parent_expr(cx, expr) else {
+        return false;
+    };
+
+    // Check if `expr` is the argument of a compiler-generated `IntoIter::into_iter(expr)`
+    if let ExprKind::Call(func, [arg]) = parent_expr.kind
+        && arg.hir_id == expr.hir_id
+        && is_path_lang_item(cx, func, LangItem::IntoIterIntoIter)
+    {
+        return true;
+    }
+
+    // Check if `expr` is used as the receiver of a method of the `Iterator`, `IntoIterator`,
+    // or `RangeBounds` traits.
+    if let ExprKind::MethodCall(_, receiver, _, _) = parent_expr.kind
+        && receiver.hir_id == expr.hir_id
+        && let Some(method_did) = cx.typeck_results().type_dependent_def_id(parent_expr.hir_id)
+        && let Some(trait_did) = cx.tcx.trait_of_item(method_did)
+        && matches!(
+            cx.tcx.get_diagnostic_name(trait_did),
+            Some(sym::Iterator | sym::IntoIterator | sym::RangeBounds)
+        )
+    {
+        return true;
+    }
+
+    // Check if `expr` is an argument of a call which requires an `Iterator`, `IntoIterator`,
+    // or `RangeBounds` trait.
+    if let ExprKind::Call(_, args) | ExprKind::MethodCall(_, _, args, _) = parent_expr.kind
+        && let Some(id) = fn_def_id(cx, parent_expr)
+        && let Some(arg_idx) = args.iter().position(|e| e.hir_id == expr.hir_id)
+    {
+        let input_idx = if matches!(parent_expr.kind, ExprKind::MethodCall(..)) {
+            arg_idx + 1
+        } else {
+            arg_idx
+        };
+        let inputs = cx
+            .tcx
+            .liberate_late_bound_regions(id, cx.tcx.fn_sig(id).instantiate_identity())
+            .inputs();
+        let expr_ty = inputs[input_idx];
+        // Check that the `expr` type is present only once, otherwise modifying just one of them might be
+        // risky if they are referenced using the same generic type for example.
+        if inputs.iter().enumerate().all(|(n, ty)|
+                                         n == input_idx
+                                         || !ty.walk().any(|arg| matches!(arg.unpack(), GenericArgKind::Type(ty) if ty == expr_ty)))
+            // Look for a clause requiring `Iterator`, `IntoIterator`, or `RangeBounds`, and resolving to `expr_type`.
+            && cx
+                .tcx
+                .param_env(id)
+                .caller_bounds()
+                .into_iter()
+                .any(|p| {
+                    if let ClauseKind::Trait(t) = p.kind().skip_binder()
+                        && t.polarity == PredicatePolarity::Positive
+                        && matches!(
+                            cx.tcx.get_diagnostic_name(t.trait_ref.def_id),
+                            Some(sym::Iterator | sym::IntoIterator | sym::RangeBounds)
+                        )
+                    {
+                        t.self_ty() == expr_ty
+                    } else {
+                        false
+                    }
+                })
+        {
+            return true;
+        }
+    }
+
+    // Check if `expr` is used for indexing, and if the switched range type could be used
+    // as well.
+    if let ExprKind::Index(outer_expr, index, _) = parent_expr.kind
+        && index.hir_id == expr.hir_id
+        // Build the switched range type (for example `RangeInclusive<usize>`).
+        && let Some(switched_range_def_id) = match original {
+            RangeLimits::HalfOpen => cx.tcx.lang_items().range_inclusive_struct(),
+            RangeLimits::Closed => cx.tcx.lang_items().range_struct(),
+        }
+        && let switched_range_ty = cx
+            .tcx
+            .type_of(switched_range_def_id)
+            .instantiate(cx.tcx, &[inner_ty.into()])
+        // Check that the switched range type can be used for indexing the original expression
+        // through the `Index` or `IndexMut` trait.
+        && let ty::Ref(_, outer_ty, mutability) = cx.typeck_results().expr_ty_adjusted(outer_expr).kind()
+        && let Some(index_def_id) = match mutability {
+            Mutability::Not => cx.tcx.lang_items().index_trait(),
+            Mutability::Mut => cx.tcx.lang_items().index_mut_trait(),
+        }
+       && implements_trait(cx, *outer_ty, index_def_id, &[switched_range_ty.into()])
+    // We could also check that the associated item of the `index_def_id` trait with the switched range type
+    // return the same type, but it is reasonable to expect so. We can't check that the result is identical
+    // in both `Index<Range<…>>` and `Index<RangeInclusive<…>>` anyway.
+    {
+        return true;
+    }
+
+    false
+}
+
 // exclusive range plus one: `x..(y+1)`
 fn check_exclusive_range_plus_one(cx: &LateContext<'_>, expr: &Expr<'_>) {
     if expr.span.can_be_used_for_suggestions()
@@ -353,6 +467,7 @@ fn check_exclusive_range_plus_one(cx: &LateContext<'_>, expr: &Expr<'_>) {
             limits: RangeLimits::HalfOpen,
         }) = higher::Range::hir(expr)
         && let Some(y) = y_plus_one(cx, end)
+        && can_switch_ranges(cx, expr, RangeLimits::HalfOpen, cx.typeck_results().expr_ty(y))
     {
         let span = expr.span;
         span_lint_and_then(
@@ -391,6 +506,7 @@ fn check_inclusive_range_minus_one(cx: &LateContext<'_>, expr: &Expr<'_>) {
             limits: RangeLimits::Closed,
         }) = higher::Range::hir(expr)
         && let Some(y) = y_minus_one(cx, end)
+        && can_switch_ranges(cx, expr, RangeLimits::Closed, cx.typeck_results().expr_ty(y))
     {
         span_lint_and_then(
             cx,

tests/ui/range_plus_minus_one.fixed

@@ -1,5 +1,10 @@
+#![warn(clippy::range_minus_one)]
+#![warn(clippy::range_plus_one)]
 #![allow(unused_parens)]
 #![allow(clippy::iter_with_drain)]
+
+use std::ops::{Index, IndexMut, Range, RangeBounds, RangeInclusive};
+
 fn f() -> usize {
     42
 }
@@ -20,8 +25,6 @@ macro_rules! macro_minus_one {
     };
 }
 
-#[warn(clippy::range_plus_one)]
-#[warn(clippy::range_minus_one)]
 fn main() {
     for _ in 0..2 {}
     for _ in 0..=2 {}
@@ -45,15 +48,13 @@ fn main() {
     //~^ range_plus_one
     for _ in 0..=(1 + f()) {}
 
+    // Those are not linted, as in the general case we cannot be sure that the exact type won't be
+    // important.
     let _ = ..11 - 1;
-    let _ = ..11;
-    //~^ range_minus_one
-    let _ = ..11;
-    //~^ range_minus_one
-    let _ = (1..=11);
-    //~^ range_plus_one
-    let _ = ((f() + 1)..=f());
-    //~^ range_plus_one
+    let _ = ..=11 - 1;
+    let _ = ..=(11 - 1);
+    let _ = (1..11 + 1);
+    let _ = (f() + 1)..(f() + 1);
 
     const ONE: usize = 1;
     // integer consts are linted, too
@@ -65,4 +66,90 @@ fn main() {
 
     macro_plus_one!(5);
     macro_minus_one!(5);
+
+    // As an instance of `Iterator`
+    (1..=10).for_each(|_| {});
+    //~^ range_plus_one
+
+    // As an instance of `IntoIterator`
+    #[allow(clippy::useless_conversion)]
+    (1..=10).into_iter().for_each(|_| {});
+    //~^ range_plus_one
+
+    // As an instance of `RangeBounds`
+    {
+        let _ = (1..=10).start_bound();
+        //~^ range_plus_one
+    }
+
+    // As a `SliceIndex`
+    let a = [10, 20, 30];
+    let _ = &a[1..=1];
+    //~^ range_plus_one
+
+    // As method call argument
+    vec.drain(2..=3);
+    //~^ range_plus_one
+
+    // As function call argument
+    take_arg(10..=20);
+    //~^ range_plus_one
+
+    // Do not lint, as the same type is used for both parameters
+    take_args(10..20 + 1, 10..21);
+
+    // Do not lint, as the range type is also used indirectly in second parameter
+    take_arg_and_struct(10..20 + 1, S { t: 1..2 });
+
+    // As target of `IndexMut`
+    let mut a = [10, 20, 30];
+    a[0..=2][0] = 1;
+    //~^ range_plus_one
+}
+
+fn take_arg<T: Iterator<Item = u32>>(_: T) {}
+fn take_args<T: Iterator<Item = u32>>(_: T, _: T) {}
+
+struct S<T> {
+    t: T,
+}
+fn take_arg_and_struct<T: Iterator<Item = u32>>(_: T, _: S<T>) {}
+
+fn no_index_by_range_inclusive(a: usize) {
+    struct S;
+
+    impl Index<Range<usize>> for S {
+        type Output = [u32];
+        fn index(&self, _: Range<usize>) -> &Self::Output {
+            &[]
+        }
+    }
+
+    _ = &S[0..a + 1];
+}
+
+fn no_index_mut_with_switched_range(a: usize) {
+    struct S(u32);
+
+    impl Index<Range<usize>> for S {
+        type Output = u32;
+        fn index(&self, _: Range<usize>) -> &Self::Output {
+            &self.0
+        }
+    }
+
+    impl IndexMut<Range<usize>> for S {
+        fn index_mut(&mut self, _: Range<usize>) -> &mut Self::Output {
+            &mut self.0
+        }
+    }
+
+    impl Index<RangeInclusive<usize>> for S {
+        type Output = u32;
+        fn index(&self, _: RangeInclusive<usize>) -> &Self::Output {
+            &self.0
+        }
+    }
+
+    S(2)[0..a + 1] = 3;
 }