Propose to exchange ranges only when it is safe to do so (rust-lang#14432)

To avoid 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.

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.

In a second commit, the `range_plus_one` and `range_minus_one` logic are
unified, in order to properly emit parentheses around the suggestion
when needed.

Fix rust-lang/rust-clippy#3307
Fix rust-lang/rust-clippy#9908

changelog: [`range_plus_one`, `range_minus_one`]: restrict lint to cases
where it is safe to switch the range type

*Edit:* as a consequence, this led to the removal of three
`#[expect(clippy::range_plus_one)]` in the Clippy sources to avoid those
false positives.

Author: Jarcho

clippy_lints/src/cognitive_complexity.rs

@@ -110,7 +110,6 @@ 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

@@ -1232,7 +1232,6 @@ 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 +4,20 @@ 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::{
+    expr_use_ctxt, 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_lint::{LateContext, LateLintPass};
-use rustc_middle::ty;
+use rustc_hir::{BinOpKind, Expr, ExprKind, HirId, LangItem, Node};
+use rustc_lint::{LateContext, LateLintPass, Lint};
+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 +29,12 @@ 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 +45,6 @@ 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 +77,11 @@ 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
@@ -344,70 +350,188 @@ fn check_range_bounds<'a, 'tcx>(cx: &'a LateContext<'tcx>, ex: &'a Expr<'_>) ->
     None
 }
 
-// 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()
-        && let Some(higher::Range {
-            start,
-            end: Some(end),
-            limits: RangeLimits::HalfOpen,
-        }) = higher::Range::hir(expr)
-        && let Some(y) = y_plus_one(cx, end)
+/// 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: &'tcx Expr<'_>,
+    original: RangeLimits,
+    inner_ty: Ty<'tcx>,
+) -> bool {
+    let use_ctxt = expr_use_ctxt(cx, expr);
+    let (Node::Expr(parent_expr), false) = (use_ctxt.node, use_ctxt.is_ty_unified) 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 == use_ctxt.child_id
+        && is_path_lang_item(cx, func, LangItem::IntoIterIntoIter)
     {
-        let span = expr.span;
-        span_lint_and_then(
-            cx,
-            RANGE_PLUS_ONE,
-            span,
-            "an inclusive range would be more readable",
-            |diag| {
-                let start = start.map_or(String::new(), |x| Sugg::hir(cx, x, "x").maybe_paren().to_string());
-                let end = Sugg::hir(cx, y, "y").maybe_paren();
-                match span.with_source_text(cx, |src| src.starts_with('(') && src.ends_with(')')) {
-                    Some(true) => {
-                        diag.span_suggestion(span, "use", format!("({start}..={end})"), Applicability::MaybeIncorrect);
-                    },
-                    Some(false) => {
-                        diag.span_suggestion(
-                            span,
-                            "use",
-                            format!("{start}..={end}"),
-                            Applicability::MachineApplicable, // snippet
-                        );
-                    },
-                    None => {},
-                }
-            },
-        );
+        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 == use_ctxt.child_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 == use_ctxt.child_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.kind(),
+                                                                          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<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+    check_range_switch(
+        cx,
+        expr,
+        RangeLimits::HalfOpen,
+        y_plus_one,
+        RANGE_PLUS_ONE,
+        "an inclusive range would be more readable",
+        "..=",
+    );
 }
 
 // inclusive range minus one: `x..=(y-1)`
-fn check_inclusive_range_minus_one(cx: &LateContext<'_>, expr: &Expr<'_>) {
+fn check_inclusive_range_minus_one<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+    check_range_switch(
+        cx,
+        expr,
+        RangeLimits::Closed,
+        y_minus_one,
+        RANGE_MINUS_ONE,
+        "an exclusive range would be more readable",
+        "..",
+    );
+}
+
+/// Check for a `kind` of range in `expr`, check for `predicate` on the end,
+/// and emit the `lint` with `msg` and the `operator`.
+fn check_range_switch<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx Expr<'_>,
+    kind: RangeLimits,
+    predicate: impl for<'hir> FnOnce(&LateContext<'_>, &Expr<'hir>) -> Option<&'hir Expr<'hir>>,
+    lint: &'static Lint,
+    msg: &'static str,
+    operator: &str,
+) {
     if expr.span.can_be_used_for_suggestions()
         && let Some(higher::Range {
             start,
             end: Some(end),
-            limits: RangeLimits::Closed,
+            limits,
         }) = higher::Range::hir(expr)
-        && let Some(y) = y_minus_one(cx, end)
+        && limits == kind
+        && let Some(y) = predicate(cx, end)
+        && can_switch_ranges(cx, expr, kind, cx.typeck_results().expr_ty(y))
     {
-        span_lint_and_then(
-            cx,
-            RANGE_MINUS_ONE,
-            expr.span,
-            "an exclusive range would be more readable",
-            |diag| {
-                let start = start.map_or(String::new(), |x| Sugg::hir(cx, x, "x").maybe_paren().to_string());
-                let end = Sugg::hir(cx, y, "y").maybe_paren();
-                diag.span_suggestion(
-                    expr.span,
-                    "use",
-                    format!("{start}..{end}"),
-                    Applicability::MachineApplicable, // snippet
-                );
-            },
-        );
+        let span = expr.span;
+        span_lint_and_then(cx, lint, span, msg, |diag| {
+            let mut app = Applicability::MachineApplicable;
+            let start = start.map_or(String::new(), |x| {
+                Sugg::hir_with_applicability(cx, x, "<x>", &mut app)
+                    .maybe_paren()
+                    .to_string()
+            });
+            let end = Sugg::hir_with_applicability(cx, y, "<y>", &mut app).maybe_paren();
+            match span.with_source_text(cx, |src| src.starts_with('(') && src.ends_with(')')) {
+                Some(true) => {
+                    diag.span_suggestion(span, "use", format!("({start}{operator}{end})"), app);
+                },
+                Some(false) => {
+                    diag.span_suggestion(span, "use", format!("{start}{operator}{end}"), app);
+                },
+                None => {},
+            }
+        });
     }
 }
 
@@ -494,7 +618,7 @@ fn check_reversed_empty_range(cx: &LateContext<'_>, expr: &Expr<'_>) {
     }
 }
 
-fn y_plus_one<'t>(cx: &LateContext<'_>, expr: &'t Expr<'_>) -> Option<&'t Expr<'t>> {
+fn y_plus_one<'tcx>(cx: &LateContext<'_>, expr: &Expr<'tcx>) -> Option<&'tcx Expr<'tcx>> {
     match expr.kind {
         ExprKind::Binary(
             Spanned {
@@ -515,7 +639,7 @@ fn y_plus_one<'t>(cx: &LateContext<'_>, expr: &'t Expr<'_>) -> Option<&'t Expr<'
     }
 }
 
-fn y_minus_one<'t>(cx: &LateContext<'_>, expr: &'t Expr<'_>) -> Option<&'t Expr<'t>> {
+fn y_minus_one<'tcx>(cx: &LateContext<'_>, expr: &Expr<'tcx>) -> Option<&'tcx Expr<'tcx>> {
     match expr.kind {
         ExprKind::Binary(
             Spanned {