Empty iterator error

compiler/rustc_error_messages/locales/en-US/lint.ftl

@@ -13,6 +13,11 @@ lint_enum_intrinsics_mem_variant =
     the return value of `mem::variant_count` is unspecified when called with a non-enum type
     .note = the type parameter of `variant_count` should be an enum, but it was instantiated with the type `{$ty_param}`, which is not an enum.
 
+lint_empty_iter_ranges = this `for` loop is never run
+    .clarification = this loop is never run because it's iterating over an empty iterator
+    .note = ranges that have a bigger start than their end will produce an empty iterator
+    .help = if you want a decreasing range sequence, create an increasing range and call `.rev()` on it
+
 lint_expectation = this lint expectation is unfulfilled
     .note = the `unfulfilled_lint_expectations` lint can't be expected and will always produce this message
 

compiler/rustc_lint/src/empty_iterator.rs

@@ -0,0 +1,81 @@
+use crate::{LateContext, LateLintPass, LintContext};
+
+use rustc_ast::ast::LitKind;
+use rustc_errors::fluent;
+use rustc_hir::{Expr, ExprField, ExprKind, MatchSource, PatKind, StmtKind};
+use rustc_span::Span;
+
+declare_lint! {
+    /// The `empty_iterator_range` lint checks for empty iterator ranges.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// for i in 10..0 { /* ... */ }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// It is usually a mistake to have a statement that has no effect.
+    pub EMPTY_ITERATOR_RANGE,
+    Warn,
+    "empty iterator range"
+}
+
+declare_lint_pass!(EmptyIteratorRange => [EMPTY_ITERATOR_RANGE]);
+
+impl<'tcx> LateLintPass<'tcx> for EmptyIteratorRange {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        let Some(for_expr) = extract_for_loop(expr) else { return };
+        let span = expr.span.with_hi(for_expr.span.hi());
+        if let ExprKind::Struct(_, expr_fields, _) = &for_expr.kind {
+            is_empty_range(cx, span, expr_fields);
+        } else if let ExprKind::MethodCall(_, expr, _, _) = &for_expr.kind {
+            if let ExprKind::Struct(_, expr_fields, _) = &expr.kind {
+                is_empty_range(cx, span, expr_fields);
+            }
+        }
+    }
+}
+
+fn extract_for_loop<'tcx>(expr: &Expr<'tcx>) -> Option<&'tcx Expr<'tcx>> {
+    if let ExprKind::DropTemps(e) = expr.kind
+    && let ExprKind::Match(iterexpr, [arm], MatchSource::ForLoopDesugar) = e.kind
+    && let ExprKind::Call(_, [arg]) = iterexpr.kind
+    && let ExprKind::Loop(block, ..) = arm.body.kind
+    && let [stmt] = block.stmts
+    && let StmtKind::Expr(e) = stmt.kind
+    && let ExprKind::Match(_, [_, some_arm], _) = e.kind
+    && let PatKind::Struct(..) = some_arm.pat.kind
+    {
+        Some(arg)
+    } else {
+        None
+    }
+}
+
+fn is_empty_range(cx: &LateContext<'_>, span: Span, expr_fields: &[ExprField<'_>]) {
+    let mut prev = 0u128;
+    for (index, expr_field) in expr_fields.iter().enumerate() {
+        if let ExprKind::Lit(lit) = &expr_field.expr.kind {
+            if let LitKind::Int(u, _) = lit.node {
+                if index == 0 {
+                    prev = u;
+                } else if prev > u {
+                    cx.struct_span_lint(
+                        EMPTY_ITERATOR_RANGE,
+                        span,
+                        fluent::lint::empty_iter_ranges,
+                        |lint| {
+                            lint.span_label(span, fluent::lint::clarification)
+                                .note(fluent::lint::note)
+                                .help(fluent::lint::help)
+                        },
+                    );
+                }
+            }
+        }
+    }
+}

compiler/rustc_lint/src/lib.rs

@@ -49,6 +49,7 @@ mod array_into_iter;
 pub mod builtin;
 mod context;
 mod early;
+mod empty_iterator;
 mod enum_intrinsics_non_enums;
 mod errors;
 mod expect;
@@ -85,6 +86,7 @@ use rustc_span::Span;
 
 use array_into_iter::ArrayIntoIter;
 use builtin::*;
+use empty_iterator::EmptyIteratorRange;
 use enum_intrinsics_non_enums::EnumIntrinsicsNonEnums;
 use hidden_unicode_codepoints::*;
 use internal::*;
@@ -226,6 +228,7 @@ macro_rules! late_lint_mod_passes {
                 InvalidAtomicOrdering: InvalidAtomicOrdering,
                 NamedAsmLabels: NamedAsmLabels,
                 OpaqueHiddenInferredBound: OpaqueHiddenInferredBound,
+                EmptyIteratorRange: EmptyIteratorRange,
             ]
         );
     };

library/core/tests/iter/range.rs

@@ -460,6 +460,8 @@ fn test_range_inclusive_size_hint() {
 
 #[test]
 fn test_double_ended_range() {
+    #![cfg_attr(not(bootstrap), allow(empty_iterator_range))]
+
     assert_eq!((11..14).rev().collect::<Vec<_>>(), [13, 12, 11]);
     for _ in (10..0).rev() {
         panic!("unreachable");

src/test/ui/lint/empty-iterator-range.rs

@@ -0,0 +1,13 @@
+#![deny(empty_iterator_range)]
+
+fn main() {
+    for _i in 10..0 {
+        //~^ ERROR this `for` loop is never run [empty_iterator_range]
+    }
+    for _i in (10..0).rev() {
+        //~^ ERROR this `for` loop is never run [empty_iterator_range]
+    }
+    for _i in (10..0).step_by(1) {
+        //~^ ERROR this `for` loop is never run [empty_iterator_range]
+    }
+}

src/test/ui/lint/empty-iterator-range.stderr

@@ -0,0 +1,34 @@
+error: this `for` loop is never run
+  --> $DIR/empty-iterator-range.rs:4:5
+   |
+LL |     for _i in 10..0 {
+   |     ^^^^^^^^^^^^^^^ this loop is never run because it's iterating over an empty iterator
+   |
+   = note: ranges that have a bigger start than their end will produce an empty iterator
+   = help: if you want a decreasing range sequence, create an increasing range and call `.rev()` on it
+note: the lint level is defined here
+  --> $DIR/empty-iterator-range.rs:1:9
+   |
+LL | #![deny(empty_iterator_range)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+
+error: this `for` loop is never run
+  --> $DIR/empty-iterator-range.rs:7:5
+   |
+LL |     for _i in (10..0).rev() {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^ this loop is never run because it's iterating over an empty iterator
+   |
+   = note: ranges that have a bigger start than their end will produce an empty iterator
+   = help: if you want a decreasing range sequence, create an increasing range and call `.rev()` on it
+
+error: this `for` loop is never run
+  --> $DIR/empty-iterator-range.rs:10:5
+   |
+LL |     for _i in (10..0).step_by(1) {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this loop is never run because it's iterating over an empty iterator
+   |
+   = note: ranges that have a bigger start than their end will produce an empty iterator
+   = help: if you want a decreasing range sequence, create an increasing range and call `.rev()` on it
+
+error: aborting due to 3 previous errors
+