Lint small gaps between ranges

compiler/rustc_lint_defs/src/builtin.rs

@@ -87,6 +87,7 @@ declare_lint_pass! {
         RUST_2021_PRELUDE_COLLISIONS,
         SEMICOLON_IN_EXPRESSIONS_FROM_MACROS,
         SINGLE_USE_LIFETIMES,
+        SMALL_GAPS_BETWEEN_RANGES,
         SOFT_UNSTABLE,
         STABLE_FEATURES,
         SUSPICIOUS_AUTO_TRAIT_IMPLS,
@@ -835,6 +836,35 @@ declare_lint! {
     "detects range patterns with overlapping endpoints"
 }
 
+declare_lint! {
+    /// The `small_gaps_between_ranges` lint detects `match` expressions that use [range patterns]
+    /// that skip over a single number.
+    ///
+    /// [range patterns]: https://doc.rust-lang.org/nightly/reference/patterns.html#range-patterns
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// let x = 123u32;
+    /// match x {
+    ///     0..100 => { println!("small"); }
+    ///     101..1000 => { println!("large"); }
+    ///     _ => { println!("larger"); }
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// It is likely a mistake to have range patterns in a match expression that miss out a single
+    /// number. Check that the beginning and end values are what you expect, and keep in mind that
+    /// with `..=` the right bound is inclusive, and with `..` it is exclusive.
+    pub SMALL_GAPS_BETWEEN_RANGES,
+    Warn,
+    "detects range patterns separated by a single number"
+}
+
 declare_lint! {
     /// The `bindings_with_variant_name` lint detects pattern bindings with
     /// the same name as one of the matched variants.

compiler/rustc_pattern_analysis/messages.ftl

@@ -11,6 +11,10 @@ pattern_analysis_overlapping_range_endpoints = multiple patterns overlap on thei
     .label = ... with this range
     .note = you likely meant to write mutually exclusive ranges
 
+pattern_analysis_small_gap_between_ranges = multiple ranges are one apart
+    .label = ... with this range
+    .note = you likely meant to match `{$range}` too
+
 pattern_analysis_uncovered = {$count ->
         [1] pattern `{$witness_1}`
         [2] patterns `{$witness_1}` and `{$witness_2}`

compiler/rustc_pattern_analysis/src/constructor.rs

@@ -193,7 +193,7 @@ impl fmt::Display for RangeEnd {
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 pub enum MaybeInfiniteInt {
     NegInfinity,
-    /// Encoded value. DO NOT CONSTRUCT BY HAND; use `new_finite`.
+    /// Encoded value. DO NOT CONSTRUCT BY HAND; use `new_finite_{int,uint}`.
     #[non_exhaustive]
     Finite(u128),
     /// The integer after `u128::MAX`. We need it to represent `x..=u128::MAX` as an exclusive range.
@@ -230,25 +230,22 @@ impl MaybeInfiniteInt {
     }
 
     /// Note: this will not turn a finite value into an infinite one or vice-versa.
-    pub fn minus_one(self) -> Self {
+    pub fn minus_one(self) -> Option<Self> {
         match self {
-            Finite(n) => match n.checked_sub(1) {
-                Some(m) => Finite(m),
-                None => panic!("Called `MaybeInfiniteInt::minus_one` on 0"),
-            },
-            JustAfterMax => Finite(u128::MAX),
-            x => x,
+            Finite(n) => n.checked_sub(1).map(Finite),
+            JustAfterMax => Some(Finite(u128::MAX)),
+            x => Some(x),
         }
     }
     /// Note: this will not turn a finite value into an infinite one or vice-versa.
-    pub fn plus_one(self) -> Self {
+    pub fn plus_one(self) -> Option<Self> {
         match self {
             Finite(n) => match n.checked_add(1) {
-                Some(m) => Finite(m),
-                None => JustAfterMax,
+                Some(m) => Some(Finite(m)),
+                None => Some(JustAfterMax),
             },
-            JustAfterMax => panic!("Called `MaybeInfiniteInt::plus_one` on u128::MAX+1"),
-            x => x,
+            JustAfterMax => None,
+            x => Some(x),
         }
     }
 }
@@ -269,18 +266,25 @@ impl IntRange {
     pub fn is_singleton(&self) -> bool {
         // Since `lo` and `hi` can't be the same `Infinity` and `plus_one` never changes from finite
         // to infinite, this correctly only detects ranges that contain exacly one `Finite(x)`.
-        self.lo.plus_one() == self.hi
+        // `unwrap()` is ok since `self.lo` will never be `JustAfterMax`.
+        self.lo.plus_one().unwrap() == self.hi
     }
 
+    /// Construct a singleton range.
+    /// `x` be a `Finite(_)` value.
     #[inline]
     pub fn from_singleton(x: MaybeInfiniteInt) -> IntRange {
-        IntRange { lo: x, hi: x.plus_one() }
+        // `unwrap()` is ok on a finite value
+        IntRange { lo: x, hi: x.plus_one().unwrap() }
     }
 
+    /// Construct a range with these boundaries.
+    /// `lo` must not be `PosInfinity` or `JustAfterMax`. `hi` must not be `NegInfinity`.
+    /// If `end` is `Included`, `hi` must also not be `JustAfterMax`.
     #[inline]
     pub fn from_range(lo: MaybeInfiniteInt, mut hi: MaybeInfiniteInt, end: RangeEnd) -> IntRange {
         if end == RangeEnd::Included {
-            hi = hi.plus_one();
+            hi = hi.plus_one().unwrap();
         }
         if lo >= hi {
             // This should have been caught earlier by E0030.

compiler/rustc_pattern_analysis/src/errors.rs

@@ -72,6 +72,36 @@ impl<'tcx> AddToDiagnostic for Overlap<'tcx> {
     }
 }
 
+#[derive(LintDiagnostic)]
+#[diag(pattern_analysis_small_gap_between_ranges)]
+#[note]
+pub struct SmallGapBetweenRanges<'tcx> {
+    #[label]
+    pub first_range: Span,
+    pub range: Pat<'tcx>,
+    #[subdiagnostic]
+    pub gap_with: Vec<GappedRange<'tcx>>,
+}
+
+pub struct GappedRange<'tcx> {
+    pub span: Span,
+    pub range: Pat<'tcx>,
+}
+
+impl<'tcx> AddToDiagnostic for GappedRange<'tcx> {
+    fn add_to_diagnostic_with<F>(self, diag: &mut Diagnostic, _: F)
+    where
+        F: Fn(&mut Diagnostic, SubdiagnosticMessage) -> SubdiagnosticMessage,
+    {
+        let GappedRange { span, range } = self;
+
+        // FIXME(mejrs) unfortunately `#[derive(LintDiagnostic)]`
+        // does not support `#[subdiagnostic(eager)]`...
+        let message = format!("this range has a small gap on `{range}`...");
+        diag.span_label(span, message);
+    }
+}
+
 #[derive(LintDiagnostic)]
 #[diag(pattern_analysis_non_exhaustive_omitted_pattern)]
 #[help]

compiler/rustc_pattern_analysis/src/lib.rs

@@ -26,15 +26,7 @@ use rustc_index::Idx;
 use rustc_middle::ty::Ty;
 
 use crate::constructor::{Constructor, ConstructorSet};
-#[cfg(feature = "rustc")]
-use crate::lints::{
-    lint_nonexhaustive_missing_variants, lint_overlapping_range_endpoints, PatternColumn,
-};
 use crate::pat::DeconstructedPat;
-#[cfg(feature = "rustc")]
-use crate::rustc::RustcMatchCheckCtxt;
-#[cfg(feature = "rustc")]
-use crate::usefulness::{compute_match_usefulness, ValidityConstraint};
 
 // It's not possible to only enable the `typed_arena` dependency when the `rustc` feature is off, so
 // we use another feature instead. The crate won't compile if one of these isn't enabled.
@@ -109,26 +101,32 @@ impl<'p, Cx: TypeCx> Copy for MatchArm<'p, Cx> {}
 /// useful, and runs some lints.
 #[cfg(feature = "rustc")]
 pub fn analyze_match<'p, 'tcx>(
-    tycx: &RustcMatchCheckCtxt<'p, 'tcx>,
+    tycx: &rustc::RustcMatchCheckCtxt<'p, 'tcx>,
     arms: &[rustc::MatchArm<'p, 'tcx>],
     scrut_ty: Ty<'tcx>,
 ) -> rustc::UsefulnessReport<'p, 'tcx> {
+    use crate::lints::PatternColumn;
+    use crate::usefulness::ValidityConstraint;
+
     // Arena to store the extra wildcards we construct during analysis.
     let wildcard_arena = tycx.pattern_arena;
     let scrut_validity = ValidityConstraint::from_bool(tycx.known_valid_scrutinee);
     let cx = MatchCtxt { tycx, wildcard_arena };
 
-    let report = compute_match_usefulness(cx, arms, scrut_ty, scrut_validity);
+    let report = usefulness::compute_match_usefulness(cx, arms, scrut_ty, scrut_validity);
 
-    let pat_column = PatternColumn::new(arms);
+    // Only run the lints if the match is exhaustive.
+    if report.non_exhaustiveness_witnesses.is_empty() {
+        let pat_column = PatternColumn::new(arms);
 
-    // Lint on ranges that overlap on their endpoints, which is likely a mistake.
-    lint_overlapping_range_endpoints(cx, &pat_column);
+        // Detect possible range-related mistakes.
+        lints::lint_likely_range_mistakes(cx, &pat_column);
 
-    // Run the non_exhaustive_omitted_patterns lint. Only run on refutable patterns to avoid hitting
-    // `if let`s. Only run if the match is exhaustive otherwise the error is redundant.
-    if tycx.refutable && report.non_exhaustiveness_witnesses.is_empty() {
-        lint_nonexhaustive_missing_variants(cx, arms, &pat_column, scrut_ty)
+        // Run the non_exhaustive_omitted_patterns lint. Only run on refutable patterns to avoid
+        // hitting `if let`s.
+        if tycx.refutable {
+            lints::lint_nonexhaustive_missing_variants(cx, arms, &pat_column, scrut_ty)
+        }
     }
 
     report

compiler/rustc_pattern_analysis/src/lints.rs

@@ -7,10 +7,7 @@ use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS;
 use rustc_span::Span;
 
 use crate::constructor::{IntRange, MaybeInfiniteInt};
-use crate::errors::{
-    NonExhaustiveOmittedPattern, NonExhaustiveOmittedPatternLintOnArm, Overlap,
-    OverlappingRangeEndpoints, Uncovered,
-};
+use crate::errors;
 use crate::rustc::{
     Constructor, DeconstructedPat, MatchArm, MatchCtxt, PlaceCtxt, RustcMatchCheckCtxt,
     SplitConstructorSet, WitnessPat,
@@ -189,9 +186,9 @@ pub(crate) fn lint_nonexhaustive_missing_variants<'a, 'p, 'tcx>(
                 NON_EXHAUSTIVE_OMITTED_PATTERNS,
                 rcx.match_lint_level,
                 rcx.scrut_span,
-                NonExhaustiveOmittedPattern {
+                errors::NonExhaustiveOmittedPattern {
                     scrut_ty,
-                    uncovered: Uncovered::new(rcx.scrut_span, rcx, witnesses),
+                    uncovered: errors::Uncovered::new(rcx.scrut_span, rcx, witnesses),
                 },
             );
         }
@@ -203,7 +200,7 @@ pub(crate) fn lint_nonexhaustive_missing_variants<'a, 'p, 'tcx>(
             let (lint_level, lint_level_source) =
                 rcx.tcx.lint_level_at_node(NON_EXHAUSTIVE_OMITTED_PATTERNS, arm.arm_data);
             if !matches!(lint_level, rustc_session::lint::Level::Allow) {
-                let decorator = NonExhaustiveOmittedPatternLintOnArm {
+                let decorator = errors::NonExhaustiveOmittedPatternLintOnArm {
                     lint_span: lint_level_source.span(),
                     suggest_lint_on_match: rcx.whole_match_span.map(|span| span.shrink_to_lo()),
                     lint_level: lint_level.as_str(),
@@ -220,9 +217,10 @@ pub(crate) fn lint_nonexhaustive_missing_variants<'a, 'p, 'tcx>(
     }
 }
 
-/// Traverse the patterns to warn the user about ranges that overlap on their endpoints.
+/// Traverse the patterns to warn the user about ranges that overlap on their endpoints or are
+/// distant by one.
 #[instrument(level = "debug", skip(cx))]
-pub(crate) fn lint_overlapping_range_endpoints<'a, 'p, 'tcx>(
+pub(crate) fn lint_likely_range_mistakes<'a, 'p, 'tcx>(
     cx: MatchCtxt<'a, 'p, 'tcx>,
     column: &PatternColumn<'a, 'p, 'tcx>,
 ) {
@@ -235,24 +233,24 @@ pub(crate) fn lint_overlapping_range_endpoints<'a, 'p, 'tcx>(
     let set = column.analyze_ctors(pcx);
 
     if matches!(ty.kind(), ty::Char | ty::Int(_) | ty::Uint(_)) {
-        let emit_lint = |overlap: &IntRange, this_span: Span, overlapped_spans: &[Span]| {
+        let emit_overlap_lint = |overlap: &IntRange, this_span: Span, overlapped_spans: &[Span]| {
             let overlap_as_pat = rcx.hoist_pat_range(overlap, ty);
             let overlaps: Vec<_> = overlapped_spans
                 .iter()
                 .copied()
-                .map(|span| Overlap { range: overlap_as_pat.clone(), span })
+                .map(|span| errors::Overlap { range: overlap_as_pat.clone(), span })
                 .collect();
             rcx.tcx.emit_spanned_lint(
                 lint::builtin::OVERLAPPING_RANGE_ENDPOINTS,
                 rcx.match_lint_level,
                 this_span,
-                OverlappingRangeEndpoints { overlap: overlaps, range: this_span },
+                errors::OverlappingRangeEndpoints { overlap: overlaps, range: this_span },
             );
         };
 
-        // If two ranges overlapped, the split set will contain their intersection as a singleton.
-        let split_int_ranges = set.present.iter().filter_map(|c| c.as_int_range());
-        for overlap_range in split_int_ranges.clone() {
+        // The two cases we are interested in will show up as a singleton after range splitting.
+        let present_int_ranges = set.present.iter().filter_map(|c| c.as_int_range());
+        for overlap_range in present_int_ranges {
             if overlap_range.is_singleton() {
                 let overlap: MaybeInfiniteInt = overlap_range.lo;
                 // Ranges that look like `lo..=overlap`.
@@ -267,29 +265,72 @@ pub(crate) fn lint_overlapping_range_endpoints<'a, 'p, 'tcx>(
                         // Don't lint when one of the ranges is a singleton.
                         continue;
                     }
-                    if this_range.lo == overlap {
+                    if overlap == this_range.lo {
                         // `this_range` looks like `overlap..=this_range.hi`; it overlaps with any
                         // ranges that look like `lo..=overlap`.
                         if !prefixes.is_empty() {
-                            emit_lint(overlap_range, this_span, &prefixes);
+                            emit_overlap_lint(overlap_range, this_span, &prefixes);
                         }
                         suffixes.push(this_span)
-                    } else if this_range.hi == overlap.plus_one() {
+                    } else if overlap.plus_one() == Some(this_range.hi) {
                         // `this_range` looks like `this_range.lo..=overlap`; it overlaps with any
                         // ranges that look like `overlap..=hi`.
                         if !suffixes.is_empty() {
-                            emit_lint(overlap_range, this_span, &suffixes);
+                            emit_overlap_lint(overlap_range, this_span, &suffixes);
                         }
                         prefixes.push(this_span)
                     }
                 }
             }
         }
+
+        let missing_int_ranges = set.missing.iter().filter_map(|c| c.as_int_range());
+        for point_range in missing_int_ranges {
+            if point_range.is_singleton() {
+                let point: MaybeInfiniteInt = point_range.lo;
+                // Ranges that look like `lo..point`.
+                let mut onebefore: SmallVec<[_; 1]> = Default::default();
+                // Ranges that look like `point+1..=hi`.
+                let mut oneafter: SmallVec<[_; 1]> = Default::default();
+                for pat in column.iter() {
+                    let this_span = *pat.data();
+                    let Constructor::IntRange(this_range) = pat.ctor() else { continue };
+
+                    if point == this_range.hi && !this_range.is_singleton() {
+                        onebefore.push(this_span)
+                    } else if point.plus_one() == Some(this_range.lo) {
+                        oneafter.push(this_span)
+                    }
+                }
+
+                if !onebefore.is_empty() && !oneafter.is_empty() {
+                    // We have some `lo..point` and some `point+1..hi` but no `point`.
+                    let point_as_pat = rcx.hoist_pat_range(point_range, ty);
+                    for span_after in oneafter {
+                        let spans_before: Vec<_> = onebefore
+                            .iter()
+                            .copied()
+                            .map(|span| errors::GappedRange { range: point_as_pat.clone(), span })
+                            .collect();
+                        rcx.tcx.emit_spanned_lint(
+                            lint::builtin::SMALL_GAPS_BETWEEN_RANGES,
+                            rcx.match_lint_level,
+                            span_after,
+                            errors::SmallGapBetweenRanges {
+                                range: point_as_pat.clone(),
+                                first_range: span_after,
+                                gap_with: spans_before,
+                            },
+                        );
+                    }
+                }
+            }
+        }
     } else {
         // Recurse into the fields.
         for ctor in set.present {
             for col in column.specialize(pcx, &ctor) {
-                lint_overlapping_range_endpoints(cx, &col);
+                lint_likely_range_mistakes(cx, &col);
             }
         }
     }

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -659,12 +659,12 @@ impl<'p, 'tcx> RustcMatchCheckCtxt<'p, 'tcx> {
                 let value = mir::Const::from_ty_const(c, cx.tcx);
                 lo = PatRangeBoundary::Finite(value);
             }
-            let hi = if matches!(range.hi, Finite(0)) {
+            let hi = if let Some(hi) = range.hi.minus_one() {
+                hi
+            } else {
                 // The range encodes `..ty::MIN`, so we can't convert it to an inclusive range.
                 end = rustc_hir::RangeEnd::Excluded;
                 range.hi
-            } else {
-                range.hi.minus_one()
             };
             let hi = cx.hoist_pat_range_bdy(hi, ty);
             PatKind::Range(Box::new(PatRange { lo, hi, end, ty }))