Auto merge of #8232 - Jarcho:match_same_arm_860, r=xFrednet

`match_same_arms` fix

fixes #860
fixes #1140

changelog: Don't lint `match_same_arms` when an interposing arm's pattern would overlap

4 files changed, 368 insertions, 53 deletions

diff --git a/clippy_lints/src/doc.rs b/clippy_lints/src/doc.rs
index 16173580fd4..703aa458f44 100644
--- a/clippy_lints/src/doc.rs
+++ b/clippy_lints/src/doc.rs
@@ -637,12 +637,6 @@ fn check_code(cx: &LateContext<'_>, text: &str, edition: Edition, span: Span) {
                 loop {
                     match parser.parse_item(ForceCollect::No) {
                         Ok(Some(item)) => match &item.kind {
-                            // Tests with one of these items are ignored
-                            ItemKind::Static(..)
-                            | ItemKind::Const(..)
-                            | ItemKind::ExternCrate(..)
-                            | ItemKind::ForeignMod(..) => return false,
-                            // We found a main function ...
                             ItemKind::Fn(box Fn {
                                 sig, body: Some(block), ..
                             }) if item.ident.name == sym::main => {
@@ -661,8 +655,13 @@ fn check_code(cx: &LateContext<'_>, text: &str, edition: Edition, span: Span) {
                                     return false;
                                 }
                             },
-                            // Another function was found; this case is ignored too
-                            ItemKind::Fn(..) => return false,
+                            // Tests with one of these items are ignored
+                            ItemKind::Static(..)
+                            | ItemKind::Const(..)
+                            | ItemKind::ExternCrate(..)
+                            | ItemKind::ForeignMod(..)
+                            // Another function was found; this case is ignored
+                            | ItemKind::Fn(..) => return false,
                             _ => {},
                         },
                         Ok(None) => break,
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 504235d0d1e..f2a07999144 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -23,6 +23,7 @@
 
 // FIXME: switch to something more ergonomic here, once available.
 // (Currently there is no way to opt into sysroot crates without `extern crate`.)
+extern crate rustc_arena;
 extern crate rustc_ast;
 extern crate rustc_ast_pretty;
 extern crate rustc_attr;
diff --git a/clippy_lints/src/matches/match_same_arms.rs b/clippy_lints/src/matches/match_same_arms.rs
index d11dda57e6f..b8591fe0db0 100644
--- a/clippy_lints/src/matches/match_same_arms.rs
+++ b/clippy_lints/src/matches/match_same_arms.rs
@@ -1,19 +1,66 @@
 use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::source::snippet;
 use clippy_utils::{path_to_local, search_same, SpanlessEq, SpanlessHash};
-use rustc_hir::{Arm, Expr, HirId, HirIdMap, HirIdSet, Pat, PatKind};
+use core::cmp::Ordering;
+use core::iter;
+use core::slice;
+use rustc_arena::DroplessArena;
+use rustc_ast::ast::LitKind;
+use rustc_errors::Applicability;
+use rustc_hir::def_id::DefId;
+use rustc_hir::{Arm, Expr, ExprKind, HirId, HirIdMap, HirIdSet, Pat, PatKind, RangeEnd};
 use rustc_lint::LateContext;
+use rustc_middle::ty;
+use rustc_span::Symbol;
 use std::collections::hash_map::Entry;
 
 use super::MATCH_SAME_ARMS;
 
-pub(crate) fn check<'tcx>(cx: &LateContext<'tcx>, arms: &'tcx [Arm<'_>]) {
+#[allow(clippy::too_many_lines)]
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, arms: &'tcx [Arm<'_>]) {
     let hash = |&(_, arm): &(usize, &Arm<'_>)| -> u64 {
         let mut h = SpanlessHash::new(cx);
         h.hash_expr(arm.body);
         h.finish()
     };
 
+    let arena = DroplessArena::default();
+    let normalized_pats: Vec<_> = arms
+        .iter()
+        .map(|a| NormalizedPat::from_pat(cx, &arena, a.pat))
+        .collect();
+
+    // The furthast forwards a pattern can move without semantic changes
+    let forwards_blocking_idxs: Vec<_> = normalized_pats
+        .iter()
+        .enumerate()
+        .map(|(i, pat)| {
+            normalized_pats[i + 1..]
+                .iter()
+                .enumerate()
+                .find_map(|(j, other)| pat.has_overlapping_values(other).then(|| i + 1 + j))
+                .unwrap_or(normalized_pats.len())
+        })
+        .collect();
+
+    // The furthast backwards a pattern can move without semantic changes
+    let backwards_blocking_idxs: Vec<_> = normalized_pats
+        .iter()
+        .enumerate()
+        .map(|(i, pat)| {
+            normalized_pats[..i]
+                .iter()
+                .enumerate()
+                .rev()
+                .zip(forwards_blocking_idxs[..i].iter().copied().rev())
+                .skip_while(|&(_, forward_block)| forward_block > i)
+                .find_map(|((j, other), forward_block)| {
+                    (forward_block == i || pat.has_overlapping_values(other)).then(|| j)
+                })
+                .unwrap_or(0)
+        })
+        .collect();
+
     let eq = |&(lindex, lhs): &(usize, &Arm<'_>), &(rindex, rhs): &(usize, &Arm<'_>)| -> bool {
         let min_index = usize::min(lindex, rindex);
         let max_index = usize::max(lindex, rindex);
@@ -42,53 +89,316 @@ pub(crate) fn check<'tcx>(cx: &LateContext<'tcx>, arms: &'tcx [Arm<'_>]) {
             }
         };
         // Arms with a guard are ignored, those can’t always be merged together
-        // This is also the case for arms in-between each there is an arm with a guard
-        (min_index..=max_index).all(|index| arms[index].guard.is_none())
-            && SpanlessEq::new(cx)
-                .expr_fallback(eq_fallback)
-                .eq_expr(lhs.body, rhs.body)
-            // these checks could be removed to allow unused bindings
-            && bindings_eq(lhs.pat, local_map.keys().copied().collect())
-            && bindings_eq(rhs.pat, local_map.values().copied().collect())
+        // If both arms overlap with an arm in between then these can't be merged either.
+        !(backwards_blocking_idxs[max_index] > min_index && forwards_blocking_idxs[min_index] < max_index)
+                && lhs.guard.is_none()
+                && rhs.guard.is_none()
+                && SpanlessEq::new(cx)
+                    .expr_fallback(eq_fallback)
+                    .eq_expr(lhs.body, rhs.body)
+                // these checks could be removed to allow unused bindings
+                && bindings_eq(lhs.pat, local_map.keys().copied().collect())
+                && bindings_eq(rhs.pat, local_map.values().copied().collect())
     };
 
     let indexed_arms: Vec<(usize, &Arm<'_>)> = arms.iter().enumerate().collect();
-    for (&(_, i), &(_, j)) in search_same(&indexed_arms, hash, eq) {
-        span_lint_and_then(
-            cx,
-            MATCH_SAME_ARMS,
-            j.body.span,
-            "this `match` has identical arm bodies",
-            |diag| {
-                diag.span_note(i.body.span, "same as this");
-
-                // Note: this does not use `span_suggestion` on purpose:
-                // there is no clean way
-                // to remove the other arm. Building a span and suggest to replace it to ""
-                // makes an even more confusing error message. Also in order not to make up a
-                // span for the whole pattern, the suggestion is only shown when there is only
-                // one pattern. The user should know about `|` if they are already using it…
-
-                let lhs = snippet(cx, i.pat.span, "<pat1>");
-                let rhs = snippet(cx, j.pat.span, "<pat2>");
-
-                if let PatKind::Wild = j.pat.kind {
-                    // if the last arm is _, then i could be integrated into _
-                    // note that i.pat cannot be _, because that would mean that we're
-                    // hiding all the subsequent arms, and rust won't compile
-                    diag.span_note(
-                        i.body.span,
-                        &format!(
-                            "`{}` has the same arm body as the `_` wildcard, consider removing it",
-                            lhs
-                        ),
-                    );
+    for (&(i, arm1), &(j, arm2)) in search_same(&indexed_arms, hash, eq) {
+        if matches!(arm2.pat.kind, PatKind::Wild) {
+            span_lint_and_then(
+                cx,
+                MATCH_SAME_ARMS,
+                arm1.span,
+                "this match arm has an identical body to the `_` wildcard arm",
+                |diag| {
+                    diag.span_suggestion(
+                        arm1.span,
+                        "try removing the arm",
+                        String::new(),
+                        Applicability::MaybeIncorrect,
+                    )
+                    .help("or try changing either arm body")
+                    .span_note(arm2.span, "`_` wildcard arm here");
+                },
+            );
+        } else {
+            let back_block = backwards_blocking_idxs[j];
+            let (keep_arm, move_arm) = if back_block < i || (back_block == 0 && forwards_blocking_idxs[i] <= j) {
+                (arm1, arm2)
+            } else {
+                (arm2, arm1)
+            };
+
+            span_lint_and_then(
+                cx,
+                MATCH_SAME_ARMS,
+                keep_arm.span,
+                "this match arm has an identical body to another arm",
+                |diag| {
+                    let move_pat_snip = snippet(cx, move_arm.pat.span, "<pat2>");
+                    let keep_pat_snip = snippet(cx, keep_arm.pat.span, "<pat1>");
+
+                    diag.span_suggestion(
+                        keep_arm.pat.span,
+                        "try merging the arm patterns",
+                        format!("{} | {}", keep_pat_snip, move_pat_snip),
+                        Applicability::MaybeIncorrect,
+                    )
+                    .help("or try changing either arm body")
+                    .span_note(move_arm.span, "other arm here");
+                },
+            );
+        }
+    }
+}
+
+#[derive(Clone, Copy)]
+enum NormalizedPat<'a> {
+    Wild,
+    Struct(Option<DefId>, &'a [(Symbol, Self)]),
+    Tuple(Option<DefId>, &'a [Self]),
+    Or(&'a [Self]),
+    Path(Option<DefId>),
+    LitStr(Symbol),
+    LitBytes(&'a [u8]),
+    LitInt(u128),
+    LitBool(bool),
+    Range(PatRange),
+    /// A slice pattern. If the second value is `None`, then this matches an exact size. Otherwise
+    /// the first value contains everything before the `..` wildcard pattern, and the second value
+    /// contains everything afterwards. Note that either side, or both sides, may contain zero
+    /// patterns.
+    Slice(&'a [Self], Option<&'a [Self]>),
+}
+
+#[derive(Clone, Copy)]
+struct PatRange {
+    start: u128,
+    end: u128,
+    bounds: RangeEnd,
+}
+impl PatRange {
+    fn contains(&self, x: u128) -> bool {
+        x >= self.start
+            && match self.bounds {
+                RangeEnd::Included => x <= self.end,
+                RangeEnd::Excluded => x < self.end,
+            }
+    }
+
+    fn overlaps(&self, other: &Self) -> bool {
+        // Note: Empty ranges are impossible, so this is correct even though it would return true if an
+        // empty exclusive range were to reside within an inclusive range.
+        (match self.bounds {
+            RangeEnd::Included => self.end >= other.start,
+            RangeEnd::Excluded => self.end > other.start,
+        } && match other.bounds {
+            RangeEnd::Included => self.start <= other.end,
+            RangeEnd::Excluded => self.start < other.end,
+        })
+    }
+}
+
+/// Iterates over the pairs of fields with matching names.
+fn iter_matching_struct_fields<'a>(
+    left: &'a [(Symbol, NormalizedPat<'a>)],
+    right: &'a [(Symbol, NormalizedPat<'a>)],
+) -> impl Iterator<Item = (&'a NormalizedPat<'a>, &'a NormalizedPat<'a>)> + 'a {
+    struct Iter<'a>(
+        slice::Iter<'a, (Symbol, NormalizedPat<'a>)>,
+        slice::Iter<'a, (Symbol, NormalizedPat<'a>)>,
+    );
+    impl<'a> Iterator for Iter<'a> {
+        type Item = (&'a NormalizedPat<'a>, &'a NormalizedPat<'a>);
+        fn next(&mut self) -> Option<Self::Item> {
+            // Note: all the fields in each slice are sorted by symbol value.
+            let mut left = self.0.next()?;
+            let mut right = self.1.next()?;
+            loop {
+                match left.0.cmp(&right.0) {
+                    Ordering::Equal => return Some((&left.1, &right.1)),
+                    Ordering::Less => left = self.0.next()?,
+                    Ordering::Greater => right = self.1.next()?,
+                }
+            }
+        }
+    }
+    Iter(left.iter(), right.iter())
+}
+
+#[allow(clippy::similar_names)]
+impl<'a> NormalizedPat<'a> {
+    #[allow(clippy::too_many_lines)]
+    fn from_pat(cx: &LateContext<'_>, arena: &'a DroplessArena, pat: &'a Pat<'_>) -> Self {
+        match pat.kind {
+            PatKind::Wild | PatKind::Binding(.., None) => Self::Wild,
+            PatKind::Binding(.., Some(pat)) | PatKind::Box(pat) | PatKind::Ref(pat, _) => {
+                Self::from_pat(cx, arena, pat)
+            },
+            PatKind::Struct(ref path, fields, _) => {
+                let fields =
+                    arena.alloc_from_iter(fields.iter().map(|f| (f.ident.name, Self::from_pat(cx, arena, f.pat))));
+                fields.sort_by_key(|&(name, _)| name);
+                Self::Struct(cx.qpath_res(path, pat.hir_id).opt_def_id(), fields)
+            },
+            PatKind::TupleStruct(ref path, pats, wild_idx) => {
+                let adt = match cx.typeck_results().pat_ty(pat).ty_adt_def() {
+                    Some(x) => x,
+                    None => return Self::Wild,
+                };
+                let (var_id, variant) = if adt.is_enum() {
+                    match cx.qpath_res(path, pat.hir_id).opt_def_id() {
+                        Some(x) => (Some(x), adt.variant_with_ctor_id(x)),
+                        None => return Self::Wild,
+                    }
                 } else {
-                    diag.span_help(i.pat.span, &format!("consider refactoring into `{} | {}`", lhs, rhs,))
-                        .help("...or consider changing the match arm bodies");
+                    (None, adt.non_enum_variant())
+                };
+                let (front, back) = match wild_idx {
+                    Some(i) => pats.split_at(i),
+                    None => (pats, [].as_slice()),
+                };
+                let pats = arena.alloc_from_iter(
+                    front
+                        .iter()
+                        .map(|pat| Self::from_pat(cx, arena, pat))
+                        .chain(iter::repeat_with(|| Self::Wild).take(variant.fields.len() - pats.len()))
+                        .chain(back.iter().map(|pat| Self::from_pat(cx, arena, pat))),
+                );
+                Self::Tuple(var_id, pats)
+            },
+            PatKind::Or(pats) => Self::Or(arena.alloc_from_iter(pats.iter().map(|pat| Self::from_pat(cx, arena, pat)))),
+            PatKind::Path(ref path) => Self::Path(cx.qpath_res(path, pat.hir_id).opt_def_id()),
+            PatKind::Tuple(pats, wild_idx) => {
+                let field_count = match cx.typeck_results().pat_ty(pat).kind() {
+                    ty::Tuple(subs) => subs.len(),
+                    _ => return Self::Wild,
+                };
+                let (front, back) = match wild_idx {
+                    Some(i) => pats.split_at(i),
+                    None => (pats, [].as_slice()),
+                };
+                let pats = arena.alloc_from_iter(
+                    front
+                        .iter()
+                        .map(|pat| Self::from_pat(cx, arena, pat))
+                        .chain(iter::repeat_with(|| Self::Wild).take(field_count - pats.len()))
+                        .chain(back.iter().map(|pat| Self::from_pat(cx, arena, pat))),
+                );
+                Self::Tuple(None, pats)
+            },
+            PatKind::Lit(e) => match &e.kind {
+                // TODO: Handle negative integers. They're currently treated as a wild match.
+                ExprKind::Lit(lit) => match lit.node {
+                    LitKind::Str(sym, _) => Self::LitStr(sym),
+                    LitKind::ByteStr(ref bytes) => Self::LitBytes(&**bytes),
+                    LitKind::Byte(val) => Self::LitInt(val.into()),
+                    LitKind::Char(val) => Self::LitInt(val.into()),
+                    LitKind::Int(val, _) => Self::LitInt(val),
+                    LitKind::Bool(val) => Self::LitBool(val),
+                    LitKind::Float(..) | LitKind::Err(_) => Self::Wild,
+                },
+                _ => Self::Wild,
+            },
+            PatKind::Range(start, end, bounds) => {
+                // TODO: Handle negative integers. They're currently treated as a wild match.
+                let start = match start {
+                    None => 0,
+                    Some(e) => match &e.kind {
+                        ExprKind::Lit(lit) => match lit.node {
+                            LitKind::Int(val, _) => val,
+                            LitKind::Char(val) => val.into(),
+                            LitKind::Byte(val) => val.into(),
+                            _ => return Self::Wild,
+                        },
+                        _ => return Self::Wild,
+                    },
+                };
+                let (end, bounds) = match end {
+                    None => (u128::MAX, RangeEnd::Included),
+                    Some(e) => match &e.kind {
+                        ExprKind::Lit(lit) => match lit.node {
+                            LitKind::Int(val, _) => (val, bounds),
+                            LitKind::Char(val) => (val.into(), bounds),
+                            LitKind::Byte(val) => (val.into(), bounds),
+                            _ => return Self::Wild,
+                        },
+                        _ => return Self::Wild,
+                    },
+                };
+                Self::Range(PatRange { start, end, bounds })
+            },
+            PatKind::Slice(front, wild_pat, back) => Self::Slice(
+                arena.alloc_from_iter(front.iter().map(|pat| Self::from_pat(cx, arena, pat))),
+                wild_pat.map(|_| &*arena.alloc_from_iter(back.iter().map(|pat| Self::from_pat(cx, arena, pat)))),
+            ),
+        }
+    }
+
+    /// Checks if two patterns overlap in the values they can match assuming they are for the same
+    /// type.
+    fn has_overlapping_values(&self, other: &Self) -> bool {
+        match (*self, *other) {
+            (Self::Wild, _) | (_, Self::Wild) => true,
+            (Self::Or(pats), ref other) | (ref other, Self::Or(pats)) => {
+                pats.iter().any(|pat| pat.has_overlapping_values(other))
+            },
+            (Self::Struct(lpath, lfields), Self::Struct(rpath, rfields)) => {
+                if lpath != rpath {
+                    return false;
+                }
+                iter_matching_struct_fields(lfields, rfields).all(|(lpat, rpat)| lpat.has_overlapping_values(rpat))
+            },
+            (Self::Tuple(lpath, lpats), Self::Tuple(rpath, rpats)) => {
+                if lpath != rpath {
+                    return false;
                 }
+                lpats
+                    .iter()
+                    .zip(rpats.iter())
+                    .all(|(lpat, rpat)| lpat.has_overlapping_values(rpat))
+            },
+            (Self::Path(x), Self::Path(y)) => x == y,
+            (Self::LitStr(x), Self::LitStr(y)) => x == y,
+            (Self::LitBytes(x), Self::LitBytes(y)) => x == y,
+            (Self::LitInt(x), Self::LitInt(y)) => x == y,
+            (Self::LitBool(x), Self::LitBool(y)) => x == y,
+            (Self::Range(ref x), Self::Range(ref y)) => x.overlaps(y),
+            (Self::Range(ref range), Self::LitInt(x)) | (Self::LitInt(x), Self::Range(ref range)) => range.contains(x),
+            (Self::Slice(lpats, None), Self::Slice(rpats, None)) => {
+                lpats.len() == rpats.len() && lpats.iter().zip(rpats.iter()).all(|(x, y)| x.has_overlapping_values(y))
             },
-        );
+            (Self::Slice(pats, None), Self::Slice(front, Some(back)))
+            | (Self::Slice(front, Some(back)), Self::Slice(pats, None)) => {
+                // Here `pats` is an exact size match. If the combined lengths of `front` and `back` are greater
+                // then the minium length required will be greater than the length of `pats`.
+                if pats.len() < front.len() + back.len() {
+                    return false;
+                }
+                pats[..front.len()]
+                    .iter()
+                    .zip(front.iter())
+                    .chain(pats[pats.len() - back.len()..].iter().zip(back.iter()))
+                    .all(|(x, y)| x.has_overlapping_values(y))
+            },
+            (Self::Slice(lfront, Some(lback)), Self::Slice(rfront, Some(rback))) => lfront
+                .iter()
+                .zip(rfront.iter())
+                .chain(lback.iter().rev().zip(rback.iter().rev()))
+                .all(|(x, y)| x.has_overlapping_values(y)),
+
+            // Enums can mix unit variants with tuple/struct variants. These can never overlap.
+            (Self::Path(_), Self::Tuple(..) | Self::Struct(..))
+            | (Self::Tuple(..) | Self::Struct(..), Self::Path(_)) => false,
+
+            // Tuples can be matched like a struct.
+            (Self::Tuple(x, _), Self::Struct(y, _)) | (Self::Struct(x, _), Self::Tuple(y, _)) => {
+                // TODO: check fields here.
+                x == y
+            },
+
+            // TODO: Lit* with Path, Range with Path, LitBytes with Slice
+            _ => true,
+        }
     }
 }
 
diff --git a/clippy_lints/src/write.rs b/clippy_lints/src/write.rs
index 532bd810a2e..f3d818cc348 100644
--- a/clippy_lints/src/write.rs
+++ b/clippy_lints/src/write.rs
@@ -581,14 +581,19 @@ impl Write {
             };
 
             let replacement: String = match lit.token.kind {
-                LitKind::Integer | LitKind::Float | LitKind::Err => continue,
                 LitKind::StrRaw(_) | LitKind::ByteStrRaw(_) if matches!(fmtstr.style, StrStyle::Raw(_)) => {
                     lit.token.symbol.as_str().replace('{', "{{").replace('}', "}}")
                 },
                 LitKind::Str | LitKind::ByteStr if matches!(fmtstr.style, StrStyle::Cooked) => {
                     lit.token.symbol.as_str().replace('{', "{{").replace('}', "}}")
                 },
-                LitKind::StrRaw(_) | LitKind::Str | LitKind::ByteStrRaw(_) | LitKind::ByteStr => continue,
+                LitKind::StrRaw(_)
+                | LitKind::Str
+                | LitKind::ByteStrRaw(_)
+                | LitKind::ByteStr
+                | LitKind::Integer
+                | LitKind::Float
+                | LitKind::Err => continue,
                 LitKind::Byte | LitKind::Char => match lit.token.symbol.as_str() {
                     "\"" if matches!(fmtstr.style, StrStyle::Cooked) => "\\\"",
                     "\"" if matches!(fmtstr.style, StrStyle::Raw(0)) => continue,