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src/librustc_const_eval/_match.rs

@@ -419,34 +419,81 @@ fn all_constructors(_cx: &mut MatchCheckCtxt, pcx: PatternContext) -> Vec<Constr
     }
 }
 
-fn max_slice_length<'a: 'b, 'b, 'tcx, I>(
+fn max_slice_length<'a, 'tcx, I>(
     _cx: &mut MatchCheckCtxt<'a, 'tcx>,
     patterns: I) -> usize
-    where I: Iterator<Item=&'b [&'a Pattern<'tcx>]>
+    where I: Iterator<Item=&'a Pattern<'tcx>>
 {
     // The exhaustiveness-checking paper does not include any details on
     // checking variable-length slice patterns. However, they are matched
     // by an infinite collection of fixed-length array patterns.
     //
-    // To check that infinite set, we notice that for every length
-    // larger than the length of the maximum fixed-length pattern,
-    // only variable-length patterns apply.
+    // Checking the infinite set directly would take an infinite amount
+    // of time. However, it turns out that for each finite set of
+    // patterns `P`, all sufficiently large array lengths are equivalent:
     //
-    // For variable length patterns, all elements after the first
-    // `prefix_len` but before the last `suffix_len` are matched by
-    // the wildcard "middle" pattern, and therefore can be added/removed
-    // without affecting the match result.
+    // Each slice `s` with a "sufficiently-large" length `l ≥ L` that applies
+    // to exactly the subset `Pₜ` of `P` can be transformed to a slice
+    // `sₘ` for each sufficiently-large length `m` that applies to exactly
+    // the same subset of `P`.
     //
-    // This means that all patterns with length at least
-    // `max(max_fixed+1,max_prefix+max_suffix)` are equivalent, so we
-    // only need to check patterns from that length and below.
+    // Because of that, each witness for reachability-checking from one
+    // of the sufficiently-large lengths can be transformed to an
+    // equally-valid witness from any other length, so we only have
+    // to check slice lengths from the "minimal sufficiently-large length"
+    // and below.
+    //
+    // Note that the fact that there is a *single* `sₘ` for each `m`
+    // not depending on the specific pattern in `P` is important: if
+    // you look at the pair of patterns
+    //     `[true, ..]`
+    //     `[.., false]`
+    // Then any slice of length ≥1 that matches one of these two
+    // patterns can be  be trivially turned to a slice of any
+    // other length ≥1 that matches them and vice-versa - for
+    // but the slice from length 2 `[false, true]` that matches neither
+    // of these patterns can't be turned to a slice from length 1 that
+    // matches neither of these patterns, so we have to consider
+    // slices from length 2 there.
+    //
+    // Now, to see that that length exists and find it, observe that slice
+    // patterns are either "fixed-length" patterns (`[_, _, _]`) or
+    // "variable-length" patterns (`[_, .., _]`).
+    //
+    // For fixed-length patterns, all slices with lengths *longer* than
+    // the pattern's length have the same outcome (of not matching), so
+    // as long as `L` is greater than the pattern's length we can pick
+    // any `sₘ` from that length and get the same result.
+    //
+    // For variable-length patterns, the situation is more complicated,
+    // because as seen above the precise value of `sₘ` matters.
+    //
+    // However, for each variable-length pattern `p` with a prefix of length
+    // `plₚ` and suffix of length `slₚ`, only the first `plₚ` and the last
+    // `slₚ` elements are examined.
+    //
+    // Therefore, as long as `L` is positive (to avoid concerns about empty
+    // types), all elements after the maximum prefix length and before
+    // the maximum suffix length are not examined by any variable-length
+    // pattern, and therefore can be added/removed without affecting
+    // them - creating equivalent patterns from any sufficiently-large
+    // length.
+    //
+    // Of course, if fixed-length patterns exist, we must be sure
+    // that our length is large enough to miss them all, so
+    // we can pick `L = max(FIXED_LEN+1 ∪ {max(PREFIX_LEN) + max(SUFFIX_LEN)})`
+    //
+    // for example, with the above pair of patterns, all elements
+    // but the first and last can be added/removed, so any
+    // witness of length ≥2 (say, `[false, false, true]`) can be
+    // turned to a witness from any other length ≥2.
 
     let mut max_prefix_len = 0;
     let mut max_suffix_len = 0;
     let mut max_fixed_len = 0;
 
     for row in patterns {
-        match *row[0].kind {
+        match *row.kind {
             PatternKind::Constant { value: ConstVal::ByteStr(ref data) } => {
                 max_fixed_len = cmp::max(max_fixed_len, data.len());
             }
@@ -504,7 +551,7 @@ pub fn is_useful<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
     let pcx = PatternContext {
         ty: rows.iter().map(|r| r[0].ty).find(|ty| !ty.references_error())
             .unwrap_or(v[0].ty),
-        max_slice_length: max_slice_length(cx, rows.iter().map(|r| &**r).chain(Some(v)))
+        max_slice_length: max_slice_length(cx, rows.iter().map(|r| r[0]).chain(Some(v[0])))
     };
 
     debug!("is_useful_expand_first_col: pcx={:?}, expanding {:?}", pcx, v[0]);