issubset: don't create a Set out of dicts

base/abstractset.jl

@@ -251,16 +251,15 @@ true
 issubset, ⊆, ⊇
 
 function issubset(l, r)
-    if haslength(r)
-        rlen = length(r)
-        if isa(l, AbstractSet)
-            # check l for too many unique elements
-            length(l) > rlen && return false
+    if haslength(r) && (isa(l, AbstractSet) || !hasfastin(r))
+        rlen = length(r) # conditions above make this length computed only when needed
+        # check l for too many unique elements
+        if isa(l, AbstractSet) && length(l) > rlen
+            return false
         end
-        # if r is big enough, convert it to a Set
-        # threshold empirically determined by repeatedly
-        # sampling using these two methods (see #26198)
-        if rlen > 70 && !isa(r, AbstractSet)
+        # when `in` would be too slow and r is big enough, convert it to a Set
+        # this threshold was empirically determined (cf. #26198)
+        if !hasfastin(r) && rlen > 70
             return issubset(l, Set(r))
         end
     end
@@ -270,6 +269,19 @@ function issubset(l, r)
     return true
 end
 
+"""
+    hasfastin(T)
+
+Determine whether the computation `x ∈ collection` where `collection::T` can be considered
+as a "fast" operation (typically constant or logarithmic complexity).
+The definition `hasfastin(x) = hasfastin(typeof(x))` is provided for convenience so that instances
+can be passed instead of types.
+However the form that accepts a type argument should be defined for new types.
+"""
+hasfastin(::Type) = false
+hasfastin(::Union{Type{<:AbstractSet},Type{<:AbstractDict},Type{<:AbstractRange}}) = true
+hasfastin(x) = hasfastin(typeof(x))
+
 ⊇(l, r) = r ⊆ l
 
 ## strict subset comparison

test/sets.jl

@@ -347,6 +347,24 @@ end
     # symdiff must NOT uniquify
     @test symdiff([1, 2, 1]) == symdiff!([1, 2, 1]) == [2]
     @test symdiff([1, 2, 1], [2, 2]) == symdiff!([1, 2, 1], [2, 2]) == [2]
+
+    # Base.hasfastin
+    @test all(Base.hasfastin, Any[Dict(1=>2), Set(1), BitSet(1), 1:9, 1:2:9,
+                                  Dict, Set, BitSet, UnitRange, StepRange])
+    @test !any(Base.hasfastin, Any[[1, 2, 3], "123",
+                                   Array, String])
+
+    # tests for Dict
+    d1 = Dict(1=>nothing, 2=>nothing)
+    d2 = Dict(1=>nothing, 3=>nothing)
+    d3 = Dict(1=>nothing, 2=>nothing, 3=>nothing)
+    @test d3 == merge(d1, d2)
+    @test !issubset(d1, d2)
+    @test !issubset(d2, d1)
+    @test !issubset(d3, d1)
+    @test !issubset(d3, d2)
+    @test issubset(d1, d3)
+    @test issubset(d2, d3)
 end
 
 @testset "unique" begin