[Utilities] skip map_indices when the variable map of IndexMap is the identity

src/Utilities/copy/index_map.jl

@@ -104,3 +104,20 @@ Base.length(map::IndexMap) = length(map.var_map) + length(map.con_map)
 function Base.iterate(map::IndexMap, args...)
     return iterate(Base.Iterators.flatten((map.var_map, map.con_map)), args...)
 end
+
+# Objects that may contain `MOI.VariableIndex` but no `MOI.ConstraintIndex`.
+const _OnlyVariableIndices = Union{
+    MOI.VariableIndex,
+    MOI.AbstractFunction,
+    MOI.AbstractFunctionModification,
+}
+
+# The variable map is often the identity, for example, when the destination
+# model of `copy_to` creates its variables in the same order as the source
+# model. In that case, there is no need to map the variables of `x`.
+function map_indices(map::IndexMap, x::X)::X where {X<:_OnlyVariableIndices}
+    if CleverDicts.is_identity(map.var_map)
+        return x
+    end
+    return map_indices(Base.Fix1(getindex, map), x)
+end

src/Utilities/functions.jl

@@ -248,6 +248,12 @@ map_indices(::Any, ::MOI.RawOptimizerAttribute, x) = x
     )::X where {T<:MOI.Index,X}
 
 Shortcut for `map_indices(vi -> variable_map[vi], x)`.
+
+!!! note
+    The returned value is not guaranteed to be a copy of `x`. For example, if
+    `variable_map` is an [`IndexMap`](@ref) whose variable map is the
+    identity, `x` may be returned without being copied. Therefore, it should
+    not be modified unless `x` is not used anymore.
 """
 function map_indices(
     variable_map::AbstractDict{T,T},

test/Utilities/test_copy.jl

@@ -1122,6 +1122,28 @@ function test_copy_names_unsupported()
     return
 end
 
+function test_map_indices_identity_index_map()
+    src = MOI.Utilities.Model{Float64}()
+    x = MOI.add_variables(src, 2)
+    dest = MOI.Utilities.Model{Float64}()
+    index_map = MOI.copy_to(dest, src)
+    @test MOI.Utilities.CleverDicts.is_identity(index_map.var_map)
+    f = 1.0 * x[1] + 2.0 * x[2]
+    @test MOI.Utilities.map_indices(index_map, f) === f
+    attr = MOI.ObjectiveFunction{typeof(f)}()
+    @test MOI.Utilities.map_indices(index_map, attr, f) === f
+    change = MOI.ScalarCoefficientChange(x[1], 2.0)
+    @test MOI.Utilities.map_indices(index_map, change) === change
+    index_map[x[1]] = x[2]
+    index_map[x[2]] = x[1]
+    @test !MOI.Utilities.CleverDicts.is_identity(index_map.var_map)
+    g = MOI.Utilities.map_indices(index_map, f)
+    @test isapprox(g, 1.0 * x[2] + 2.0 * x[1])
+    @test MOI.Utilities.map_indices(index_map, change) ==
+          MOI.ScalarCoefficientChange(x[2], 2.0)
+    return
+end
+
 end  # module
 
 TestCopy.runtests()