Same-shape fast path for restructure(x::Array, y::Array)

Project.toml

@@ -1,6 +1,6 @@
 name = "ArrayInterface"
 uuid = "4fba245c-0d91-5ea0-9b3e-6abc04ee57a9"
-version = "7.28.0"
+version = "7.28.1"
 
 [deps]
 Adapt = "79e6a3ab-5dfb-504d-930d-738a2a938a0e"

src/ArrayInterface.jl

@@ -674,6 +674,14 @@ function restructure(x::Array, y)
     reshape(convert(Array, y), Base.size(x)...)
 end
 
+function restructure(x::Array, y::Array)
+    # When `y` already has `x`'s shape it is its own restructuring. `reshape` is not a no-op
+    # here: unlike `vec`, it has no same-shape short-circuit and always mints a fresh `Array`
+    # header (sharing the data), so returning `y` avoids that per-call allocation.
+    Base.size(x) == Base.size(y) && return y
+    reshape(convert(Array, y), Base.size(x)...)
+end
+
 abstract type AbstractDevice end
 abstract type AbstractCPU <: AbstractDevice end
 struct CPUPointer <: AbstractCPU end

test/core.jl

@@ -153,6 +153,32 @@ end
         @test size(yr) == ()
         @test yr == y
     end
+
+    @testset "same-shape Array fast path" begin
+        # When `y` already has `x`'s shape, `restructure` returns `y` itself. `reshape` has no
+        # same-shape short-circuit (unlike `vec`), so it would otherwise mint a fresh Array
+        # header (sharing the data) on every call.
+        for dims in ((3,), (2,3), (2,2,2))
+            x = rand(dims...)
+            y = rand(dims...)
+            @test ArrayInterface.restructure(x, y) === y
+        end
+        # ... and does so without allocating
+        function _restructure_alloc()
+            x = rand(4); y = rand(4)
+            ArrayInterface.restructure(x, y)
+            @allocated ArrayInterface.restructure(x, y)
+        end
+        @test _restructure_alloc() == 0
+
+        # a genuine shape change still reshapes to `x`'s shape (unchanged behavior)
+        x = rand(2,2)
+        y = rand(4)
+        yr = ArrayInterface.restructure(x, y)
+        @test yr isa Matrix{Float64}
+        @test size(yr) == (2,2)
+        @test vec(yr) == y
+    end
 end
 
 @testset "isstructured" begin