Make copyto!(A,I) work for rectangular matrices (#28790)

* Make copyto!(A,I) work for rectangular matrices

* add tests for `copyto!`

add tests for `copyto!(::Matrix, ::UniformScaling)`

* fix whitespace

stdlib/LinearAlgebra/src/uniformscaling.jl

@@ -207,10 +207,9 @@ isapprox(A::AbstractMatrix, J::UniformScaling; kwargs...) = isapprox(J, A; kwarg
 
 function copyto!(A::AbstractMatrix, J::UniformScaling)
     @assert !has_offset_axes(A)
-    size(A,1)==size(A,2) || throw(DimensionMismatch("a UniformScaling can only be copied to a square matrix"))
     fill!(A, 0)
     λ = J.λ
-    for i = 1:size(A,1)
+    for i = 1:min(size(A,1),size(A,2))
         @inbounds A[i,i] = λ
     end
     return A

stdlib/LinearAlgebra/test/uniformscaling.jl

@@ -86,6 +86,13 @@ let
         @test cond(J) == (λ ≠ zero(λ) ? one(real(λ)) : oftype(real(λ), Inf))
     end
 
+    @testset "copyto!" begin
+        A = Matrix{Int}(undef, (3,3))
+        @test copyto!(A, I) == one(A)
+        B = Matrix{ComplexF64}(undef, (1,2))
+        @test copyto!(B, J) == [λ zero(λ)]
+    end
+
     @testset "binary ops with matrices" begin
         B = bitrand(2, 2)
         @test B + I == B + Matrix(I, size(B))