Move core Size to core package

Author: N5N3

src/StaticArraysCore.jl

@@ -42,5 +42,6 @@ const StaticVecOrMat{T} = Union{StaticVector{<:Any, T}, StaticMatrix{<:Any, <:An
 
 include("staticarrays.jl")
 include("utils.jl")
+include("staticsize.jl")
 
 end # module

src/staticsize.jl

@@ -0,0 +1,50 @@
+"""
+    Dynamic()
+
+Used to signify that a dimension of an array is not known statically.
+"""
+struct Dynamic end
+
+const StaticDimension = Union{Int, Dynamic}
+
+"""
+    Size(dims::Int...)
+
+`Size` is used extensively throughout the `StaticArrays` API to describe _compile-time_
+knowledge of the size of an array. The dimensions are stored as a type parameter and are
+statically propagated by the compiler, resulting in efficient, type-inferrable code. For
+example, to create a static matrix of zeros, use `A = zeros(SMatrix{3,3})`. The static
+size of `A` can be obtained by `Size(A)`. (rather than `size(zeros(3,3))`, which returns
+`Base.Tuple{2,Int}`).
+
+Note that if dimensions are not known statically (e.g., for standard `Array`s),
+[`Dynamic()`](@ref) should be used instead of an `Int`.
+
+    Size(a::AbstractArray)
+    Size(::Type{T<:AbstractArray})
+
+The `Size` constructor can be used to extract static dimension information from a given
+array. For example:
+
+```julia-repl
+julia> Size(zeros(SMatrix{3, 4}))
+Size(3, 4)
+
+julia> Size(zeros(3, 4))
+Size(StaticArrays.Dynamic(), StaticArrays.Dynamic())
+```
+
+This has multiple uses, including "trait"-based dispatch on the size of a statically-sized
+array. For example:
+
+```julia
+det(x::StaticMatrix) = _det(Size(x), x)
+_det(::Size{(1,1)}, x::StaticMatrix) = x[1,1]
+_det(::Size{(2,2)}, x::StaticMatrix) = x[1,1]*x[2,2] - x[1,2]*x[2,1]
+# and other definitions as necessary
+```
+
+"""
+struct Size{S}
+    Size{S}() where {S} = new{S::Tuple{Vararg{StaticDimension}}}()
+end