Add a comment about the internal _inline_meta macro

Author: mbauman

base/abstractarray.jl

@@ -337,15 +337,21 @@ end
 
 zero{T}(x::AbstractArray{T}) = fill!(similar(x), zero(T))
 
-## iteration support for arrays ##
+## iteration support for arrays by iterating over `eachindex` in the array ##
+# Allows fast iteration by default for both LinearFast and LinearSlow arrays
+
+# While the definitions for LinearFast are all simple enough to inline on their
+# own, LinearSlow's CartesianRange is more complicated and requires explicit
+# inlining. The real @inline macro is not available this early in the bootstrap,
+# so this internal macro splices the meta Expr directly into the function body.
 macro _inline_meta()
     Expr(:meta, :inline)
 end
 start(A::AbstractArray) = (@_inline_meta(); itr = eachindex(A); (itr, start(itr)))
 next(A::AbstractArray,i) = (@_inline_meta(); (idx, s) = next(i[1], i[2]); (A[idx], (i[1], s)))
 done(A::AbstractArray,i) = done(i[1], i[2])
 
-# eachindex iterates over all indices. LinearSlow is later in bootstrap
+# eachindex iterates over all indices. LinearSlow definitions are later.
 eachindex(A::AbstractArray) = (@_inline_meta; eachindex(linearindexing(A), A))
 eachindex(::LinearFast, A::AbstractArray) = 1:length(A)