use eachindex instead of 1:length

Co-authored-by: Frames White <oxinabox@ucc.asn.au>

src/rulesets/SparseArrays/sparsematrix.jl

@@ -164,6 +164,6 @@ end
 
 function _diagm_back(p, ȳ)
     k, v = p
-    d = diag(unthunk(ȳ), k)[1:length(v)] # handle if diagonal was smaller than matrix
+    d = diag(unthunk(ȳ), k)[eachindex(v)]  # handle if diagonal was smaller than matrix
     return Tangent{typeof(p)}(second = d)
 end