Use view instead of getindex; set up for constant prop type stability

NEWS.md

@@ -646,7 +646,9 @@ Deprecated or removed
   * Using Bool values directly as indices is now deprecated and will be an error in the future. Convert
     them to `Int` before indexing if you intend to access index `1` for `true` and `0` for `false`.
 
-  * `slicedim` has been deprecated in favor of `selectdim` ([#26009]).
+  * `slicedim(A, d, i)` has been deprecated in favor of `copy(selectdim(A, d, i)`. The new
+    `selectdim` function now always returns a view into `A`; in many cases the `copy` is
+	not necessary ([#26009]).
 
   * `whos` has been renamed `varinfo`, and now returns a markdown table instead of printing
     output ([#12131]).

base/abstractarraymath.jl

@@ -100,8 +100,9 @@ imag(x::AbstractArray{<:Real}) = zero(x)
 """
     selectdim(A, d::Integer, i)
 
-Return all the data of `A` where the index for dimension `d` equals `i`. Equivalent to
-`A[:,:,...,i,:,:,...]` where `i` is in position `d`.
+Return a view of all the data of `A` where the index for dimension `d` equals `i`.
+
+Equivalent to `view(A,:,:,...,i,:,:,...)` where `i` is in position `d`.
 
 # Examples
 ```jldoctest
@@ -110,17 +111,18 @@ julia> A = [1 2 3 4; 5 6 7 8]
  1  2  3  4
  5  6  7  8
 
-julia> selectdim(A,2,3)
-2-element Array{Int64,1}:
+julia> selectdim(A, 2, 3)
+2-element view(::Array{Int64,2}, Base.OneTo(2), 3) with eltype Int64:
  3
  7
 ```
 """
-function selectdim(A::AbstractArray, d::Integer, i)
+@inline selectdim(A::AbstractArray, d::Integer, i) = _selectdim(A, d, i, setindex(axes(A), i, d))
+@noinline function _selectdim(A, d, i, idxs)
     d >= 1 || throw(ArgumentError("dimension must be ≥ 1"))
     nd = ndims(A)
-    d > nd && (i == 1 || throw_boundserror(A, (ntuple(k->Colon(),nd)..., ntuple(k->1,d-1-nd)..., i)))
-    A[setindex(axes(A), i, d)...]
+    d > nd && (i == 1 || throw(BoundsError(A, (ntuple(k->Colon(),d-1)..., i))))
+    return view(A, idxs...)
 end
 
 """

base/deprecated.jl

@@ -26,7 +26,7 @@ macro deprecate(old, new, ex=true)
         newname = Expr(:quote, new)
         Expr(:toplevel,
             ex ? Expr(:export, esc(old)) : nothing,
-            :(function $(esc(old))(args...)
+            :(@__doc__ function $(esc(old))(args...)
                   $meta
                   depwarn($"`$old` is deprecated, use `$new` instead.", $oldmtname)
                   $(esc(new))(args...)
@@ -51,7 +51,7 @@ macro deprecate(old, new, ex=true)
         end
         Expr(:toplevel,
             ex ? Expr(:export, esc(oldsym)) : nothing,
-            :($(esc(old)) = begin
+            :($(@__doc__ esc(old)) = begin
                   $meta
                   depwarn($"`$oldcall` is deprecated, use `$newcall` instead.", $oldmtname)
                   $(esc(new))
@@ -1360,8 +1360,21 @@ end
 # issue #25928
 @deprecate wait(t::Task) fetch(t)
 
-# PR #26008
-@deprecate slicedim(A::AbstractArray, d::Integer, i) selectdim(A, d, i)
+"
+`slicedim` is deprecated in favor of `selectdim`. Whereas `slicedim` previously
+used `getindex` to select its data, `selectdim` now always uses `view`.
+"
+@deprecate slicedim(A::AbstractArray, d::Integer, i) copy(selectdim(A, d, i))
+function slicedim(A::AbstractVector, d::Integer, i::Number)
+    if d == 1
+        # slicedim would have returned a scalar value, selectdim always returns views
+        depwarn("`slicedim(A::AbstractVector, d::Integer, i)` is deprecated, use `selectdim(A, d, i)[]` instead.", :slicedim)
+        selectdim(A, d, i)[]
+    else
+        depwarn("`slicedim(A::AbstractArray, d::Integer, i)` is deprecated, use `copy(selectdim(A, d, i))` instead.", :slicedim)
+        copy(selectdim(A, d, i))
+    end
+end
 
 # PR 25062
 @deprecate(link_pipe(pipe; julia_only_read = true, julia_only_write = true),