rc docs: hack Documenter to insert rc docs in the version selector.

doc/make.jl

@@ -233,11 +233,33 @@ end
 
 const devurl = "v$(VERSION.major).$(VERSION.minor)-dev"
 
+# Hack to make rc docs visible in the version selector
+struct Versions versions end
+function Documenter.Writers.HTMLWriter.expand_versions(dir::String, v::Versions)
+    # Find all available docs
+    available_folders = readdir(dir)
+    cd(() -> filter!(!islink, available_folders), dir)
+    filter!(x -> occursin(Base.VERSION_REGEX, x), available_folders)
+
+    # Look for docs for an "active" release candidate and insert it
+    vnums = [VersionNumber(x) for x in available_folders]
+    master_version = maximum(vnums)
+    filter!(x -> x.major == 1 && x.minor == master_version.minor-1, vnums)
+    rc = maximum(vnums)
+    if !isempty(rc.prerelease) && occursin(r"^rc", rc.prerelease[1])
+        src = "v$(rc)"
+        @assert src ∈ available_folders
+        push!(v.versions, src => src, pop!(v.versions))
+    end
+
+    return Documenter.Writers.HTMLWriter.expand_versions(dir, v.versions)
+end
+
 deploydocs(
     repo = "github.com/JuliaLang/docs.julialang.org.git",
     deploy_config = BuildBotConfig(),
     target = joinpath(buildroot, "doc", "_build", "html", "en"),
     dirname = "en",
     devurl = devurl,
-    versions = ["v#.#", devurl => devurl]
+    versions = Versions(["v#.#", devurl => devurl]),
 )