improvement: add redirects config, and documentation explaining it

Author: zachdaniel

Diff for: README.md

@@ -448,6 +448,35 @@ Similarly to the example above, if your Markdown includes Mermaid graph specific
 
 For more details and configuration options, see the [Mermaid usage docs](https://mermaid-js.github.io/mermaid/#/usage).
 
+## Changing documentation over time
+
+As your project grows, your documentation may very likely change, even structurally. There are a few important things to consider in this regard:
+
+- Links to your *extras* will break if you change or move file names.
+- Links to your *modules, and mix tasks* will change if you change your code.
+- Links to *functions* are actually links to modules with anchor links. If you change the function name, the link does
+  not break but will leave users at the top of the module's documentation.
+
+Because these docs are static files, when a user gets to a page that is not found, they will see a generic 404 page.
+They will not be redirected to your packages home page. This can potentially be jarring for users.
+
+With this in mind, it is a good idea to preserve links to your old documentation. We do this with the `redirects` configuration.
+This can solve for everything but the function names changing.
+
+For this example, we've changed the module `MyApp.MyModule` to `MyApp.My.Module`, and the extra `get-started.md` to `quickstart.md`
+
+```elixir
+defp docs do
+  [
+    ...,
+    redirects: %{
+      MyApp.MyModule => MyApp.My.Module,
+      "get-started" => "quickstart"
+    }
+  ]
+end
+```
+
 ## Contributing
 
 The easiest way to test changes to ExDoc is to locally rebuild the app and its own documentation:

Diff for: lib/ex_doc/config.ex

@@ -39,6 +39,7 @@ defmodule ExDoc.Config do
             package: nil,
             proglang: :elixir,
             project: nil,
+            redirects: %{},
             retriever: ExDoc.Retriever,
             skip_undefined_reference_warnings_on:
               &__MODULE__.skip_undefined_reference_warnings_on/1,
@@ -78,6 +79,7 @@ defmodule ExDoc.Config do
           output: nil | Path.t(),
           package: :atom | nil,
           project: nil | String.t(),
+          redirects: %{optional(String.t()) => String.t()},
           retriever: atom(),
           skip_undefined_reference_warnings_on: (String.t() -> boolean),
           skip_code_autolink_to: (String.t() -> boolean),

Diff for: lib/ex_doc/formatter/html.ex

@@ -47,7 +47,8 @@ defmodule ExDoc.Formatter.HTML do
         generate_search(nodes_map, config) ++
         generate_not_found(nodes_map, config) ++
         generate_list(nodes_map.modules, nodes_map, config) ++
-        generate_list(nodes_map.tasks, nodes_map, config) ++ generate_index(config)
+        generate_list(nodes_map.tasks, nodes_map, config) ++
+        generate_redirects(config, ".html")
 
     generate_build(Enum.sort(all_files), build)
     config.output |> Path.join("index.html") |> Path.relative_to_cwd()
@@ -187,12 +188,6 @@ defmodule ExDoc.Formatter.HTML do
     File.write!(build, entries)
   end
 
-  defp generate_index(config) do
-    index_file = "index.html"
-    main_file = "#{config.main}.html"
-    generate_redirect(index_file, config, main_file)
-    [index_file]
-  end
 
   defp generate_not_found(nodes_map, config) do
     filename = "404.html"
@@ -390,6 +385,35 @@ defmodule ExDoc.Formatter.HTML do
     |> Enum.sort_by(fn extra -> GroupMatcher.group_index(groups, extra.group) end)
   end
 
+  def generate_redirects(config, ext) do
+    config.redirects
+    |> Map.put_new("index", config.main)
+    |> Enum.map(fn {from, to} ->
+      source = stringify_redirect_item(from) <> ext
+      destination = stringify_redirect_item(to) <> ext
+      generate_redirect(source, config, destination)
+
+      source
+    end)
+  end
+
+  defp stringify_redirect_item(item) when is_binary(item) do
+    item
+  end
+
+  defp stringify_redirect_item(item) when is_atom(item) do
+    inspected = inspect(item)
+
+    case to_string(item) do
+      "Elixir." <> ^inspected -> inspected
+      other -> other
+    end
+  end
+
+  defp stringify_redirect_item(item) do
+    raise ArgumentError, "Redirect source and destination must be a string or an atom, got: #{inspect(item)}"
+  end
+
   defp disambiguate_id(extra, discriminator) do
     Map.put(extra, :id, "#{extra.id}-#{discriminator}")
   end

Diff for: mix.exs

@@ -87,6 +87,7 @@ defmodule ExDoc.Mixfile do
           "Cheatsheet.cheatmd",
           "CHANGELOG.md"
         ] ++ test_dev_examples(Mix.env()),
+      redirects: redirects(Mix.env()),
       source_ref: "v#{@version}",
       source_url: @source_url,
       groups_for_modules: [
@@ -107,6 +108,15 @@ defmodule ExDoc.Mixfile do
   defp test_dev_examples(:dev), do: Path.wildcard("test/examples/*")
   defp test_dev_examples(_), do: []
 
+  defp redirects(:dev) do
+    %{
+      "old-admonition" => "admonition",
+      Exdoc.OldMarkdown => ExDoc.Markdown
+    }
+  end
+
+  defp redirects(_), do: %{}
+
   defp clean_test_fixtures(_args) do
     File.rm_rf("test/tmp")
   end