Add redirects config (#1952)

Author: zachdaniel

Diff for: README.md

@@ -448,6 +448,49 @@ 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 their name.
+- 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, the behavior of a missing page will depend on where they are hosted.
+In particular, [hexdocs.pm](https://hexdocs.pm) will show a 404 page.
+
+You can improve the developer experience on everything but function names changing
+by using the `redirects` configuration. For example, if you changed the module `MyApp.MyModule`
+to `MyApp.My.Module` and the extra `get-started.md` to `quickstart.md`, you can
+setup the following redirects:
+
+<!-- tabs-open -->
+
+### Elixir
+
+For this example, we've changed the module `MyApp.MyModule` to `MyApp.My.Module`, and the extra `get-started.md` to `quickstart.md`
+
+```elixir
+redirects: %{
+  "MyApp.MyModule" => "MyApp.My.Module",
+  "get-started" => "quickstart"
+}
+```
+
+### Erlang
+
+For this example, we've changed the module `:my_module` to `:my_module2`, and the extra `get-started.md` to `quickstart.md`
+
+```erlang
+{redirects, [
+  {"my_module", "my_module2"},
+  {"get-started", "quickstart"}
+]}.
+```
+
+<!-- tabs-close -->
+
 ## 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()} | [{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()
@@ -187,13 +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"
     config = set_canonical_url(config, filename)
@@ -390,6 +384,25 @@ 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.new()
+    |> Map.put_new("index", config.main)
+    |> Enum.map(fn {from, to} ->
+      unless is_binary(from),
+        do: raise("expected a string for the source of a redirect, got: #{inspect(from)}")
+
+      unless is_binary(to),
+        do: raise("expected a string for the destination of a redirect, got: #{inspect(to)}")
+
+      source = from <> ext
+      destination = to <> ext
+      generate_redirect(source, config, destination)
+
+      source
+    end)
+  end
+
   defp disambiguate_id(extra, discriminator) do
     Map.put(extra, :id, "#{extra.id}-#{discriminator}")
   end

Diff for: test/ex_doc/formatter/html_test.exs

@@ -326,6 +326,56 @@ defmodule ExDoc.Formatter.HTMLTest do
     end
   end
 
+  describe "generates redirects" do
+    test "redirects are generated based on the configuration", %{tmp_dir: tmp_dir} = context do
+      generate_docs(
+        doc_config(context,
+          extras: ["test/fixtures/LICENSE"],
+          redirects: %{
+            "/old-license" => "license"
+          }
+        )
+      )
+
+      assert File.read!(tmp_dir <> "/html/old-license.html") == """
+             <!DOCTYPE html>
+             <html>
+               <head>
+                 <meta charset="utf-8">
+                 <title>Elixir v1.0.1 — Documentation</title>
+                 <meta http-equiv="refresh" content="0; url=license.html">
+                 <meta name="generator" content="ExDoc v0.34.2">
+               </head>
+               <body></body>
+             </html>
+             """
+    end
+
+    test "redirects accept a list", %{tmp_dir: tmp_dir} = context do
+      generate_docs(
+        doc_config(context,
+          extras: ["test/fixtures/LICENSE"],
+          redirects: [
+            {"/old-license", "license"}
+          ]
+        )
+      )
+
+      assert File.read!(tmp_dir <> "/html/old-license.html") == """
+             <!DOCTYPE html>
+             <html>
+               <head>
+                 <meta charset="utf-8">
+                 <title>Elixir v1.0.1 — Documentation</title>
+                 <meta http-equiv="refresh" content="0; url=license.html">
+                 <meta name="generator" content="ExDoc v0.34.2">
+               </head>
+               <body></body>
+             </html>
+             """
+    end
+  end
+
   describe "generates extras" do
     @extras [
       "test/fixtures/LICENSE",