Insert <link rel="canonical"> elements

This prevents duplicate search results in case copies of the material
is being left visible on the web. See

  https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls

for details. The implementation here is very simple and should go away
when google/mdbook-i18n-helpers#70 is
implemented.

.github/workflows/publish.yml

@@ -55,6 +55,28 @@ jobs:
               echo "::endgroup::"
           done
 
+      # This helps guard against duplicate search results in case test
+      # instances are left around on the public Internet. See
+      # https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls
+      # for details.
+      #
+      # For simplicity, we replace a fixed string instead of
+      # attempting to parse the HTML in some clever way.
+      # https://github.com/google/mdbook-i18n-helpers/issues/70 will
+      # eventually make this obsolete.
+      - name: 'Add link rel="canonical" elements'
+        run: |
+          canonical="https://google.github.io/comprehensive-rust"
+          cd book/html
+          for page in **/*.html; do
+              if [[ ${page:t} -eq "index.html" ]]; then
+                  link=${page:h}
+              else
+                  link=$page
+              fi
+              sed -i 's|<!-- LINK-REL-CANONICAL -->|<link rel="canonical" href="'$canonical/$link'">|'
+        shell: zsh {0}
+
       - name: Setup Pages
         uses: actions/configure-pages@v2
 

theme/index.hbs

@@ -29,6 +29,10 @@
         <meta name="viewport" content="width=device-width, initial-scale=1">
         <meta name="theme-color" content="#ffffff" />
 
+        {{! Replaced with a <link rel="canonical"> element in
+            publish.yml. Remove when after fixing this issue:
+            https://github.com/google/mdbook-i18n-helpers/issues/70 }}
+        <!-- LINK-REL-CANONICAL -->
         {{#if favicon_svg}}
         <link rel="icon" href="{{ path_to_root }}favicon.svg">
         {{/if}}