How to insert file on matching pattern with filename?

Question

Suppose I have 3 files:

index.html

And here is foo:
<!-- include: includes/foo.html -->
This can be done similarly:
<code>
<!-- include: includes/bar.txt -->
</code>

includes/foo.html

FOO CONTENTS
FOO CONTENTS
FOO CONTENTS

includes/bar.txt

BAR CONTENTS
BAR CONTENTS
BAR CONTENTS
BAR CONTENTS
BAR CONTENTS

How can I replace the includes automatically with a command like awk 'some magic code' index.html > compiled.html?

compiled.html

And here is foo:
FOO CONTENTS
FOO CONTENTS
This can be done similarly:
<code>
BAR CONTENTS
BAR CONTENTS
BAR CONTENTS
BAR CONTENTS
BAR CONTENTS
</code>

Additional comments

• If no such file is found, a warning or error (preferred) should occur (<!-- include: includes/does-not-exist.txt -->)
• The <!-- include: ... --> line must not be in compiled.html
• I am flexible on the include syntax. It could be <include "includes/foo.html"> if it's easier.
• I want something like "pointer files", but without a hardcoded filename and without the pointer file line being included
• The command must not require me to manually specify all of the includes. (awk 'some magic code' index.html includes/foo.html includes/bar.txt > compiled.html is not allowed)
• It just needs to work on most filenames and should not require too many tools (for example, I'd rather not install PHP server just to use <? include "includes/bar.txt" ?>)

Answer 1

With perl:

perl -0777 -pe 'while (s{<!-- include: (.*?) -->}{
  open I, "<", $1; <I>}ge) {}' index.html > compiled.html

(the while loop to make it recursive to allow includes to include more files).

The capture group (.*?) allow filename retrieving to include them in the File Handle with $1.

With error handling:

perl -0777 -pe '
  while (
    s{<!-- include: (.*?) -->}{
      if (open I, "<", $1) {
        <I>;
      } else {
        warn "$1: $!\n";
        $ret = 1; 
        "<!-- failed-include: $1 -->"
      }
    }ge
  ) {}
  END {exit $ret}' index.html > compiled.html