chore: strip TOC from readme in rustdoc

Rustdoc already puts a mini-toc in the sidebar, so filling up the page
above the fold with a full TOC seems pointless.

Also moves the munge script into a separate file for better editability.

Author: coriolinus

.cargo/munge-readme.pl

@@ -0,0 +1,27 @@
+#!/usr/bin/env perl
+use strict;
+use warnings;
+
+local $/ = undef;   # slurp mode; no input separators -> one big input
+$_ = <>;            # read whole input (files on command line, fallback to stdin)
+
+# replace "ignore" code blocks with "text"
+s/```ignore/```text/g;
+# remove lines with admonition markers
+s/^.*\[!(NOTE|TIP|IMPORTANT|WARNING|CAUTION)\].*\n//mg;
+
+# handle TOC markers
+my $start = "<!-- mdformat-toc start";
+my $end   = "<!-- mdformat-toc end -->";
+
+if (index($_, $start) == -1 && index($_, $end) == -1) {
+    # both absent → do nothing
+}
+elsif (index($_, $start) == -1 || index($_, $end) == -1) {
+    die "Error: only one TOC marker present\n";
+}
+else {
+    s/$start.*?$end//s;
+}
+
+print;

.cargo/rustdoc-wrapper.sh

@@ -3,13 +3,8 @@
 # that are not understood by rustdoc. Remove them so rustdoc doesn't complain.
 if [ "$CARGO_CRATE_NAME" = "core_crypto" ]; then
     tempfile=$(mktemp)
-    # we do not want to expand the contained backticks as expressions here;
-    # they are code fences
-    #shellcheck disable=SC2016
-     perl -ne '
-         s/```ignore/```text/g;
-         print unless /\[!(NOTE|TIP|IMPORTANT|WARNING|CAUTION)\]/
-     ' "$CARGO_MANIFEST_DIR/../README.md" > "$tempfile"
+    cwd="$(dirname "$(realpath "$0")")"
+    perl "$cwd/munge-readme.pl" "$CARGO_MANIFEST_DIR/../README.md" > "$tempfile"
     export STRIPPED_README_PATH="$tempfile"
 fi
 rustdoc "$@"