ci: Use some tricks to format macro bodies

We have a lot of syntax contained within macro bodies, which `rustfmt`
doesn't touch. Add a hack that replaces relevant macro invocations with
functions, formats, then resets.

ci/style.sh

@@ -13,7 +13,48 @@ rustc ci/style.rs && ./style src
 
 command -v rustfmt
 rustfmt -V
-cargo fmt --all -- ${check:+"$check"}
+
+# Save a list of all source files
+tmpfile="file-list~" # trailing tilde for gitignore
+find src -name '*.rs' > "$tmpfile"
+
+# Before formatting, replace all macro identifiers with a function signature.
+# This allows `rustfmt` to format it.
+while IFS= read -r file; do
+    if [ "$file" = "src/macros.rs" ]; then
+        # Too much special syntax in `macros.rs` that we don't want to format
+        continue
+    fi
+
+    # Suffix all braced macros except `macro_rules!` with `_fmt_tmp()`. Also
+    # remove the bang.
+    perl -pi -e 's/(?!macro_rules)\b(\w+)!\s*\{/fn $1_fmt_tmp() {/g' "$file"
+
+    # Replace `#[cfg(...)]` within `cfg_if` with `cfg_tmp!([...])` which
+    # `rustfmt` will format. We put braces within the parens so it is easy to
+    # match (matching parentheses would catch the first closing `)` which
+    # wouldn't be correct for something like `all(any(...), ...)`).
+    perl -pi -0777 -e 's/if #\[cfg\((.*?)\)\]/if cfg_tmp!([$1])/gms' "$file"
+
+    # We have some instances of `{const}` that make macros happy. Replace this
+    # with just the keyword, plus an indicator comment on the preceding line
+    # (which is where rustfmt puts it. Also rust-lang/rustfmt#5464).
+    perl -pi -e 's/^(\s*)(.*)\{const\}/$1\/\* FMT-CONST \*\/\n$1$2const/g' "$file"
+
+    # We need to invoke `rustfmt` directly since `cargo fmt` can't figure out
+    # the module tree with the hacks in place.
+    rustfmt --config-path rustfmt.toml "$file" ${check:+"$check"}
+
+    # Reset everything
+    perl -pi -e 's/fn (\w+)_fmt_tmp\(\)/$1!/g' "$file"
+    perl -pi -0777 -e 's/cfg_tmp!\(\[(.*?)\]\)/#[cfg($1)]/gms' "$file"
+
+    # Rustfmt sometimes puts our `/* FMT-CONST */` comment on the preceding line
+    # so we need to do a multiline match.
+    perl -pi -0777 -e 's/\/\* FMT-CONST \*\/(?:\n\s*)?(.*?)const/$1\{const\}/gms' "$file"
+done < "$tmpfile"
+
+rm "$tmpfile"
 
 if shellcheck --version ; then
     find . -name '*.sh' -print0 | xargs -0 shellcheck

rustfmt.toml

@@ -1 +1,2 @@
 error_on_line_overflow = true
+edition = "2021"