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 +13,53 @@ 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
+
+    # Turn all braced macro `foo! { /* ... */ }` invocations into
+    # `fn foo_fmt_tmp() { /* ... */ }`.
+    perl -pi -e 's/(?!macro_rules)\b(\w+)!\s*\{/fn $1_fmt_tmp() {/g' "$file"
+
+    # Replace `if #[cfg(...)]` within `cfg_if` with `if cfg_tmp!([...])` which
+    # `rustfmt` will format. We put brackets within the parens so it is easy to
+    # match (trying to match 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 but aren't
+    # valid syntax. 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"
+
+    # Format the file. We need to invoke `rustfmt` directly since `cargo fmt`
+    # can't figure out the module tree with the hacks in place.
+    failed=false
+    rustfmt --config-path rustfmt.toml "$file" ${check:+"$check"} || failed=true
+
+    # Restore all changes to the files.
+    perl -pi -e 's/fn (\w+)_fmt_tmp\(\)/$1!/g' "$file"
+    perl -pi -0777 -e 's/cfg_tmp!\(\[(.*?)\]\)/#[cfg($1)]/gms' "$file"
+    perl -pi -0777 -e 's/\/\* FMT-CONST \*\/(?:\n\s*)?(.*?)const/$1\{const\}/gms' "$file"
+
+    # Defer emitting the failure until after the files get reset
+    if [ "$failed" != "false" ]; then
+        echo "Formatting failed"
+        exit 1
+    fi
+done < "$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"