Improve make::struct_ field_list whitespace #37039

Job	Run time
analysis-stats	11m 18s
changes	4s
proc-macro-srv	1m 43s
Rust (windows-latest)	7m 41s
Rust (macos-latest)	5m 42s
Rust (ubuntu-latest)	5m 28s
rustfmt	12s
miri	37s
Rust Cross (powerpc-unknown-linux-gnu)	1m 19s
Typo Check	6s
Rust Cross (x86_64-unknown-linux-musl)	1m 19s
Rust Cross (wasm32-unknown-unknown)	43s
TypeScript	0s
cancel-if-matrix-failed	2s
conclusion	3s
	36m 17s

Provide feedback