Rollup merge of #140181 - nnethercote:rm-underscore_const, r=compiler-errors

Remove `synstructure::Structure::underscore_const` calls.

The `synstructure` docs say "This method is a no-op, underscore consts are used by default now." The behaviour change occurred going from `synstructure` version 0.13.0 to 0.13.1.

r? ``@SparrowLii``

Author: matthiaskrgr