Auto merge of #55986 - cjgillot:issue-45510, r=nikomatsakis

Allow to dispatch fn traits depending on number of parameters

Hello,

By following @eddyb's advise on issue #45510, I managed to have the snippets of code in #45510 and #18952 passing without breaking older diagnostics.

EDIT: the codegen tests breakage I experienced is due to the poor quality of my laptop.

If any kind reviewer has any advice, you are very welcome.