fix #24968: more friend error message for Self in fn args

[XuefengWu]

fix#24968
report more friendly error message for Self when fn args
copy from #25096
r? @nrc@arielb1

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nrc (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[nrc]

@bors r+ 574a8cd

[nrc]

Thanks for making the changes!

[bors]

⌛ Testing commit 574a8cd with merge 49601db...

[bors]

💔 Test failed - auto-mac-64-opt

[XuefengWu]

@nrc fix a related test.

[alexcrichton]

@bors: r=nrc 7fe60c1

[bors]

⌛ Testing commit 7fe60c1 with merge 0d707d1...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt