Improve OwnedSlice and use it in HIR (take 2)

[petrochenkov]

cc #30095

r? @nrc

[nrc]

lgtm, how did you fix the segfault?

@bors: r+

[bors]

📌 Commit e3da2a9 has been approved by nrc

[petrochenkov]

how did you fix the segfault?

By not doing this (I also reduced the changes to a necessary minimum, just to be sure).

[petrochenkov]

Also, rr wasn't especially more helpful than vanilla gdb, because it is a release-only segfault - it's still not clear what happens even if the error location is known, the amount of available debuginfo is vanishingly small. After spending some time trying to do assembly level debugging I gave up and spent some machine time instead by bisecting the changes and running make check.

[bors]

⌛ Testing commit e3da2a9 with merge 3d15039...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt