debuginfo: Build scope map for consts

[klutzy]

Fixes #22432.

[rust-highfive]

r? @eddyb

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[klutzy]

cc @michaelwoerister

[klutzy]

This is fixup of #21744: it optimized rustc_trans::trans::expr::trans_into s.t. if expr is ExprPath referring const, get const expr and directly call trans_into. But debuginfo was not updated so it ignored const-ExprPath while building scope_map.

[eddyb]

My bad - but this needs a test, otherwise it could regress again.

[michaelwoerister]

Thanks for the PR. Unfortunately this solution will lead to wrong debug locations if the same constant is referred to more than once within the same function. The scope map uses node IDs as keys and for a constant expression these are always the same, regardless of where it gets expanded to. So later occurences will overwrite scope map entries of former ones.

I already have an idea how to solve this problem differently.

[michaelwoerister]

OK, so my idea is to provide debuginfo::with_debug_loc_override() function that allows to fixate emitted debug locations to a given value for a limited scope. This can then be used in expr::trans_into(), where it allows to set all emitted debug locations to the location of the constant's occurrence. To me this seems like a reasonably clean solution.

@klutzy Let me know if you have a better idea or want to implement my suggestion. Otherwise I'll implement it.

[michaelwoerister]

@klutzy Since I haven't heard from you, I implemented my suggested solution (PR #23074).

[klutzy]

@michaelwoerister I haven't have sufficient time to investigate this further, sorry. Thanks for making #23074!