Could this have introduced the following double quoting in the Mercurial wrapper:

$ less $(command -v hg)                 
#! /nix/store/65l6hr8snf4v823f974k97jc65i7bhvf-bash-4.4-p12/bin/bash -e
export TK_LIBRARY='"/nix/store/gwzzcr61sk5arbi9vzh2c3pnz4k2z930-tk-8.6.6/lib/tk8.6"'
export HG='"/nix/store/6ji054br1di9iwm4md73gr8p12lvp60y-mercurial-4.3.2/bin/hg"'
export PATH='"/nix/store/gwzzcr61sk5arbi9vzh2c3pnz4k2z930-tk-8.6.6/bin"'${PATH:+':'}$PATH
exec -a "$0" "/nix/store/6ji054br1di9iwm4md73gr8p12lvp60y-mercurial-4.3.2/bin/.hg-wrapped"  "${extraFlagsArray[@]}" "$@"

Note the export HG='"…"' pattern of double quoting. This makes the variables point at the wrong things—I ran into this because my editor script for committing uses "$HG".

Out of curiosity I ran a search for e.g. ^export\s+\w+='" (as well as one for "") in my store and there’s actually quite a couple of hits. Is there an outstanding issue for this?

No, this only changes $out/nix-support/propagated-build-inputs.

@mickk-on-cpp Don't think I've seen extra quoting like this before for wrappers mercurial/default.nix. Should be safe to remove if you look at other usages of wrapProgram.

Sorry for the noise, I have opened #33625.

No problem. 😃