RFC: post-2.4 redesign of v2 --bindir UI

[hvr]

This is a bit of a follow-up to #5837 (which is intended to provide an incremental pain remedy for 2.4.x) which is expected to result in the following situation for v2-install:

We have two bindir "install" paths settings,

• --symlink-bindir=DIR (used when --bindir-method=symlink)
• --bindir=DIR (used when --bindir-method=copy)

This is mostly for historical reasons as cabal has had these two configuration properties for about a decade.

However, it doesn't seem to make much sense in the nix-style paradigm to have two separate folders; instead I suggest to redesign this for cabal 3.0 to

1. deprecate --symlink-bindir=DIR
2. Use a single --bindir=DIR configuration field
3. Use --bindir-method={copy,symlink,auto} to control how executables are "installed" into the folder denoted by --bindir

This would IMO be a less confusing UI as you need to be careful which bindir field you set; there's now only one!

---

So e.g. if you're preparing some binary dist for an executable (for which you know it is relocatable as it doesn't have any ties to the nix-style store), you can just say

cabal install --bindir=./bindist/bin --bindir-method=copy  mypackage

and if you instead want to prepare some local bin-folder with symlinks to some build tools, you can instead say

cabal install --bindir=${HOME}/buildtools-bin --bindir-method=symlink alex happy

and if you use the auto method (i.e. try to use symlinks and fallback to copy if not possible), now you can say

cabal install --bindir=${HOME}/buildtools-bin --bindir-method=auto alex happy

With the UI as implemented for #5837 for cabal 2.4 however, you'd have to use the following incantations respectively:

cabal v2-install --bindir=./bindist/bin --bindir-method=copy  mypackage

cabal v2-install --symlink-bindir=${HOME}/buildtools-bin --bindir-method=symlink alex happy

# NB: we need to set both bindir locations!
cabal v2-install --bindir=${HOME}/buildtools-bin --symlink-bindir=${HOME}/buildtools-bin --bindir-method=auto alex happy

[bgamari]

Sounds right to me.

[fgaz]

+1.

This requires:

• a bindir in NewInstallFlags, the proper place to put it
• NewInstallFlags in SavedConfig, so bindir remains configurable in ~/.cabal/config