Tilde not expanded in paths

[bgamari]

Describe the bug
I've noticed a variety of places where cabal-install accepts paths (e.g. on the command-line) yet doesn't expand tilde as is typical.

To Reproduce
Steps to reproduce the behavior:

$ cabal v2-install cabal-install --installdir=~/.cabal/bin
Resolving dependencies...
Up to date
~/.cabal/bin: createDirectory: does not exist (No such file or directory)

I know there have been other cases although I can't think of them off-hand.

Expected behavior
The path ~/.cabal/bin be expanded to $HOME/.cabal/bin.

System information

• NixOS 20.03
• cabal-install 3.2.0.0

[phadej]

We could fix this, but only after #6667. Before it's essentially impossible to get all the cases, as this have to be done in software. Tilde is expanded by shell. An example:

% ghc-pkg list --package-db=~/.cabal/store/ghc-8.6.5/package.db
ghc-pkg: ~/.cabal/store/ghc-8.6.5/package.db: getDirectoryContents:openDirStream: does not exist (No such file or directory)

% ghc-pkg list --package-db=$HOME/.cabal/store/ghc-8.6.5/package.db
/home/phadej/.cabal/store/ghc-8.6.5/package.db
    p-0.1

Until then, drop the equals:

% cabal v2-install cabal-install --installdir ~/.cabal/bin
Resolving dependencies...
Build profile: -w ghc-8.6.5 -O1
In order, the following will be built (use -v for more details):
 - hackage-security-0.6.0.1 (lib) (requires build)
 - cabal-install-3.2.0.0 (exe:cabal) (requires build)
Starting     hackage-security-0.6.0.1 (lib)
Building     hackage-security-0.6.0.1 (lib)
Installing   hackage-security-0.6.0.1 (lib)
Completed    hackage-security-0.6.0.1 (lib)
Starting     cabal-install-3.2.0.0 (all, legacy fallback)
Building     cabal-install-3.2.0.0 (all, legacy fallback)
Installing   cabal-install-3.2.0.0 (all, legacy fallback)
Completed    cabal-install-3.2.0.0 (all, legacy fallback)
Symlinking 'cabal'