A new repository uses zlib storage, which doesn't need a requirement

  $ hg init default

  $ cd default

  $ cat .hg/requires

  dotencode

  fncache

  generaldelta

  revlogv1

  store

  $ touch foo

  $ hg -q commit -A -m 'initial commit with a lot of repeated repeated repeated text to trigger compression'

  $ hg debugrevlog -c | grep 0x78

      0x78 (x)  :   1 (100.00%)

      0x78 (x)  : 110 (100.00%)

  $ cd ..

Unknown compression engine to format.compression aborts

  $ hg --config experimental.format.compression=unknown init unknown

  abort: compression engine unknown defined by experimental.format.compression not available

  (run "hg debuginstall" to list available compression engines)

  [255]

A requirement specifying an unknown compression engine results in bail

  $ hg init unknownrequirement

  $ cd unknownrequirement

  $ echo exp-compression-unknown >> .hg/requires

  $ hg log

  abort: repository requires features unknown to this Mercurial: exp-compression-unknown!

  (see https://mercurial-scm.org/wiki/MissingRequirement for more information)

  [255]

  $ cd ..

#if zstd

  $ hg --config experimental.format.compression=zstd init zstd

  $ cd zstd

  $ cat .hg/requires

  dotencode

  exp-compression-zstd

  fncache

  generaldelta

  revlogv1

  store

  $ touch foo

  $ hg -q commit -A -m 'initial commit with a lot of repeated repeated repeated text'

  $ hg debugrevlog -c | grep 0x28

      0x28      :  1 (100.00%)

      0x28      : 98 (100.00%)

  $ cd ..

Specifying a new format.compression on an existing repo won't introduce data

with that engine or a requirement

  $ cd default

  $ touch bar

  $ hg --config experimental.format.compression=zstd -q commit -A -m 'add bar with a lot of repeated repeated repeated text'

  $ cat .hg/requires

  dotencode

  fncache

  generaldelta

  revlogv1

  store

  $ hg debugrevlog -c | grep 0x78

      0x78 (x)  :   2 (100.00%)

      0x78 (x)  : 199 (100.00%)

#endif