Now I get the following, as expected:

  $ EMACS_GENERATE_DYNVARS=1 emacs -Q -batch -f batch-byte-compile foo.el
  Generating /tmp/foo.elc.dynvars

  $ cat foo.elc.dynvars
  (foo-y . "/tmp/foo.el")
  (foo-x . "/tmp/foo.el")

  $ EMACS_DYNVARS_FILE=foo.elc.dynvars emacs -Q -batch -f batch-byte-compile bar.el

  In bar-x:
  bar.el:2:2: Warning: ‘foo-x’ lexically bound in lambda here but declared dynamic in: /tmp/foo.el
  bar.el:3:8: Warning: ‘foo-y’ lexically bound in let here but declared dynamic in: /tmp/foo.el

WDYT?

Side question: if I replace

  (let ((foo-y 1))
    (ignore foo-y)
    (setq foo-y 2))

with

  (let ((foo-y 1))
    (ignore foo-y)
    (bar-x foo-y))

in bar.el (i.e. foo-y is only read, never set), the .dynvars warning
about foo-y disappears.  I assume this is due to an earlier cconv /
constant propagation pass, is that right?  If so, I'm curious
whether/how the .dynvars feature could be adapted to cope, e.g. by
taking place at an earlier stage.

Thanks,
-- 
Basil

In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.18.0, Xaw3d scroll bars) of 2023-10-18 built on tia
Repository revision: 06fc5c24170b820939d3d51071b2957354edcb65
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101008
System Description: Debian GNU/Linux trixie/sid

Configured using:
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 --enable-checking=structs --with-file-notification=yes
 --with-x-toolkit=lucid --with-x'

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