Compose: giving tmpfs options with volumes syntax

[jouve]

Description of the issue

I see 2 ways of mounting a tmpfs in docker-compose:

tmpfs:
  - /my/mount/point

volumes:
  - type: tmpfs
    target: /my/mount/point

Now if I want to give options for the mount point, I can do it with the first syntax:

tmpfs:
  - /my/mount/point:exec,size=1G

but I can't do it with the other syntax (or it's not documented):

      - type: tmpfs
        target: /var/tmp/portage
        ???

Context information (for bug reports)

Output of "docker-compose version"

docker-compose version 1.21.0, build 5920eb0
docker-py version: 3.2.1
CPython version: 3.6.3
OpenSSL version: OpenSSL 1.0.2g  1 Mar 2016

Output of "docker version"

Client:
 Version:       18.04.0-ce
 API version:   1.37
 Go version:    go1.9.4
 Git commit:    3d479c0
 Built: Tue Apr 10 18:21:20 2018
 OS/Arch:       linux/amd64
 Experimental:  false
 Orchestrator:  swarm

Server:
 Engine:
  Version:      18.04.0-ce
  API version:  1.37 (minimum version 1.12)
  Go version:   go1.9.4
  Git commit:   3d479c0
  Built:        Tue Apr 10 18:19:27 2018
  OS/Arch:      linux/amd64
  Experimental: true

Output of "docker-compose config"

services:
  toto:
    image: alpine:3.7
    tmpfs:
    - /var/tmp/portage:exec,size=1G
version: '2.3

[shin-]

https://docs.docker.com/compose/compose-file/compose-file-v2/#long-syntax

[jouve]

@shin- Then how do you give "exec" ?

Is there a hidden/undocumented keyword like for size ?

 - type: tmpfs
   target: /target/dir
  tempfs:
    size: 1G
    ??? exec ???

or is it a limitation a the mount syntax https://docs.docker.com/storage/tmpfs/#specify-tmpfs-options ?