[ndeloof]

What this PR does, in one sentence

Adds format.IsShortSyntaxExpressible, which tells whether a volume can be written as a plain [SOURCE:]TARGET[:OPTIONS] string and mean exactly the same thing.

Context

volumes entries come in two syntaxes, and an implementation that hands them to an engine has to know which ones it can send as a string (the Binds of the Docker API) and which need more (the Mount API). Today each implementation guesses it option by option: Docker Compose keeps a list of the options that force the Mount API, and a volume nobody listed (for instance a target containing a colon, docker/compose#12993) silently takes the wrong path or is misread. The answer is a property of the volume, and it is the parser's to give.

What the PR brings

• format.ShortSyntax(v) renders the volume in the short syntax and reports whether it could; format.IsShortSyntaxExpressible(v) is the boolean. A volume is expressible when some spelling of it parses back, with ParseVolume, as the very same volume, so the answer cannot drift from the parser.
• What is not expressible: what the short syntax has no place for (tmpfs, image, subpath, labels, consistency, extensions, a recursive bind, a bind that doesn't create its host path, nocopy on a bind…), and what it would read otherwise (a colon in a source or a target, a bind source that is not a path, a one-letter volume name read as a drive letter). z/Z/propagation on a named volume are expressible, as the parser puts them in the volume's bind.
• Purely additive: nothing in the loader, the model or the schema changes, and the function is not called by compose-go itself. Tests cover each class above, plus the round trip (what it renders parses back to the same volume).

This is deliberately small and independent of the mounts proposal in #979, so the check can be used, and discussed, on its own.

🤖 Generated with Claude Code

[glours]

LGTM