Wishlist: Make cache key elements explicit

Type: Feature Task Name: CacheBeta@1

Environment

Azure Pipelines - but this is a feature wishlist item, not a bug, Hosted agent

Issue Description

The current method of specifying a cache key has several issues.

First, it’s very easy to generate invalid YAML with it, just by wanting to put literal strings first:

key: "v1" | some-file.txt

This is invalid because parsers see the double quote and assume that they value will be entirely quoted. There is a disgusting workaround, of course:

key: '"v1" | some-file.txt'

But this is a real eyesore!

The other issue is that we have to guess whether the task will interpret any given element as a path or not.

I propose a more explicit system for specifying the key where each key element is an array element:

key:
  - literal: v1
  - path: some-file.txt
  - literal: some-file-name

This is entirely unambiguous and doesn’t require any odd double-quoting. As a bonus, this makes it trivial to introduce new types of key elements in the future if that is ever needed.

[autarch]

You will pry Emacs from my cold, dead hands, which will be curled into some unnatural Ctrl-Alt-Shift-$Letter shape.

[samuelkoppes]

We do not have a good way to implement this in YAML without introducing further syntactic complexity.