Can't set document properties based on context

[frozolotl]

Description

There currently is no way to set any document properties using context-dependent functions.
If one tries to define a show-rule as follows, the PDF export will not have a title set. Using context document.title does still output the set title, though.

// NOTE: You probably don't want to actually do such a `show: body => context …` construction.
#show: body => context {
  set document(title: [Title]) if query(<my-label>).len() == 0
  body
}
#context document.title
// uncomment if desired: <my-label>

Proposal

There should probably be a way to define document properties based on context.

If that is not feasible, then context set document() should throw a compiler error.

Reproduction URL

No response

Operating system

No response

Typst version

• I am using the latest version of Typst

[Andrew15-5]

Here is the minified version:

#let template(doc) = {
  // context if state("bundle").get() != true {
  context if query(<sub-document-begin>).len() == 0 {
    set document(title: [Title])
  }
  // set ...
  // set ...
  // set ...
  // show ...
  // show ...
  // show ...
  doc
}
#show: template
Text

[Andrew15-5]

Similar issue: #2326.
(set page() inside context vs. set document() (inside if statement that is) inside context)