[kixelated]

Problem

A &dyn Debug or &dyn Display, and since #445 a Box, Arc, or Rc of one, translate as a function writing the value, but a &mut dyn Debug or &mut dyn Display was refused as "a coercion to &mut dyn Debug, which is not supported yet". The trait objects quest names it. moq builds none; this is std coverage the quest calls for.

Approach

A &mut dyn Debug or &mut dyn Display is the same function a &dyn one is. Debug::fmt and Display::fmt take &self, so nothing writes through the borrow. A value JS copies is read through its rt.Place as the function writes, not as it is made, so it shows what the place holds then. The change is one line: fmt_object accepts either mutability of reference.

Tests: fmt::objects in the lang fixture's differential test against native wasm32 now also covers:

• A Vec<&mut dyn Debug> of a number and a String that JS copies, the crate's type, a Box<dyn Debug>, and a tuple holding the crate's type over a Cell. Each is written with {:?} and {:#?}.
• The Cell set while the borrows live, then written again through a reborrowed &mut dyn Debug and a &dyn Debug coerced from it.
• &mut dyn Display of a number written after the number changes, of the crate's type centered, and of a String after a push.
• The box's drop.

Census at the moq pin, before → after: unchanged (692 / 186 / 119 for moq-net / model / kio), since moq builds none.

Impact

• The translator's output: &mut dyn Debug and &mut dyn Display translate instead of failing.
• No breaking change.

Alternatives

• Reading a value JS copies out of its place as the object is made: the same output, since the exclusive borrow lets nothing write the place meanwhile, but a copy per object where reading as it writes needs none.

Follow-ups

• Debug of a Cell is refused, as Debug of a RefCell is (quest/m3/ts-refcell-debug.md, from #447). Met writing the fixture; the fixture writes the cell through the crate's own wrapper.
• String + &str is an "unsupported call", met in a scratch test: new quest quest/m3/ts-string-add.md.

Review

Codex gpt-6.1-sol at medium effort, via quest review, round 1 on head 5e7bb518e42eb51a5681431456e74e5d41bb7454: no findings. Head bfb9718c adds only the ts-string-add quest; just check passes.

🤖 Generated with Claude Code

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