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I understood that Drupal/php is compiled to WASM and then run on a WASM runtime. So no WASM inside Drupal, but Drupal run as, and in WASM.
Exactly. The Drupal code is unmodified and runs on top of the PHP interpreter compiled to Wasm [0]. The Drupal PHP code is not compiled to Wasm. - [0] https://github.com/vmware-labs/webassembly-language-runtimes
Is that possible, though? To compile PHP to WASM seems possible¹, but not sure if Drupal can be compiled: drupal does (did? back when I did Drupal it certainly did. I certainly did) some really funky metaprogramming and eval() stuff. Like keeping PHP in the database.