I'm again at a framework where files may be contributed to the app's installation directory.

The debug_assertions constant may be used to detect cargo build --release, but I wanted some consensus on what is best to do here.

In my framework the user could do this at the Cargo manifest:

[package.metadata.fl]
files = [
    { from = "assets/**/*.webp", to = "img" }
]

This would make it so the source path ~/assets/flowers/red.webp may be read during runtime using the app://img/flowers/red.webp URL (and the actual file will be exported at that path as well).

When debugging (e.g. cargo running an app), I want to avoid exporting these files, therefore bootstrapping the application with the (from, to) entries so I do some magic when resolving app: URLs for the user. (In the HTML5 target, app: is resolved via HTTP requests, while in native targets I'd use std::fs and the executable path).

What leaves me confused is whether I detect cargo run solely through debug_assertions or if there's something more reliable to use (because, then, even a simple cargo build can't be used correctly as I'd not export files when debugging with the debug_assertions approach).

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