Silence unknown prop warnings

text/0000-silence-unknown-prop-warnings.md

@@ -0,0 +1,98 @@
+- Start Date: 2024-01-09
+- RFC PR:
+- Svelte Issue:
+
+# Silence unknown prop warnings
+
+## Summary
+
+Make it possible to silence unknown prop warnings on a per component basis using
+
+```svelte
+<svelte:options silenceUnknownPropWarnings={true} />
+```
+
+## Motivation
+
+As of now, when a parent component renders a child component with props that are not present on the child component
+
+```svelte
+// Parent.svelte
+<script>
+import Child from './Child.svelte'
+</script>
+
+<Child foo="foo" bar="bar" />
+```
+
+```svelte
+// Child.svelte
+<script>
+export let foo;
+</script>
+
+<p>{foo}</p>
+```
+
+a warning is printed to the console
+
+```console
+<Child> was created with unknown prop 'bar'
+```
+
+This is a sensible default behaviour in most cases and this RFC does not intend to change any of this.
+
+However, take for example our use case at [svelte flow](https://svelteflow.dev/) (a library that renders flowgraphs). With our current architecture, it's possible to mount any user-provided Svelte component as a node. These nodes receive information (which is managed by svelte flow) about their current state through props (its position in the flow, its label, if it's selected or not, etc.). It is not mandatory for a user-provided custom component to implement all of these props - some are only needed in special situations.
+
+For example, if we want to render 10 custom node components in svelte flow, where the user-provided child component only implements 5 of the 10 possible passed props, the browser console will be flooded by 50 unknown prop warnings.
+
+This issue [has already been discussed here](https://github.com/sveltejs/svelte/issues/5892#issuecomment-762660755).
+
+AFAIK the only workaround for this is to actually implement all props, and use them to prevent further unused prop warnings.
+
+```svelte
+// Child.svelte
+<script>
+export let foo;
+export let bar;
+bar; // this statement prevents unused prop warnings
+</script>
+
+<p>{foo}</p>
+```
+
+Here is [a more extreme example in the svelte flow documentation.](https://svelteflow.dev/learn/guides/custom-nodes#suppress-unknown-prop-warnings)
+
+## Detailed design
+
+### Technical Background
+
+`<svelte:options />` already exists as way to communicate certain per component configurations to the compiler.
+
+### Implementation
+
+The idea of this RFC is to have the possibility to set this at the top of a component:
+
+```svelte
+<svelte:options silenceUnknownPropWarnings={true} />
+```
+
+which suppresses all unknown prop warnings for possibly unsound calls to children.
+
+Concerning the question: where should prop warnings be silenced? Should the parent silence prop warnings (parent accepts improper use of child components) or should the child silence them (child accepts unproper calls by parent component)?
+
+To keep in line with the original goal of the unknown prop warning, which is to raise awareness to the improper usage of child components, the proposed option should be provided inside the parent component.
+
+As already discussed, having unknown prop warnings as a default is sensible and disabling it globally would defeat its purpose. Having a way to opt-in on specific components (ones that are expected to have unknown props) would be a good way forward.
+
+## Alternatives
+
+Two other possible names for this option include:
+
+```svelte
+<svelte:options suppressUnknownPropWarnings={true} />
+```
+
+```svelte
+<svelte:options disableUnknownPropWarnings={true} />
+```