[MLIR][docs] Mention declarePromisedInterface in Interfaces doc

[frederik-h]

No description provided.

[llvmbot]

@llvm/pr-subscribers-mlir

Author: Frederik Harwath (frederik-h)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/88689.diff

1 Files Affected:

• (modified) mlir/docs/Interfaces.md (+13)

diff --git a/mlir/docs/Interfaces.md b/mlir/docs/Interfaces.md
index 536e7613e50936..4fb6f8a88bfa8d 100644
--- a/mlir/docs/Interfaces.md
+++ b/mlir/docs/Interfaces.md
@@ -299,6 +299,19 @@ owner of the dialect containing the object nor the owner of the interface are
 aware of an interface implementation, which can lead to duplicate or
 diverging implementations.
 
+Forgetting to register an external model can lead to bugs which are hard
+to track down. The `declarePromisedInterface` function can be used
+to declare that an external model implementation for an operation
+must eventually be provided.
+```
+  void MyDialect::initialize() {
+    declarePromisedInterface<SomeInterface, SomeOp>();
+     ...
+  }
+```
+Now attempting to use the interface, e.g in a cast, without a prior
+registration of the external model will lead to a runtime error.
+
 #### Dialect Fallback for OpInterface
 
 Some dialects have an open ecosystem and don't register all of the possible

[ftynse]

Would be helpful to list what the message looks like so quick googling references to this page.

[frederik-h]

Would be helpful to list what the message looks like so quick googling references to this page.

Thanks for the review. I have now added an example of the error message.

[joker-eph]

Thanks!

[joker-eph]

Can you please rewrap the doc to 80 columns though?

In general I use mdformat --wrap 80 for this.

[frederik-h]

Can you please rewrap the doc to 80 columns though?

In general I use mdformat --wrap 80 for this.

I have only rewrapped the new changes introduced by this PR since applying it to the whole document would introduce lots of unrelated changes, i.e. the rest of the document is not formatted in that way yet.