Add unwrap_or api for bxl.Result

Summary: RFC: https://fb.workplace.com/groups/buck2dev/permalink/3899367573684619/

Reviewed By: JakobDegen

Differential Revision: D68863757

fbshipit-source-id: 00ee1c5550364342d693708a5a3f54af820db69f

app/buck2_bxl/src/bxl/starlark_defs/result.rs

@@ -144,6 +144,17 @@ fn result_methods(builder: &mut MethodsBuilder) {
         }
     }
 
+    /// If the result is an `Ok`, return the inner value, otherwise return the default
+    fn unwrap_or<'v>(
+        this: ValueTypedComplex<'v, StarlarkResult<'v>>,
+        #[starlark(require = pos)] default: Value<'v>,
+    ) -> starlark::Result<Value<'v>> {
+        match this.unpack() {
+            either::Either::Left(x) => Ok(x.unwrap_or(default)),
+            either::Either::Right(x) => Ok(x.unwrap_or(default)),
+        }
+    }
+
     /// Unwrap the error, returning the inner error if the result is `Err`.
     /// If the result is an `Ok`, it will fail
     fn unwrap_err<'v>(
@@ -180,6 +191,13 @@ impl<'v, V: ValueLike<'v>> StarlarkResultGen<V> {
         }
     }
 
+    fn unwrap_or(&self, default: Value<'v>) -> Value<'v> {
+        match self {
+            StarlarkResultGen::Ok(val) => val.to_value(),
+            StarlarkResultGen::Err(_) => default,
+        }
+    }
+
     fn unwrap_err(&self) -> buck2_error::Result<StarlarkError> {
         match self {
             StarlarkResultGen::Ok(val) => {

tests/core/docs/test_builtin_docs_data/buck2-golden-docs/bxl/Result.md

@@ -4,3 +4,4 @@
 ## Result.is\_ok
 ## Result.unwrap
 ## Result.unwrap\_err
+## Result.unwrap\_or