nbnoexcept.diff

diff --git a/mlir/include/mlir/Bindings/Python/NanobindAdaptors.h b/mlir/include/mlir/Bindings/Python/NanobindAdaptors.h
index 2dd35c097c796..8dcf91e5806bd 100644
--- a/mlir/include/mlir/Bindings/Python/NanobindAdaptors.h
+++ b/mlir/include/mlir/Bindings/Python/NanobindAdaptors.h
@@ -20,6 +20,7 @@
 #define MLIR_BINDINGS_PYTHON_NANOBINDADAPTORS_H
 
 #include <cstdint>
+#include <optional>
 
 #include "mlir-c/Diagnostics.h"
 #include "mlir-c/IR.h"
@@ -43,18 +44,14 @@ namespace detail {
 /// with a raw handle (unowned). The returned object's lifetime may not extend
 /// beyond the apiObject handle without explicitly having its refcount increased
 /// (i.e. on return).
-static nanobind::object mlirApiObjectToCapsule(nanobind::handle apiObject) {
+static std::optional<nanobind::object>
+mlirApiObjectToCapsule(nanobind::handle apiObject) {
   if (PyCapsule_CheckExact(apiObject.ptr()))
     return nanobind::borrow<nanobind::object>(apiObject);
   nanobind::object api =
       nanobind::getattr(apiObject, MLIR_PYTHON_CAPI_PTR_ATTR, nanobind::none());
-  if (api.is_none()) {
-    std::string repr = nanobind::cast<std::string>(nanobind::repr(apiObject));
-    throw nanobind::type_error(
-        (llvm::Twine("Expected an MLIR object (got ") + repr + ").")
-            .str()
-            .c_str());
-  }
+  if (api.is_none())
+    return {};
   return api;
 }
 
@@ -67,12 +64,9 @@ static nanobind::object mlirApiObjectToCapsule(nanobind::handle apiObject) {
 template <>
 struct type_caster<MlirAffineMap> {
   NB_TYPE_CASTER(MlirAffineMap, const_name("MlirAffineMap"))
-  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToAffineMap(capsule.ptr());
-    if (mlirAffineMapIsNull(value)) {
-      return false;
-    }
+  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToAffineMap(capsule->ptr());
     return !mlirAffineMapIsNull(value);
   }
   static handle from_cpp(MlirAffineMap v, rv_policy,
@@ -90,9 +84,9 @@ struct type_caster<MlirAffineMap> {
 template <>
 struct type_caster<MlirAttribute> {
   NB_TYPE_CASTER(MlirAttribute, const_name("MlirAttribute"))
-  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToAttribute(capsule.ptr());
+  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToAttribute(capsule->ptr());
     return !mlirAttributeIsNull(value);
   }
   static handle from_cpp(MlirAttribute v, rv_policy,
@@ -111,9 +105,9 @@ struct type_caster<MlirAttribute> {
 template <>
 struct type_caster<MlirBlock> {
   NB_TYPE_CASTER(MlirBlock, const_name("MlirBlock"))
-  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToBlock(capsule.ptr());
+  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToBlock(capsule->ptr());
     return !mlirBlockIsNull(value);
   }
 };
@@ -122,7 +116,7 @@ struct type_caster<MlirBlock> {
 template <>
 struct type_caster<MlirContext> {
   NB_TYPE_CASTER(MlirContext, const_name("MlirContext"))
-  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) {
+  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
     if (src.is_none()) {
       // Gets the current thread-bound context.
       // TODO: This raises an error of "No current context" currently.
@@ -132,8 +126,8 @@ struct type_caster<MlirContext> {
                 .attr("Context")
                 .attr("current");
     }
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToContext(capsule.ptr());
+    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
+    value = mlirPythonCapsuleToContext(capsule->ptr());
     return !mlirContextIsNull(value);
   }
 };
@@ -142,9 +136,9 @@ struct type_caster<MlirContext> {
 template <>
 struct type_caster<MlirDialectRegistry> {
   NB_TYPE_CASTER(MlirDialectRegistry, const_name("MlirDialectRegistry"))
-  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToDialectRegistry(capsule.ptr());
+  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToDialectRegistry(capsule->ptr());
     return !mlirDialectRegistryIsNull(value);
   }
   static handle from_cpp(MlirDialectRegistry v, rv_policy,
@@ -162,15 +156,15 @@ struct type_caster<MlirDialectRegistry> {
 template <>
 struct type_caster<MlirLocation> {
   NB_TYPE_CASTER(MlirLocation, const_name("MlirLocation"))
-  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) {
+  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
     if (src.is_none()) {
       // Gets the current thread-bound context.
       src = nanobind::module_::import_(MAKE_MLIR_PYTHON_QUALNAME("ir"))
                 .attr("Location")
                 .attr("current");
     }
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToLocation(capsule.ptr());
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToLocation(capsule->ptr());
     return !mlirLocationIsNull(value);
   }
   static handle from_cpp(MlirLocation v, rv_policy,
@@ -188,9 +182,9 @@ struct type_caster<MlirLocation> {
 template <>
 struct type_caster<MlirModule> {
   NB_TYPE_CASTER(MlirModule, const_name("MlirModule"))
-  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToModule(capsule.ptr());
+  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToModule(capsule->ptr());
     return !mlirModuleIsNull(value);
   }
   static handle from_cpp(MlirModule v, rv_policy,
@@ -209,12 +203,13 @@ template <>
 struct type_caster<MlirFrozenRewritePatternSet> {
   NB_TYPE_CASTER(MlirFrozenRewritePatternSet,
                  const_name("MlirFrozenRewritePatternSet"))
-  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToFrozenRewritePatternSet(capsule.ptr());
+  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToFrozenRewritePatternSet(capsule->ptr());
     return value.ptr != nullptr;
   }
-  static handle from_cpp(MlirFrozenRewritePatternSet v, rv_policy, handle) {
+  static handle from_cpp(MlirFrozenRewritePatternSet v, rv_policy,
+                         handle) noexcept {
     nanobind::object capsule = nanobind::steal<nanobind::object>(
         mlirPythonFrozenRewritePatternSetToCapsule(v));
     return nanobind::module_::import_(MAKE_MLIR_PYTHON_QUALNAME("rewrite"))
@@ -228,9 +223,9 @@ struct type_caster<MlirFrozenRewritePatternSet> {
 template <>
 struct type_caster<MlirOperation> {
   NB_TYPE_CASTER(MlirOperation, const_name("MlirOperation"))
-  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToOperation(capsule.ptr());
+  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToOperation(capsule->ptr());
     return !mlirOperationIsNull(value);
   }
   static handle from_cpp(MlirOperation v, rv_policy,
@@ -250,9 +245,9 @@ struct type_caster<MlirOperation> {
 template <>
 struct type_caster<MlirValue> {
   NB_TYPE_CASTER(MlirValue, const_name("MlirValue"))
-  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToValue(capsule.ptr());
+  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToValue(capsule->ptr());
     return !mlirValueIsNull(value);
   }
   static handle from_cpp(MlirValue v, rv_policy,
@@ -273,9 +268,9 @@ struct type_caster<MlirValue> {
 template <>
 struct type_caster<MlirPassManager> {
   NB_TYPE_CASTER(MlirPassManager, const_name("MlirPassManager"))
-  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToPassManager(capsule.ptr());
+  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToPassManager(capsule->ptr());
     return !mlirPassManagerIsNull(value);
   }
 };
@@ -284,9 +279,9 @@ struct type_caster<MlirPassManager> {
 template <>
 struct type_caster<MlirTypeID> {
   NB_TYPE_CASTER(MlirTypeID, const_name("MlirTypeID"))
-  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToTypeID(capsule.ptr());
+  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToTypeID(capsule->ptr());
     return !mlirTypeIDIsNull(value);
   }
   static handle from_cpp(MlirTypeID v, rv_policy,
@@ -306,9 +301,9 @@ struct type_caster<MlirTypeID> {
 template <>
 struct type_caster<MlirType> {
   NB_TYPE_CASTER(MlirType, const_name("MlirType"))
-  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToType(capsule.ptr());
+  bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToType(capsule->ptr());
     return !mlirTypeIsNull(value);
   }
   static handle from_cpp(MlirType t, rv_policy,
@@ -462,9 +457,10 @@ class mlir_attribute_subclass : public pure_subclass {
     nanobind::object newCf = nanobind::cpp_function(
         [superCls, isaFunction, captureTypeName](
             nanobind::object cls, nanobind::object otherAttribute) {
-          MlirAttribute rawAttribute =
-              nanobind::cast<MlirAttribute>(otherAttribute);
-          if (!isaFunction(rawAttribute)) {
+          MlirAttribute rawAttribute;
+          if (!nanobind::try_cast<MlirAttribute>(otherAttribute,
+                                                 rawAttribute) ||
+              !isaFunction(rawAttribute)) {
             auto origRepr =
                 nanobind::cast<std::string>(nanobind::repr(otherAttribute));
             throw std::invalid_argument(
@@ -543,8 +539,9 @@ class mlir_type_subclass : public pure_subclass {
     nanobind::object newCf = nanobind::cpp_function(
         [superCls, isaFunction, captureTypeName](nanobind::object cls,
                                                  nanobind::object otherType) {
-          MlirType rawType = nanobind::cast<MlirType>(otherType);
-          if (!isaFunction(rawType)) {
+          MlirType rawType;
+          if (!nanobind::try_cast<MlirType>(otherType, rawType) ||
+              !isaFunction(rawType)) {
             auto origRepr =
                 nanobind::cast<std::string>(nanobind::repr(otherType));
             throw std::invalid_argument((llvm::Twine("Cannot cast type to ") +
@@ -625,8 +622,9 @@ class mlir_value_subclass : public pure_subclass {
     nanobind::object newCf = nanobind::cpp_function(
         [superCls, isaFunction, captureValueName](nanobind::object cls,
                                                   nanobind::object otherValue) {
-          MlirValue rawValue = nanobind::cast<MlirValue>(otherValue);
-          if (!isaFunction(rawValue)) {
+          MlirValue rawValue;
+          if (!nanobind::try_cast<MlirValue>(otherValue, rawValue) ||
+              !isaFunction(rawValue)) {
             auto origRepr =
                 nanobind::cast<std::string>(nanobind::repr(otherValue));
             throw std::invalid_argument((llvm::Twine("Cannot cast value to ") +
diff --git a/mlir/test/python/dialects/python_test.py b/mlir/test/python/dialects/python_test.py
index fd678f8321fd9..694616696a9e2 100644
--- a/mlir/test/python/dialects/python_test.py
+++ b/mlir/test/python/dialects/python_test.py
@@ -361,7 +361,9 @@ def testCustomAttribute():
         try:
             TestAttr(42)
         except TypeError as e:
-            assert "Expected an MLIR object" in str(e)
+            assert "Expected an MLIR object (got 42)" in str(e)
+        except ValueError as e:
+            assert "Cannot cast attribute to TestAttr (from 42)" in str(e)
         else:
             raise
 
@@ -406,7 +408,9 @@ def testCustomType():
         try:
             TestType(42)
         except TypeError as e:
-            assert "Expected an MLIR object" in str(e)
+            assert "Expected an MLIR object (got 42)" in str(e)
+        except ValueError as e:
+            assert "Cannot cast type to TestType (from 42)" in str(e)
         else:
             raise
 
diff --git a/mlir/test/python/lib/PythonTestModuleNanobind.cpp b/mlir/test/python/lib/PythonTestModuleNanobind.cpp
index dd0a9f2b66ea6..108df8da86a7e 100644
--- a/mlir/test/python/lib/PythonTestModuleNanobind.cpp
+++ b/mlir/test/python/lib/PythonTestModuleNanobind.cpp
@@ -113,8 +113,10 @@ NB_MODULE(_mlirPythonTestNanobind, m) {
       .attr(MLIR_PYTHON_CAPI_VALUE_CASTER_REGISTER_ATTR)(
           mlirRankedTensorTypeID)(
           nanobind::cpp_function([valueCls](const nb::object &valueObj) {
-            nb::object capsule = mlirApiObjectToCapsule(valueObj);
-            MlirValue v = mlirPythonCapsuleToValue(capsule.ptr());
+            std::optional<nb::object> capsule =
+                mlirApiObjectToCapsule(valueObj);
+            assert(capsule.has_value() && "capsule is not null");
+            MlirValue v = mlirPythonCapsuleToValue(capsule.value().ptr());
             MlirType t = mlirValueGetType(v);
             // This is hyper-specific in order to exercise/test registering a
             // value caster from cpp (but only for a single test case; see