transformations: (mlir-opt) separate mlir-opt pass errors (#3687)

We currently have two statements that can raise an error when calling
out to `mlir-opt`, one for errors the executable raises, the other for
parsing in xDSL, these should be reported separately.

tests/filecheck/mlir-conversion/with-mlir/mlir_opt_fail.mlir

@@ -1,4 +1,4 @@
-// RUN: xdsl-opt %s -p mlir-opt{arguments='--hello','--mlir-print-op-generic'} --print-op-generic --verify-diagnostics | filecheck %s
+// RUN: xdsl-opt %s -p mlir-opt{arguments='--hello','--mlir-print-op-generic'} --print-op-generic --verify-diagnostics | filecheck %s --check-prefix=CHECK-PARSING
 // RUN: xdsl-opt %s -p mlir-opt[this-probably-will-never-be-an-MLIR-pass-name] --print-op-generic --verify-diagnostics | filecheck %s
 // RUN: xdsl-opt %s -p mlir-opt{executable='"false"'} --print-op-generic --verify-diagnostics | filecheck %s
 
@@ -10,4 +10,5 @@
 }) : () -> ()
 
 
-// CHECK:         Error executing mlir-opt pass
+// CHECK-PARSING:         Error parsing mlir-opt pass output
+// CHECK:                 Error executing mlir-opt pass

xdsl/transforms/mlir_opt.py

@@ -9,7 +9,7 @@
 from xdsl.passes import ModulePass
 from xdsl.printer import Printer
 from xdsl.rewriter import Rewriter
-from xdsl.utils.exceptions import DiagnosticException
+from xdsl.utils.exceptions import DiagnosticException, ParseError
 
 
 @dataclass(frozen=True)
@@ -44,20 +44,18 @@ def apply(self, ctx: MLContext, op: ModuleOp) -> None:
 
         try:
             completed_process.check_returncode()
+        except subprocess.CalledProcessError as e:
+            raise DiagnosticException("Error executing mlir-opt pass") from e
 
-            # Get the stdout output
-            stdout_output = completed_process.stdout
-
-            parser = Parser(ctx, stdout_output)
+        # Get the stdout output
+        stdout_output = completed_process.stdout
+        parser = Parser(ctx, stdout_output)
 
+        try:
             new_module = parser.parse_module()
+        except ParseError as e:
+            raise DiagnosticException("Error parsing mlir-opt pass output") from e
 
-            op.detach_region(op.body)
-            op.add_region(
-                Rewriter().move_region_contents_to_new_regions(new_module.body)
-            )
-            op.attributes = new_module.attributes
-        except Exception as e:
-            raise DiagnosticException(
-                "Error executing mlir-opt pass:", completed_process.stderr
-            ) from e
+        op.detach_region(op.body)
+        op.add_region(Rewriter().move_region_contents_to_new_regions(new_module.body))
+        op.attributes = new_module.attributes