Lower operation assembly formats through a shared intermediate representation

[jonathanvdc]

Problem

Operation assembly-format handling is spread across multiple emitters. TryParseEmitter, BuildCustomAssemblySyntaxEmitter, and EmitterHelpers.AppendBodySyntaxFields each switch over the same assembly-format element kinds and duplicate pieces of directive semantics.

The generator should lower operation assembly formats once into a reusable intermediate representation, then emit parse/build/write code mechanically from that lowered form.

Current Hotspots

• src/MLIR.Generators/Emitters/AssemblyFormat/TryParseEmitter.cs
• src/MLIR.Generators/Emitters/AssemblyFormat/BuildCustomAssemblySyntaxEmitter.cs
• src/MLIR.Generators/Emitters/Common/EmitterHelpers.cs: AppendBodySyntaxFields
• src/MLIR.Generators/Emitters/Common/AssemblyFormatTraversal.cs
• src/MLIR.Generators/Emitters/Operations/OperationBodySyntaxEmitter.cs

Proposed Direction

Create a lowering layer that maps ODS assembly-format elements to a shared representation, for example:

public sealed class LoweredFormatElement
{
    public IReadOnlyList<BodySyntaxField> Fields { get; }
    public CodeTemplate Parse { get; }
    public CodeTemplate Build { get; }
    public CodeTemplate Write { get; }
    public CodeTemplate? PresenceCondition { get; }
}

The lowering layer should own directive semantics for literals, variables, attr-dict, prop-dict, regions, operands, successors, type, qualified, results, functional-type, optional groups, and oilists.

Acceptance Criteria

• Assembly-format directive handling is centralized in one lowering layer.
• TryParseEmitter and BuildCustomAssemblySyntaxEmitter no longer independently switch over every directive kind.
• Body syntax field generation consumes the same lowered representation as parse/build emission.
• Unsupported directives are identified during lowering with clear fallback behavior or diagnostics.
• Existing generated operation custom assembly parse/build behavior remains stable.

Suggested Tests

• Generator tests for each supported directive kind.
• Optional group tests with literal, operand, attribute, unit attribute, and region anchors.
• Oilist tests with multiple clauses and repeated-clause rejection.
• Fallback test for an unsupported directive.

Notes

This is the operation-side equivalent of unifying attr/type assembly-format emission. It should make later removal of ad hoc directive hacks much easier.