<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/54136>54136</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [MLIR] Attribute assemblyFormat doesn't handle `custom`
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          bzcheeseman
      </td>
    </tr>
</table>

<pre>
    I would like the following to work: 
```let assemblyFormat = "`<` custom<snip>($type) custom<snip>($bytes) `>`";``` 

but it fails with the following: 
```error: 'custom' not supported (yet)```

To reproduce, use the following Tablegen Attr def with a dialect of choice (most of the functions necessary won't exist but the TableGen generation should still fail):
```
def MyAttr : AttrDef<MyDialect, "MyAttr"> {
  let mnemonic = "my_attr";

  let parameters = (ins "unsigned":$type, "std::string":$bytes);
  let assemblyFormat = "`<` custom<DecodedEnum>($type) custom<PrintedBytes>($bytes) `>`";
}```
</pre>
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