Support arrays in `mono_reflection_create_custom_attr_data_args_noalloc`

[elinor-fung]

mono_reflection_create_custom_attr_data_args_noalloc does not currently support arrays. If a custom attribute has an array field/property, the non-AOT-compatible version that creates managed objects (mono_reflection_create_custom_attr_data_args) needs to be used instead. The _noalloc version should be updated to allow for improved AOT support.

When this is done, reading of UnmanagedCallConvAttribute should use this functionality.

cc @lambdageek @CoffeeFlux

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Issue Details

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mono_reflection_create_custom_attr_data_args_noalloc does not currently support arrays. If a custom attribute has an array field/property, the non-AOT-compatible version that creates managed objects (mono_reflection_create_custom_attr_data_args) needs to be used instead. The _noalloc version should be updated to allow for improved AOT support.

When this is done, reading of UnmanagedCallConvAttribute should use this functionality.

cc @lambdageek @CoffeeFlux

Author:	elinor-fung
Assignees:	-
Labels:	area-VM-meta-mono, runtime-mono
Milestone:	-

[lambdageek]

This is going to be slightly painful, but doable - I wonder if we should actually make _noalloc default and then make a wrapper that converts an unmanaged version of the data into managed objects.

Something like

typedef struct MonoCustomAttrValueArray MonoCustomAttrValueArray;

typedef struct MonoCustomAttrValue {
  union {
    gpointer prim; /* int/enum/MonoType/string */
    MonoCustomAttrValueArray *array_val;
  };
} MonoCustomAttrValue;

struct MonoCustomAttrValueArray {
  int len;
 MonoCustomAttrValue values[MONO_ZERO_LEN_ARRAY];
};

/* returns a prim value directly, or a MonoCustomAttrValueArray* for arrays.  Allocated from allocator, or g_malloc if allocator is NULL. */
gpointer load_cattr_value (MonoMemoryManager *allocator, MonoType *type, char *p, char *boundp, char **endp, MonoError *error);

/* convert a native custom attribute value into a managed representation. returns FALSE if the value is a primitive and doesn't need conversion, or TRUE if conversion was performed and *out_obj is the result */
gboolean convert_cattr_value_to_object (gpointer val, MonoType *type, MonoObject **out_obj);

Then mono_reflection_create_custom_attr_data_args_noalloc can be rebuilt on top of this.

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I kind of don't like "shove everything into a gpointer" but it's probably hard to clean up. the general approach of using the MonoType* of the overall expected value to describe how to analyze the value is ok, though.

[lambdageek]

This affects the CallConv property of [UnmanagedCallersOnly] which makes it difficult to use function pointers with Mono AOT on Windows (where we need to often specify a non-default CallConv) /cc @lateralusX

[lambdageek]

We should do a more targeted solution since this is primarily affecting the AOT compiler when we're looking for well-known attributes like UnmanagedCallersOnly where we know it's always a Type[] value on a property.

So we could add a load cattr value function as I suggest above, but we wouldn't have to update all existing callers, and we could focus on just the attribute value types that we expect to see there.