Add static IntrinsicSizeInBytes(parameters) for parameter-dependent struct sizes

[Copilot]

For dynamically-sized structs where size depends only on parameters (not field values), users currently must instantiate a view to compute size. This adds a static IntrinsicSizeInBytes(params...) function to compute size from parameters alone.

Changes

• header_generator.py:

  • _expression_only_depends_on_parameters() - detects if size expression uses only parameters/constants
  • _StaticParameterFieldRenderer - renders parameter refs as function args instead of view accessors
  • _generate_static_size_from_parameters() - generates the static function when applicable

• generated_code_templates: Added declaration/definition templates for static parameterized size function

• parameters_test.cc: Test coverage for Axes::IntrinsicSizeInBytes()

Example

struct Axes(axes: UInt:4):
  0 [+axes*4]  Axis(AxisType.GENERIC)[]  values

Generated:

namespace Axes {
inline constexpr ::std::size_t IntrinsicSizeInBytes(::std::int32_t axes);
}

// Usage: compute size before allocating
std::size_t size = Axes::IntrinsicSizeInBytes(3);  // Returns 12
std::vector<char> buffer(size);
auto view = MakeAxesView(3, buffer.data(), buffer.size());

Only generated when size depends solely on parameters—structs with constant size or field-dependent size are unchanged.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>static IntrinsicSizeInBytes(parameters) to compute expected size of struct</issue_title>
<issue_description>This issue was copied from the upstream repository google/emboss.

Original issue: google#227
Original state: open, created at: 2025-08-19T18:01:51Z, updated at: 2025-08-19T18:01:51Z, by @studgeek

---

Original description

I have seen a common need where code has to compute a struct size outside of emboss to properly allocate that space. It then applies an emboss view on that allocated space.

For non-dynamically-sized structs the code can use static IntrinsicSizeInBytes to get the needed size. But for dynamically-sized structs (that determine size based on parameters or fields) the code usually has to do its own computation of struct size outside of emboss.

I have been thinking that for dynamically-sized structs where parameters are enough to determine the size, it would be handy if emboss generated a parameterized version of the static IntrinsicSizeInBytes function that can take the struct's parameters and use them to compute the size.

So

struct Bar(x: UInt:8, y: Int:32)

would generate something like

template <typename T>
auto MakeBarView(std::uint8_t x, std::int32_t y, T *data, size_t size);

and also generate the new

static constexpr auto IntrinsicSizeInBytes(std::uint8_t x, std::int32_t y) const;

Above assumes the parameters are enough to compute the size. If not, then no IntrinsicSizeInBytes would be created. I think recognizing these cases is probably one of the trickt parts of any implementation.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes static IntrinsicSizeInBytes(parameters) to compute expected size of struct #4

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