Add struct types to Halide

[alexreinking]

This PR adds struct types to Halide. This allows for inputs and outputs to be defined in terms of popular wire formats, even if they aren't prepared for efficient computation. For example, you can define GGML's q5_0 and q8_0 formats like so:

Type q5_0 = Type::Struct({{"d", Float(16)}, {"qh", UInt(32)}, {"qs", UInt(8), 16}});
Type q8_0 = Type::Struct({{"d", Float(16)}, {"qs", Int(8), 32}});

Then you can write a pipeline that consumes them:

ImageParam x{q4_0_type(), 1, "x"};
ImageParam y{q8_0_type(), 1, "y"};

Var b("b"), k("k"), u("u");
RDom r(0, x.dim(0).extent(), 0, 32, "r");  // block, quant

// Dequantize one 4-bit weight to float: quant k is a nibble of the
// packed byte k % 16 -- its low nibble for k < 16, its high nibble
// otherwise -- biased by -8 to signed [-8, 7], times the block delta.
Expr nib = field(x(b), "qs")[k % 16];  // uint8
Func x_wt("x_wt");
x_wt(k, b) = cast<float>(field(x(b), "d")) * 
             (cast<int32_t>(select(k < 16, nib % 16, nib / 16)) - 8);

// Dequantize one int8 activation to float: quant k is int8 k, times the
// block delta.
Func y_wt("y_wt");
y_wt(k, b) = cast<float>(field(y(b), "d")) * 
             cast<int32_t>(field(y(b), "qs")[k]);

// Dequantize each side and sum every product.
Func qdot{"qdot"};
qdot() = 0.0f;
qdot() += x_wt(r[1], r[0]) * y_wt(r[1], r[0]);

As you can see, struct types support both individual elements, as well as fixed-size array elements. Fields are accessed with a named field(expr, "name") intrinsic. The tests include additional cases for struct-typed fields and output structs.

This adjusts the Type and halide_type_t representations in the following ways:

1. Type: like Handle, a Struct type has a compile-time known field layout.
2. halide_type_t: structs get their own kind, and the reserved field records the number of bytes in the struct.

All layouts are expected to be packed. No padding is inserted around or in between field elements.

Breaking changes

None—it's a new feature.

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)

[codecov]

Codecov Report

❌ Patch coverage is 56.88705% with 313 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.74%. Comparing base (3fcc674) to head (5462034).

Files with missing lines	Patch %	Lines
src/CodeGen_Vulkan_Dev.cpp	0.00%	125 Missing ⚠️
src/IROperator.cpp	59.23%	34 Missing and 19 partials ⚠️
src/LowerStructTypes.cpp	74.27%	35 Missing and 18 partials ⚠️
src/Type.cpp	72.80%	10 Missing and 21 partials ⚠️
src/CodeGen_D3D12Compute_Dev.cpp	0.00%	18 Missing ⚠️
src/Type.h	80.00%	2 Missing and 3 partials ⚠️
src/FuseGPUThreadLoops.cpp	0.00%	3 Missing and 1 partial ⚠️
src/Deserialization.cpp	81.25%	1 Missing and 2 partials ⚠️
src/IROperator.h	57.14%	3 Missing ⚠️
src/IRPrinter.cpp	75.00%	1 Missing and 2 partials ⚠️
... and 9 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9416      +/-   ##
==========================================
- Coverage   70.92%   70.74%   -0.18%     
==========================================
  Files         261      262       +1     
  Lines       79989    80594     +605     
  Branches    19494    19653     +159     
==========================================
+ Hits        56733    57018     +285     
- Misses      17446    17637     +191     
- Partials     5810     5939     +129     

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[abadams]

Nice to haves:

• user-error earlier that lowering if you do something like add two structs
• project field looks through tags so you can say select(x < 10, one_struct, likely(a_different_struct))

Far future:

• The ability to user-define the semantics of various math ops on struct types (e.g. complex multiplication)