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Richard Smith
1b2ae912fc
Add support for compile-time functions. `eval fn` is analogous to C++ `constexpr`, and is evaluated at compile time when it has compile-time arguments. `musteval fn` is analogous to C++ `consteval`, and requires that its arguments be available at compile time and is always evaluated at compile time. For now we require the modifier to match across redeclarations of the function. The specific modifier syntax here is a placeholder and not yet part of an approved design. Limitations: Only very basic support for evaluation is provided. So far there's no support for mutable state or `if` expressions, but otherwise control flow and passing and returning values should work. Carbon evaluation recursion is modeled by C++ recursion for now, so you can overflow the toolchain stack easily. Functions that use in-place initialization will generally not work yet, as they are modeled as passing a non-compile-time-constant reference to a temporary to the call. Add missing categorization of `name_binding_decl` as `NotExpr` to match other similar declaration instructions like `FunctionDecl`, so that we can uniformly skip over them when they occur within function bodies. Assisted-by: Gemini 3 Pro and Flash via Antigravity