141546c355
Previously, we would generate a single struct with the layout of the dataful variant plus an extra field whose name contained the value of the niche (this would only really work for things like `Option<&_>` where we can determine that the `None` case maps to `0` but for enums that have multiple tag only variants, this doesn't work). Now, we generate a union of two structs, one which is the layout of the dataful variant and one which just has a way of reading the discriminant. We also generate an enum which maps the discriminant value to the tag only variants. We also encode information about the range of values which correspond to the dataful variant in the type name and then use natvis to determine which union field we should display to the user. As a result of this change, all niche-layout enums render correctly in WinDbg and Visual Studio!
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc dev guide.