Can you expand on this? What are they? I would've thought rustc outputs only 1 IR?
* AST - abstract syntax tree, created from lexing/parsing
* HIR - high level IR
* THIR - typed high level IR
* MIR - mid-level IR (this is the one that enabled non-lexical lifetimes)
* LLVM IR - given to LLVM to generate the final binary.
If you want to learn more, the rustc dev guide has a bunch of details https://rustc-dev-guide.rust-lang.org/hir.html
> I would've thought rustc outputs only 1 IR?
Yeah, I mean these are in stages. One goes to the next goes to the next, and there's only one output, but that doesn't mean there aren't a bunch in the middle.
Different IRs are good at different things. MIR is based around the control-flow graph, which is also what non-lexical lifetimes are based on, so it's much easier to implement against MIR than it would have been against HIR.
1. AST 2. HIR 3. THIR (side-table lookups) 4. GCC Generic
We basically skip MIR in gccrs.
Its pretty sensible to have other IR's, we have many passes in gccrs simplifying things so the graph of what your working with is simpler and simpler each time.
I mean in GCC for C++ for example they use GENERIC and add a bunch of custom tree-codes such as LAMBDA_EXPR or TEMPLATE stuff for example then they keep substituting etc and finally as part of handing off to GCC middle-end it triggers the gimplification of all of these custom tree codes. So even the C++ front-end you could argue has two IR's.
Please correct me if I'm wrong; I'm not trying to pretend I am any kind of expert here.
> It's used for type checking and error verification; once that's done, it can be translated and handed to the GCC mid-layer.
Sounds the same serial thing to me.