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cmse: fix 'region variables should not be hashed' tracking issue: https://github.com/rust-lang/rust/issues/81391 fixes https://github.com/rust-lang/rust/issues/131639 Some background: the `cmse-nonsecure-call` calling convention is used for a call from "secure" to "non-secure" code. To make sure that "non-secure" cannot read any secrets, restrictions are put on the signatures of functions with this calling convention: they can only use 4 arguments for passing arguments, and one register for passing a result. No arguments are passed via the stack, and all other registers are cleared before the call. We check during `hir_ty_lowering` that the signature follows these rules. We do that by determining and then inspecting the layout of the type. That works well overall, but can run into asserts when the type itself is ill-formed. This PR fixes one such case. I believe that the fix here, just erasing the regions, is the right shape, but there may be some nuance that I'm missing. r? types
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.