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802343fa47
Remove unsized arg handling in `ArgAbiBuilderMethods::store_fn_arg` implementations ... since it is unreachable and would ICE anyway. These branches are unreachable with how `store_fn_arg` is currently used (where it is called, unsized arguments are either: 1. not (yet) supported, or 2. handled differently)[^1], and even if they were reachable, they would ICE anyway, since they call [`OperandValue::store`](https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_codegen_ssa/mir/operand.rs.html#855-861), which calls [`OperandValue::store_with_flags`](https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_codegen_ssa/mir/operand.rs.html#887-926) which [panics on any unsized layout](https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_codegen_ssa/mir/operand.rs.html#900-903). Also updates the `bug!` message in `store_arg` to not suggest `store_fn_arg` for unsized args. [^1]: `store_fn_arg` is only nontrivially[^2] called in `compiler/rustc_codegen_ssa/src/mir/mod.rs` for: Line 428 `extern "rust-call"` tuple (un)splitting, which does not support unsized arguments, Line 496 which is only for sized `PassMode::Indirect` (`meta_attrs: None`) arguments, and Line 521 which is only for non-`PassMode::Indirect` arguments which can never be unsized. [^2]: `<Bx as ArgAbiBuilderMethods>::store_fn_arg` is what is actually called, but codegen_llvm and codegen_gcc's builders both delegate to their own `codegen_crate::ArgAbiExt::store_fn_arg`, which contain the actual implementations that are changed in this PR.
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.