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Don't allocate on SimplifyCfg/Locals/Const on every MIR pass Hey! 👋🏾 This is a first PR attempt to see if I could speed up some rustc internals. Thought process: ```rust pub struct SimplifyCfg { label: String, } ``` in [compiler/src/rustc_mir_transform/simplify.rs](https://github.com/rust-lang/rust/blob/7908a1d65496b88626e4b7c193c81d777005d6f3/compiler/rustc_mir_transform/src/simplify.rs#L39) fires multiple times per MIR analysis. This means that a likely string allocation is happening in each of these runs, which may add up, as they are not being lazily allocated or cached in between the different passes. ...yes, I know that adding a global static array is probably not the future-proof solution, but I wanted to lob this now as a proof of concept to see if it's worth shaving off a few cycles and then making more robust.