rust/compiler/rustc_mir_transform/src/match_branches.rs, branch beta

mir-opt: Do not transform non-int type in match_branches

2025-05-26T10:15:54+00:00

mir-opt: Use one MirPatch in MatchBranchSimplification

2025-04-20T12:56:41+00:00

Rollup merge of #139042 - compiler-errors:do-not-optimize-switchint, r=saethlin

2025-04-19T19:30:46+00:00

Do not remove trivial `SwitchInt` in analysis MIR This PR ensures that we don't prematurely remove trivial `SwitchInt` terminators which affects both the borrow-checking and runtime semantics (i.e. UB) of the code. Previously the `SimplifyCfg` optimization was removing `SwitchInt` terminators when they was "trivial", i.e. when all arms branched to the same basic block, even if that `SwitchInt` terminator had the side-effect of reading an operand which (for example) may not be initialized or may point to an invalid place in memory. This behavior is unlike all other optimizations, which are only applied after "analysis" (i.e. borrow-checking) is finished, and which Miri disables to make sure the compiler doesn't silently remove UB. Fixing this code "breaks" (i.e. unmasks) code that used to borrow-check but no longer does, like: ```rust fn foo() { let x; let (0 | _) = x; } ``` This match expression should perform a read because `_` does not shadow the `0` literal pattern, and the compiler should have to read the match scrutinee to compare it to 0. I've checked that this behavior does not actually manifest in practice via a crater run which came back clean: https://github.com/rust-lang/rust/pull/139042#issuecomment-2767436367 As a side-note, it may be tempting to suggest that this is actually a good thing or that we should preserve this behavior. If we wanted to make this work (i.e. trivially optimize out reads from matches that are redundant like `0 | _`), then we should be enabling this behavior *after* fixing this. However, I think it's kinda unprincipled, and for example other variations of the code don't even work today, e.g.: ```rust fn foo() { let x; let (0.. | _) = x; } ```

In `rustc_mir_tranform`, iterate over index newtypes instead of ints

2025-04-12T11:53:07+00:00

Do not optimize out SwitchInt before borrowck, or if Zmir-preserve-ub

2025-04-08T21:05:20+00:00

Stop relying on rustc_type_ir in non-type-system crates

2025-03-15T06:42:48+00:00

Move `MirPatch` from `rustc_middle` to `rustc_mir_transform`.

2025-02-14T05:15:57+00:00

Because it's only used in `rustc_mir_transform`. (Presumably it is currently in `rustc_middle` because lots of other MIR-related stuff is, but that's not a hard requirement.) And because `rustc_middle` is huge and it's always good to make it smaller.

Disable non-required MIR opts with `optimize(none)`

2025-01-23T17:40:41+00:00

Co-authored-by: Waffle Lapkin

MatchBranchSimplification: Consider empty-unreachable otherwise branch

2024-12-27T10:57:46+00:00

Remove -Zfuel.

2024-11-26T10:45:21+00:00