Change:
<ins> - // MIR for `inner` before ConstProp + // MIR for `inner` after ConstProp fn inner(_1: u32) -> i64 { debug fields => _1;                  // in scope 0 at $DIR/issue-101973.rs:+0:14: +0:20 let mut _0: i64;                     // return place in scope 0 at $DIR/issue-101973.rs:+0:30: +0:33 let mut _2: i32;                     // in scope 0 at $DIR/issue-101973.rs:+1:5: +1:65 let mut _3: u32;                     // in scope 0 at $DIR/issue-101973.rs:+1:5: +1:58 let mut _4: u32;                     // in scope 0 at $DIR/issue-101973.rs:+1:5: +1:17 let mut _5: u32;                     // in scope 0 at $DIR/issue-101973.rs:+1:10: +1:16 let mut _6: u32;                     // in scope 0 at $DIR/issue-101973.rs:+1:31: +1:57 let mut _7: u32;                     // in scope 0 at $DIR/issue-101973.rs:+1:31: +1:52 let mut _8: u32;                     // in scope 0 at $DIR/issue-101973.rs:+1:32: +1:45 let mut _9: u32;                     // in scope 0 at $DIR/issue-101973.rs:+1:33: +1:39 let mut _10: (u32, bool);            // in scope 0 at $DIR/issue-101973.rs:+1:32: +1:45 let mut _11: (u32, bool);            // in scope 0 at $DIR/issue-101973.rs:+1:31: +1:57 scope 1 (inlined imm8) {             // at $DIR/issue-101973.rs:14:5: 14:17 debug x => _5;                   // in scope 1 at $DIR/issue-101973.rs:5:13: 5:14 let mut _12: u32;                // in scope 1 at $DIR/issue-101973.rs:7:12: 7:27 let mut _13: u32;                // in scope 1 at $DIR/issue-101973.rs:7:12: 7:20 let mut _14: u32;                // in scope 1 at $DIR/issue-101973.rs:7:13: 7:14 let mut _15: (u32, bool);        // in scope 1 at $DIR/issue-101973.rs:7:12: 7:20 scope 2 { debug out => _4;             // in scope 2 at $DIR/issue-101973.rs:6:9: 6:16 } } scope 3 (inlined core::num::<impl u32>::rotate_right) { // at $DIR/issue-101973.rs:14:5: 14:58 debug self => _4;                // in scope 3 at $SRC_DIR/core/src/num/uint_macros.rs:LL:COL debug n => _6;                   // in scope 3 at $SRC_DIR/core/src/num/uint_macros.rs:LL:COL let mut _16: u32;                // in scope 3 at $SRC_DIR/core/src/num/uint_macros.rs:LL:COL let mut _17: u32;                // in scope 3 at $SRC_DIR/core/src/num/uint_macros.rs:LL:COL } bb0: { StorageLive(_2);                 // scope 0 at $DIR/issue-101973.rs:+1:5: +1:65 StorageLive(_3);                 // scope 0 at $DIR/issue-101973.rs:+1:5: +1:58 StorageLive(_4);                 // scope 0 at $DIR/issue-101973.rs:+1:5: +1:17 StorageLive(_5);                 // scope 0 at $DIR/issue-101973.rs:+1:10: +1:16 _5 = _1;                         // scope 0 at $DIR/issue-101973.rs:+1:10: +1:16 _4 = const 0_u32;                // scope 1 at $DIR/issue-101973.rs:6:19: 6:23 StorageLive(_12);                // scope 2 at $DIR/issue-101973.rs:7:12: 7:27 StorageLive(_13);                // scope 2 at $DIR/issue-101973.rs:7:12: 7:20 StorageLive(_14);                // scope 2 at $DIR/issue-101973.rs:7:13: 7:14 _14 = _5;                        // scope 2 at $DIR/issue-101973.rs:7:13: 7:14 _15 = CheckedShr(_14, const 0_i32); // scope 2 at $DIR/issue-101973.rs:7:12: 7:20 assert(!move (_15.1: bool), "attempt to shift right by `{}`, which would overflow", const 0_i32) -> bb3; // scope 2 at $DIR/issue-101973.rs:7:12: 7:20 } bb1: { _8 = move (_10.0: u32);          // scope 0 at $DIR/issue-101973.rs:+1:32: +1:45 StorageDead(_9);                 // scope 0 at $DIR/issue-101973.rs:+1:44: +1:45 _7 = BitAnd(move _8, const 15_u32); // scope 0 at $DIR/issue-101973.rs:+1:31: +1:52 StorageDead(_8);                 // scope 0 at $DIR/issue-101973.rs:+1:51: +1:52 _11 = CheckedShl(_7, const 1_i32); // scope 0 at $DIR/issue-101973.rs:+1:31: +1:57 assert(!move (_11.1: bool), "attempt to shift left by `{}`, which would overflow", const 1_i32) -> bb2; // scope 0 at $DIR/issue-101973.rs:+1:31: +1:57 } bb2: { _6 = move (_11.0: u32);          // scope 0 at $DIR/issue-101973.rs:+1:31: +1:57 StorageDead(_7);                 // scope 0 at $DIR/issue-101973.rs:+1:56: +1:57 StorageLive(_16);                // scope 3 at $SRC_DIR/core/src/num/uint_macros.rs:LL:COL _16 = _4;                        // scope 3 at $SRC_DIR/core/src/num/uint_macros.rs:LL:COL StorageLive(_17);                // scope 3 at $SRC_DIR/core/src/num/uint_macros.rs:LL:COL _17 = _6;                        // scope 3 at $SRC_DIR/core/src/num/uint_macros.rs:LL:COL _3 = rotate_right::<u32>(move _16, move _17) -> bb4; // scope 3 at $SRC_DIR/core/src/num/uint_macros.rs:LL:COL // mir::Constant // + span: $SRC_DIR/core/src/num/uint_macros.rs:LL:COL // + literal: Const { ty: extern "rust-intrinsic" fn(u32, u32) -> u32 {rotate_right::<u32>}, val: Value(<ZST>) } } bb3: { _13 = move (_15.0: u32);         // scope 2 at $DIR/issue-101973.rs:7:12: 7:20 StorageDead(_14);                // scope 2 at $DIR/issue-101973.rs:7:19: 7:20 _12 = BitAnd(move _13, const 255_u32); // scope 2 at $DIR/issue-101973.rs:7:12: 7:27 StorageDead(_13);                // scope 2 at $DIR/issue-101973.rs:7:26: 7:27 _4 = BitOr(_4, move _12);        // scope 2 at $DIR/issue-101973.rs:7:5: 7:27 StorageDead(_12);                // scope 2 at $DIR/issue-101973.rs:7:26: 7:27 StorageDead(_5);                 // scope 0 at $DIR/issue-101973.rs:+1:16: +1:17 StorageLive(_6);                 // scope 0 at $DIR/issue-101973.rs:+1:31: +1:57 StorageLive(_7);                 // scope 0 at $DIR/issue-101973.rs:+1:31: +1:52 StorageLive(_8);                 // scope 0 at $DIR/issue-101973.rs:+1:32: +1:45 StorageLive(_9);                 // scope 0 at $DIR/issue-101973.rs:+1:33: +1:39 _9 = _1;                         // scope 0 at $DIR/issue-101973.rs:+1:33: +1:39 _10 = CheckedShr(_9, const 8_i32); // scope 0 at $DIR/issue-101973.rs:+1:32: +1:45 assert(!move (_10.1: bool), "attempt to shift right by `{}`, which would overflow", const 8_i32) -> bb1; // scope 0 at $DIR/issue-101973.rs:+1:32: +1:45 } bb4: { StorageDead(_17);                // scope 3 at $SRC_DIR/core/src/num/uint_macros.rs:LL:COL StorageDead(_16);                // scope 3 at $SRC_DIR/core/src/num/uint_macros.rs:LL:COL StorageDead(_6);                 // scope 0 at $DIR/issue-101973.rs:+1:57: +1:58 StorageDead(_4);                 // scope 0 at $DIR/issue-101973.rs:+1:57: +1:58 _2 = move _3 as i32 (Misc);      // scope 0 at $DIR/issue-101973.rs:+1:5: +1:65 StorageDead(_3);                 // scope 0 at $DIR/issue-101973.rs:+1:64: +1:65 _0 = move _2 as i64 (Misc);      // scope 0 at $DIR/issue-101973.rs:+1:5: +1:72 StorageDead(_2);                 // scope 0 at $DIR/issue-101973.rs:+1:71: +1:72 return;                          // scope 0 at $DIR/issue-101973.rs:+2:2: +2:2 } } </ins>

Discussion:
This code normally prints but when compiled with it produces instead. This was noticed because MIR optimization is enabled by default on the current nightly. This is the LLVM IR for the function with . Notice how the inputs to (used by ) are , which is incorrect.
prioritize
label regression-from-stable-to-nightly, I-unsound, A-mir-opt, T-compiler
Can someone execute the snippet with and upload the resulting directory somewhere? That should allow me to debug from my phone
I’ve made a gist originally, too, but it looks like you can’t see all the files there. (Afaict only 300 of the 401 files show up.) Here’s a repo Edit: One downside of a repo (compared to a gist) is that I might want to get rid of it eventually; AFAIK, this shouldn’t be needed for too long anyway though.
With an assertion instead of println: Individual builds from , point to cc
Ouch. That probably means the unsoundness was already possible before my PR (as explained there, the check it removes was ineffective), but is easier to trigger now. Cc something is wrong in ConstProp handling of shifts and/or casts and/or bit-ops. Is there a reproducer that does not need shifts and casts and a bitop? That would make the issue easier to track down.
FWIW it prints 0xfffffffff0000000 in Miri so the interpreter is probably fine, and it's probably a problem with the ConstProp pass itself.
The original code was much more complex. I minimized it as much as I could, but at this point even removing a useless will cause the bug to no longer trigger.
Yeah I quite appreciate that it does fit in a few lines now. :)
The is not needed, but indeed the bug needs to trigger. And inlining. Very strange. ConstProp code is super hard to read and full of subtle (usually barely documented) invariants, so... this could be an interesting one to track down.
The inlined code looks fine, but it has the basic blocks in a strange order, which might explain why the ConstProp bug only occurs if inlining happens first.

The code just before ConstProp has this One possible explanation would be if ConstProp iterated the basic blocks in order, then by the time it reaches , the last assignment to it saw is in bb0. There is another assignment to in bb3, which is executed before bb2, but maybe somehow my change made it so that this is not taken into account properly any more.
Turns out the actual problem was an early return that I which skipped more code than it should have -- fixed by
WG-prioritization assigning priority (). label -I-prioritize +P-critical