Aggregate rvalue in optimized mir

[bjorn3]

#![allow(warnings)]

fn needs_slice(s: &[u8]) {}

fn gives_array(a: u8) {
    needs_slice(&[a])
}

fn main() {}

(Playground)

Mir from tcx.optimized_mir:

fn gives_array(_1: u8) -> (){
    let mut _0: ();                      // return place
    let mut _2: &[u8];
    let mut _3: &[u8; 1];
    let mut _4: &[u8; 1];
    let mut _5: [u8; 1];
    let mut _6: u8;

    bb0: {                              
        StorageLive(_2);                 // bb0[0]: scope 0 at src/main.rs:6:17: 6:21
        StorageLive(_3);                 // bb0[1]: scope 0 at src/main.rs:6:17: 6:21
        StorageLive(_4);                 // bb0[2]: scope 0 at src/main.rs:6:17: 6:21
        StorageLive(_5);                 // bb0[3]: scope 0 at src/main.rs:6:18: 6:21
        StorageLive(_6);                 // bb0[4]: scope 0 at src/main.rs:6:19: 6:20
        _6 = _1;                         // bb0[5]: scope 0 at src/main.rs:6:19: 6:20
        _5 = [move _6];                  // bb0[6]: scope 0 at src/main.rs:6:18: 6:21
        StorageDead(_6);                 // bb0[7]: scope 0 at src/main.rs:6:20: 6:21
        _4 = &_5;                        // bb0[8]: scope 0 at src/main.rs:6:17: 6:21
        _3 = _4;                         // bb0[9]: scope 0 at src/main.rs:6:17: 6:21
        _2 = move _3 as &[u8] (Unsize);  // bb0[10]: scope 0 at src/main.rs:6:17: 6:21
        StorageDead(_3);                 // bb0[11]: scope 0 at src/main.rs:6:20: 6:21
        _0 = const needs_slice(move _2) -> bb1; // bb0[12]: scope 0 at src/main.rs:6:5: 6:22
                                         // ty::Const
                                         // + ty: for<'r> fn(&'r [u8]) {needs_slice}
                                         // + val: Value(ByVal(Undef))
                                         // mir::Constant
                                         // + span: src/main.rs:6:5: 6:16
                                         // + ty: for<'r> fn(&'r [u8]) {needs_slice}
                                         // + literal: const needs_slice
    }

    bb1: {                              
        StorageDead(_2);                 // bb1[0]: scope 0 at src/main.rs:6:21: 6:22
        StorageDead(_4);                 // bb1[1]: scope 0 at src/main.rs:7:1: 7:2
        StorageDead(_5);                 // bb1[2]: scope 0 at src/main.rs:7:1: 7:2
        return;                          // bb1[3]: scope 0 at src/main.rs:7:2: 7:2
    }
}

This contains an aggregate (_5 = [move _6];), while the docs of Rvalue::Aggregate seem to imply that it shouldn't exist after optimizations:

These rvalues can be optimized away after type-checking and before lowering.

[bjorn3]

This is a dupe of #48193