MIR building generates mistyped assignments for opaques in dead code

enum Never {}
fn foo() -> impl Sized {
    let ret = 'block: {
        break 'block foo();
    };
    let _: Never = ret;
    ret
}

results in the following MIR:

// MIR for `foo` after built

| User Type Annotations
| 0: user_ty: Canonical { value: Ty(Never), max_universe: U0, variables: [], defining_opaque_types: [DefId(0:6 ~ main[7eb4]::foo::{opaque#0})] }, span: src/main.rs:6:12: 6:17, inferred_ty: Never
| 1: user_ty: Canonical { value: Ty(Never), max_universe: U0, variables: [], defining_opaque_types: [DefId(0:6 ~ main[7eb4]::foo::{opaque#0})] }, span: src/main.rs:6:12: 6:17, inferred_ty: Never
|
fn foo() -> impl Sized {
    let mut _0: impl Sized;
    let _1: impl Sized;
    scope 1 {
        debug ret => _1;
        scope 2 {
        }
    }

    bb0: {
        StorageLive(_1);
        _1 = foo() -> [return: bb1, unwind: bb6];
    }

    bb1: {
        goto -> bb3;
    }

    bb2: {
        _1 = const ();
        goto -> bb4;
    }

    bb3: {
        goto -> bb4;
    }

    bb4: {
        FakeRead(ForLet(None), _1);
        PlaceMention(_1);
        AscribeUserType((_1 as Never), +, UserTypeProjection { base: UserType(1), projs: [] });
        _0 = move _1;
        StorageDead(_1);
        return;
    }

    bb5: {
        FakeRead(ForMatchedPlace(None), _1);
        unreachable;
    }

    bb6 (cleanup): {
        resume;
    }
}

notice the _1 = const (); which assigns unit to impl Sized even though its type is Never.

This should result in a type error and therefore an ICE, but apparently we never even get to test that as the block is entirely dead code 🤔 cc @oli-obk

https://github.com/rust-lang/rust/blob/fbce03b195c02e425fbb12276b8f02349048a75f/compiler/rustc_mir_build/src/build/block.rs#L342-L354

Start with the reproducer and the MIR output in the issue, then inspect compiler/rustc_mir_build/src/build/block.rs around lines 342-354. Run the example to confirm how the dead block produces _1 = const (); for an opaque value. Done means the invalid assignment is no longer generated, with the resulting type-checking behavior verified.