[move] Fixed locals disassembly

[awelc]

Description

It looks like locals are currently not disassembled correctly when running Move disassembler and this PR contains a fix. In particular, consider the following Move code:

module test::M1;

public enum SomeEnum has drop, copy {
    NamedVariant { field: u64 },
    PositionalVariant(u64),
}

public fun foo(e: SomeEnum, p1: u64, p2: u64): (u64, u64) {
    match (e) {
        SomeEnum::NamedVariant { field } => {
            let mut res = field + p1;
            res = res + p2;
            (res, res)
        },
        SomeEnum::PositionalVariant(field) => (field, field)
    }
}

#[test]
public fun test() {
    let e = SomeEnum::NamedVariant { field: 42 };
    foo(e, 42, 7);
}

When running disassembler on the M1 module, the following 3 locals are reported in the disassembly:

L0:	__match_tmp%#unpack_subject#1#1#0: SomeEnum
L1:	field#3#0: u64
L2:	res#1#0: u64

However, 6 locals are reported in the source map:

%#7
%#8
__match_tmp%#match_subject#2#2#0
__match_tmp%#unpack_subject#1#1#0
field#3#0
res#1#0

The same number of locals is also reported in the trace generated out of the test function in M1.

It looks like the disassembler incorrectly assumes that function parameters are included in the locals and only locals beyond the (optional) initial number of parameters should be counted. After making the adjustment to ignore the parameters when disassembling locals, the following is reported in the disassembly which is consistent with the number (and names) of the locals in the source map and with the types of the locals in the trace:

L0:	%#7: u64
L1:	%#8: u64
L2:	__match_tmp%#match_subject#2#2#0: &SomeEnum
L3:	__match_tmp%#unpack_subject#1#1#0: SomeEnum
L4:	field#3#0: u64
L5:	res#1#0: u64

Test plan

All tests should pass

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[tzakian]

I don't think this is quite right. There's a slight mismatch here due to an underlying problem/mismatch between source maps and compiled modules:

• In the bytecode all parameters are laid out in locals (so the first parameter_len local indices in the bytecode are the parameters).
• In the function source map, we add parameters in to the parameters field, but not the locals.

I see two possibilities here:

1. Pass local_idx + parameter_len here (since this is the index into the bytecode, so we want to skip the parameters here).
2. Update the source map generation to also add the parameters to the locals (and in this case the parameters field in the function source map should be removed).

I prefer option (2) but that's a lot more work than just updating the index here, so if you need this to unblock you updating the index just here is probably fine.

[awelc]

I don't think this is going to work as in disassemble_locals we actually iterate over locals indexes from the source map:

        for (local_idx, (name, _)) in function_source_map.locals.iter().enumerate() {
               ...
        }

If we use local_idx +parameter_len to get the type, we will miss some of them as locals signature only contains locals types:

L0:	%#7: SomeEnum
L1:	%#8: u64
L2:	__match_tmp%#match_subject#2#2#0: u64
L3:	__match_tmp%#unpack_subject#1#1#0: ERROR[Unable to get type for local at index 6]
L4:	field#3#0: ERROR[Unable to get type for local at index 7]
L5:	res#1#0: ERROR[Unable to get type for local at index 8]

That being said, with the current solution locals indexes displayed with locals disassembly do not match what's in the disassembled bytecode due to what you observed ("in the bytecode all parameters are laid out in locals") and we need to bump them up using parameter_len (note that now MoveLoc and StLoc disassembly's indexes much parameter and local indexes):

public foo(e#0#0: SomeEnum, p1#0#0: u64, p2#0#0: u64): u64 * u64 {
L3:	%#7: u64
L4:	%#8: u64
L5:	__match_tmp%#match_subject#2#2#0: &SomeEnum
L6:	__match_tmp%#unpack_subject#1#1#0: SomeEnum
L7:	field#3#0: u64
L8:	res#1#0: u64
B0:
	0: MoveLoc[0](e#0#0: SomeEnum)
	1: StLoc[6](__match_tmp%#unpack_subject#1#1#0: SomeEnum)

...

[tzakian]

Changes look good to me!