opcode-macros 0.1.0

# opcode-macros

This crate provides a `opcode_match` macro, which generates a complex match statement
for opcodes encoded in bit fields.

## Format

The basic format is like this:

```rust
use opcode_macros::opcode_match;
let opcode = 0u8;
mod namespace {
    pub const A_1: u8 = 0;
    pub const A_2: u8 = 0;
    pub const B_1: u8 = 0;
    pub const B_2: u8 = 0;
}
let result =
opcode_match! {
    opcode as u8 in namespace,
    [[A_1: a1, A_2: a2], [B_1: b1, B_2: b2]] => {
        // Code
        1
    }
    _ => {
        // Code
        0
    }
}
;
assert_eq!(result, 1);
```

It generates something like this:

```rust
let opcode = 0u8;
mod namespace {
    pub const A_1: u8 = 0;
    pub const A_2: u8 = 0;
    pub const B_1: u8 = 0;
    pub const B_2: u8 = 0;
}
const A_1_B_1: u8 = namespace::A_1 | namespace::B_1;
const A_1_B_2: u8 = namespace::A_1 | namespace::B_2;
const A_2_B_1: u8 = namespace::A_2 | namespace::B_1;
const A_2_B_2: u8 = namespace::A_2 | namespace::B_2;
match opcode {
    A_1_B_1 => { /* Code */ }
    A_1_B_2 => { /* Code */ }
    A_2_B_1 => { /* Code */ }
    A_2_B_2 => { /* Code */ }
    _ => { /* Code */ }
}
```

### Match Arm Headers

Match arm headers is something like `[[A_1: a1, A_2: a2], [B_1: b1, B_2: b2]]`.

For example, in eBPF opcodes, `BPF_ALU | BPF_K | BPF_ADD` is an opcode for
32-bit addition with constants, while `BPF_ALU | BPF_K | BPF_SUB` is an opcode
for 32-bit subtraction with constants. To match against these opcodes,
we use the following code:

```rust
use opcode_macros::opcode_match;
use ebpf_consts::*;
let opcode = 0x04u8;
let mut result = 0u64;
let dst = 10u64;
let imm = 10u64;
opcode_match! {
    opcode as u8 in ebpf_consts,
    [[BPF_ALU: _], [BPF_K: _],
     [BPF_ADD: add, BPF_SUB: sub]] => {
        result = dst.#"wrapping_{}"2(imm);
    }
    _ => {}
}
assert_eq!(result, 20);
```

We will talk about the templating rules later.

In the example above, you can also use some other variants:
- `[BPF_ADD: "add", BPF_SUB: "sub"]`
- `[BPF_ADD: [add], BPF_SUB: [sub]]`
- `[BPF_ADD: ["add"], BPF_SUB: ["sub"]]`
- `[BPF_ADD: ["add", "extra1"], BPF_SUB: ["sub", "extra2"]]`

If you want to substitutes parts of the code with symbols like "+",
you will need to quote the symbols like `[BPF_ADD: "+"]`.

### Code Template

#### Substitution

This is not a real life example.

```rust
use opcode_macros::opcode_match;
use ebpf_consts::*;
use core::ops::Add;
let opcode = 0u8;
opcode_match! {
    opcode as u8 in ebpf_consts,
    [
        // Group 0
        [BPF_K:   ["const", 1, 2, "Constant Operation"]],
        // Group 1
        [BPF_ADD: ["add", 3, 4, "Addition Operation"]],
    ] => {
        // Use the first token in group 0 as a string
        assert_eq!(#0, "const");
        // Use the fourth token in group 0 as a string
        assert_eq!(#:3:0, "Constant Operation");
        assert_eq!(#:0:1, "add");
        assert_eq!(#:3:1, "Addition Operation");

        // Use raw tokens
        assert_eq!(#:1:1, "3");
        assert_eq!(#:1:=1, 3);
        // 30.add(40) == 70, where #=1 is just #:0:=1
        let value = 30isize;
        assert_eq!(value.#=1(40), 70);

        // With in-token substitution: add -> wrapping_add
        assert_eq!(value.#"wrapping_{}"1(40), 70);
    }
    _ => panic!(),
}
```

#### Conditional Blocks

```rust
use opcode_macros::opcode_match;
use ebpf_consts::*;

let opcode = BPF_X | BPF_ALU;
opcode_match! {
    opcode as u8 in ebpf_consts,
    [[BPF_X: x, BPF_K: k], [BPF_ALU: "alu", BPF_ALU64: "alu64"]] => {
        #?((x))
            println!("In V1 branch");
            assert_eq!(#0, "x");                ##

        #?((k))
            println!("In V2 && {} branch", #1);
            assert_eq!(#0, "k");                ##

        #?((!"k"))
            println!("In V2 && {} branch", #1);
            assert_eq!(#0, "x");                ##
        println!("Common");
    }
    _ => panic!(),
};
```

The grammar is `#?((cond1, cond2)|(cond3|cond4)|...) CODE ##`,
making the code only injected if the opcode matches
`(cond1 && cond2) || (cond3 && cond4) || ...`.
A condition can be negated with an exclamation mark `!cond1`.

We don't allow nested conditions.