Add Bool builtin type, branches, and some more stuff

* obj::Bool builtin type is used for truthiness and decision-making
* Branches are compiled and seem to be working for basic integer
  comparison
* Updated version of Shredder to what is current as of writing
* CheckTruth VM instruction that will explicitly set the condition flag
* Probably some other stuff

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/vm/consts.rs

@@ -7,9 +7,6 @@ pub struct ConstPool {
     pool: Vec<ObjRef>,
 }
 
-// Constant pools are constant, and live for the lifetime of the entire program.
-impl shredder::EmptyScan for ConstPool {}
-
 impl ConstPool {
     pub fn new() -> Self {
         Default::default()

src/vm/inst.rs

@@ -1,5 +1,4 @@
 use crate::{obj::prelude::*, vm::consts::*};
-use shredder::EmptyScan;
 
 #[derive(Debug, PartialEq, Clone, Copy)]
 pub enum Inst {
@@ -36,6 +35,9 @@ pub enum Inst {
     ///
     SetAttr(Sym),
 
+    /// Pops the top value off of the stack and checks if it is a `Bool` of a true value.
+    CheckTruth,
+
     /// Jump to a given address in the current function unconditionally.
     Jump(usize),
 
@@ -131,6 +133,7 @@ impl Inst {
             Inst::PopGlobal(_) => "POP_GLOBAL",
             Inst::GetAttr(_) => "GET_ATTR",
             Inst::SetAttr(_) => "SET_ATTR",
+            Inst::CheckTruth => "CHECK_TRUTH",
             Inst::Jump(_) => "JUMP",
             Inst::JumpTrue(_) => "JUMP_TRUE",
             Inst::Call(_) => "CALL",
@@ -154,4 +157,4 @@ impl Inst {
     }
 }
 
-impl EmptyScan for Inst {}
+unsafe impl shredder::marker::GcDrop for Inst {}

src/vm/mod.rs

@@ -254,6 +254,15 @@ impl<'c> Vm<'c> {
                     todo!("TODO: throw an error for attributes that can't be set");
                 }
             }
+            Inst::CheckTruth => {
+                let source = self.pop().expect("no source available for CheckTruth");
+                read_obj_downcast!(let obj: Option<&Bool> = &source);
+                if let Some(obj) = obj {
+                    self.condition = obj.value();
+                } else {
+                    unreachable!("Expected a boolean for CheckTruth but got {:?} instead", source);
+                }
+            }
             Inst::Jump(addr) => {
                 next_pc = addr;
             }
@@ -272,12 +281,12 @@ impl<'c> Vm<'c> {
                     .expect("TODO: throw an error for missing __call__ attr");
                 signal = Some(Signal::Call(callee, args));
             }
-            Inst::Index => todo!(),
+            Inst::Index => todo!("Inst::Index"),
             Inst::Return => {
                 signal = Some(Signal::Return);
             }
-            Inst::UnNeg => todo!(),
-            Inst::UnPos => todo!(),
+            Inst::UnNeg => todo!("Inst::UnNeg"),
+            Inst::UnPos => todo!("Inst::UnPos"),
             Inst::BinPlus => {
                 let rhs = self.pop().unwrap();
                 let lhs = self.pop().unwrap();