fix(tvix/eval): add operation to assert boolean type r/4491

This operation is required because both sides of the logical operators
are strictly evaluated by Nix, even if the resulting value is not used
further.

For example, in our implementation of `&&`, if the left-hand side is
`true`, then the result of the expression is simply the right-hand
side value. This value must be asserted to be a boolean for the
semantics of the language to work correctly.

Change-Id: I34f5364f2a444753fa1d8b0a1a2b2d9cdf7c6700
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6157
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: grfn <grfn@gws.fyi>

4 files changed, 24 insertions, 0 deletions

diff --git a/tvix/eval/src/compiler.rs b/tvix/eval/src/compiler.rs
index 90a6155b6b..1bfd765a07 100644
--- a/tvix/eval/src/compiler.rs
+++ b/tvix/eval/src/compiler.rs
@@ -350,6 +350,7 @@ impl Compiler {
         self.compile(node.rhs().unwrap())?;
 
         self.patch_jump(end_idx);
+        self.chunk.add_op(OpCode::OpAssertBool);
 
         Ok(())
     }
@@ -370,6 +371,7 @@ impl Compiler {
         self.chunk.add_op(OpCode::OpPop);
         self.compile(node.rhs().unwrap())?;
         self.patch_jump(end_idx);
+        self.chunk.add_op(OpCode::OpAssertBool);
 
         Ok(())
     }
@@ -390,6 +392,7 @@ impl Compiler {
         self.chunk.add_op(OpCode::OpPop);
         self.compile(node.rhs().unwrap())?;
         self.patch_jump(end_idx);
+        self.chunk.add_op(OpCode::OpAssertBool);
 
         Ok(())
     }
diff --git a/tvix/eval/src/opcode.rs b/tvix/eval/src/opcode.rs
index f577bc985d..4831a71ebd 100644
--- a/tvix/eval/src/opcode.rs
+++ b/tvix/eval/src/opcode.rs
@@ -53,4 +53,7 @@ pub enum OpCode {
 
     // Strings
     OpInterpolate(usize),
+
+    // Type assertion operators
+    OpAssertBool,
 }
diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs
index 4a34379748..99eb4c657b 100644
--- a/tvix/eval/src/value/mod.rs
+++ b/tvix/eval/src/value/mod.rs
@@ -91,6 +91,10 @@ impl Value {
             }),
         }
     }
+
+    pub fn is_bool(&self) -> bool {
+        matches!(self, Value::Bool(_))
+    }
 }
 
 impl Display for Value {
diff --git a/tvix/eval/src/vm.rs b/tvix/eval/src/vm.rs
index ed040cf917..3e8509187b 100644
--- a/tvix/eval/src/vm.rs
+++ b/tvix/eval/src/vm.rs
@@ -187,6 +187,20 @@ impl VM {
                         self.ip += offset;
                     }
                 }
+
+                // These assertion operations error out if the stack
+                // top is not of the expected type. This is necessary
+                // to implement some specific behaviours of Nix
+                // exactly.
+                OpCode::OpAssertBool => {
+                    let val = self.peek(0);
+                    if !val.is_bool() {
+                        return Err(Error::TypeError {
+                            expected: "bool",
+                            actual: val.type_of(),
+                        });
+                    }
+                }
             }
 
             if self.ip == self.chunk.code.len() {