refactor(tvix/eval): decouple local depth & initialisation tracking

In order to resolve recursive references correctly, these two can not
be initialised the same way as a potentially large number of (nested!)
locals can be declared without initialising their depth.

This would lead to issues with detecting things like shadowed
variables, so making both bits explicit is preferable.

Change-Id: I100cdf1724faa4a2b5a0748429841cf8ef206252
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6325
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/compiler/mod.rs

@@ -29,7 +29,7 @@ use crate::opcode::{CodeIdx, Count, JumpOffset, OpCode, StackIdx, UpvalueIdx};
 use crate::value::{Closure, Lambda, Value};
 use crate::warnings::{EvalWarning, WarningKind};
 
-use self::scope::{Depth, Local, LocalPosition, Scope, Upvalue};
+use self::scope::{Local, LocalPosition, Scope, Upvalue};
 
 /// Represents the result of compiling a piece of Nix code. If
 /// compilation was successful, the resulting bytecode can be passed
@@ -752,7 +752,7 @@ impl Compiler {
             // Calculate the with_idx without taking into account
             // uninitialised variables that are not yet in their stack
             // slots.
-            .filter(|l| matches!(l.depth, Depth::At(_)))
+            .filter(|l| l.initialised)
             .count()
             - 1;
 
@@ -897,9 +897,7 @@ impl Compiler {
         // TL;DR - iterate from the back while things belonging to the
         // ended scope still exist.
         while !self.scope().locals.is_empty()
-            && self.scope().locals[self.scope().locals.len() - 1]
-                .depth
-                .above(self.scope().scope_depth)
+            && self.scope().locals[self.scope().locals.len() - 1].above(self.scope().scope_depth)
         {
             pops += 1;
 
@@ -944,13 +942,8 @@ impl Compiler {
         }
 
         let mut shadowed = false;
-        for local in self.scope().locals.iter().rev() {
-            if local.depth.below(self.scope().scope_depth) {
-                // Shadowing identifiers from higher scopes is allowed.
-                break;
-            }
-
-            if local.name == name {
+        for other in self.scope().locals.iter().rev() {
+            if other.name == name && other.depth == depth {
                 shadowed = true;
                 break;
             }
@@ -965,7 +958,8 @@ impl Compiler {
 
         self.scope_mut().locals.push(Local {
             name,
-            depth: Depth::Unitialised,
+            depth,
+            initialised: false,
             node: Some(node),
             phantom: false,
             used: false,
@@ -978,7 +972,8 @@ impl Compiler {
     fn declare_phantom(&mut self) {
         let depth = self.scope().scope_depth;
         self.scope_mut().locals.push(Local {
-            depth: Depth::At(depth),
+            depth,
+            initialised: true,
             name: "".into(),
             node: None,
             phantom: true,
@@ -990,8 +985,8 @@ impl Compiler {
     fn mark_initialised(&mut self, name: &str) {
         let depth = self.scope().scope_depth;
         for local in self.scope_mut().locals.iter_mut().rev() {
-            if matches!(local.depth, Depth::Unitialised) && local.name == name {
-                local.depth = Depth::At(depth);
+            if !local.initialised && local.depth == depth && local.name == name {
+                local.initialised = true;
                 return;
             }
         }