chore(experimental): Improve variable not defined error message in comptime interpreter (#4889)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*

Slightly improves the error message for when a variable in the
interpreter isn't found.

Since all ids are already given by name resolution, the only time one
cannot be found is if it is not a comptime variable, so I've narrowed
the error message a bit.

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [ ] I have tested the changes locally.
- [ ] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

Author: jfecher

compiler/noirc_frontend/src/ast/statement.rs

@@ -40,7 +40,7 @@ pub enum StatementKind {
     Break,
     Continue,
     /// This statement should be executed at compile-time
-    CompTime(Box<StatementKind>),
+    Comptime(Box<StatementKind>),
     // This is an expression with a trailing semi-colon
     Semi(Expression),
     // This statement is the result of a recovered parse error.
@@ -87,10 +87,10 @@ impl StatementKind {
                 }
                 self
             }
-            StatementKind::CompTime(mut statement) => {
+            StatementKind::Comptime(mut statement) => {
                 *statement =
                     statement.add_semicolon(semi, span, last_statement_in_block, emit_error);
-                StatementKind::CompTime(statement)
+                StatementKind::Comptime(statement)
             }
             // A semicolon on a for loop is optional and does nothing
             StatementKind::For(_) => self,
@@ -685,7 +685,7 @@ impl Display for StatementKind {
             StatementKind::For(for_loop) => for_loop.fmt(f),
             StatementKind::Break => write!(f, "break"),
             StatementKind::Continue => write!(f, "continue"),
-            StatementKind::CompTime(statement) => write!(f, "comptime {statement}"),
+            StatementKind::Comptime(statement) => write!(f, "comptime {statement}"),
             StatementKind::Semi(semi) => write!(f, "{semi};"),
             StatementKind::Error => write!(f, "Error"),
         }

compiler/noirc_frontend/src/hir/comptime/errors.rs

@@ -1,4 +1,4 @@
-use crate::{node_interner::DefinitionId, Type};
+use crate::Type;
 use acvm::FieldElement;
 use noirc_errors::Location;
 
@@ -9,7 +9,7 @@ use super::value::Value;
 pub enum InterpreterError {
     ArgumentCountMismatch { expected: usize, actual: usize, call_location: Location },
     TypeMismatch { expected: Type, value: Value, location: Location },
-    NoValueForId { id: DefinitionId, location: Location },
+    NonComptimeVarReferenced { name: String, location: Location },
     IntegerOutOfRangeForType { value: FieldElement, typ: Type, location: Location },
     ErrorNodeEncountered { location: Location },
     NonFunctionCalled { value: Value, location: Location },

compiler/noirc_frontend/src/hir/comptime/hir_to_ast.rs

@@ -66,8 +66,8 @@ impl StmtId {
             HirStatement::Expression(expr) => StatementKind::Expression(expr.to_ast(interner)),
             HirStatement::Semi(expr) => StatementKind::Semi(expr.to_ast(interner)),
             HirStatement::Error => StatementKind::Error,
-            HirStatement::CompTime(statement) => {
-                StatementKind::CompTime(Box::new(statement.to_ast(interner).kind))
+            HirStatement::Comptime(statement) => {
+                StatementKind::Comptime(Box::new(statement.to_ast(interner).kind))
             }
         };
 

compiler/noirc_frontend/src/hir/comptime/interpreter.rs

@@ -246,13 +246,7 @@ impl<'a> Interpreter<'a> {
         location: Location,
     ) -> IResult<()> {
         self.type_check(typ, &argument, location)?;
-        for scope in self.scopes.iter_mut().rev() {
-            if let Entry::Occupied(mut entry) = scope.entry(id) {
-                entry.insert(argument);
-                return Ok(());
-            }
-        }
-        Err(InterpreterError::NoValueForId { id, location })
+        self.mutate(id, argument, location)
     }
 
     /// Mutate an existing variable, potentially from a prior scope
@@ -263,17 +257,12 @@ impl<'a> Interpreter<'a> {
                 return Ok(());
             }
         }
-        Err(InterpreterError::NoValueForId { id, location })
+        let name = self.interner.definition(id).name.clone();
+        Err(InterpreterError::NonComptimeVarReferenced { name, location })
     }
 
     fn lookup(&self, ident: &HirIdent) -> IResult<Value> {
-        for scope in self.scopes.iter().rev() {
-            if let Some(value) = scope.get(&ident.id) {
-                return Ok(value.clone());
-            }
-        }
-
-        Err(InterpreterError::NoValueForId { id: ident.id, location: ident.location })
+        self.lookup_id(ident.id, ident.location)
     }
 
     fn lookup_id(&self, id: DefinitionId, location: Location) -> IResult<Value> {
@@ -283,7 +272,12 @@ impl<'a> Interpreter<'a> {
             }
         }
 
-        Err(InterpreterError::NoValueForId { id, location })
+        // Justification for `NonComptimeVarReferenced`:
+        // If we have an id to lookup at all that means name resolution successfully
+        // found another variable in scope for this name. If the name is in scope
+        // but unknown by the interpreter it must be because it was not a comptime variable.
+        let name = self.interner.definition(id).name.clone();
+        Err(InterpreterError::NonComptimeVarReferenced { name, location })
     }
 
     fn type_check(&self, typ: &Type, value: &Value, location: Location) -> IResult<()> {
@@ -1023,7 +1017,7 @@ impl<'a> Interpreter<'a> {
             HirStatement::Break => self.evaluate_break(statement),
             HirStatement::Continue => self.evaluate_continue(statement),
             HirStatement::Expression(expression) => self.evaluate(expression),
-            HirStatement::CompTime(statement) => self.evaluate_comptime(statement),
+            HirStatement::Comptime(statement) => self.evaluate_comptime(statement),
             HirStatement::Semi(expression) => {
                 self.evaluate(expression)?;
                 Ok(Value::Unit)

compiler/noirc_frontend/src/hir/comptime/scan.rs

@@ -1,15 +1,15 @@
 //! This module is for the scanning of the Hir by the interpreter.
-//! In this initial step, the Hir is scanned for `CompTime` nodes
+//! In this initial step, the Hir is scanned for `Comptime` nodes
 //! without actually executing anything until such a node is found.
 //! Once such a node is found, the interpreter will call the relevant
 //! evaluate method on that node type, insert the result into the Ast,
 //! and continue scanning the rest of the program.
 //!
-//! Since it mostly just needs to recur on the Hir looking for CompTime
+//! Since it mostly just needs to recur on the Hir looking for Comptime
 //! nodes, this pass is fairly simple. The only thing it really needs to
 //! ensure to do is to push and pop scopes on the interpreter as needed
 //! so that any variables defined within e.g. an `if` statement containing
-//! a `CompTime` block aren't accessible outside of the `if`.
+//! a `Comptime` block aren't accessible outside of the `if`.
 use crate::{
     hir_def::{
         expr::{
@@ -30,7 +30,7 @@ use super::{
 
 #[allow(dead_code)]
 impl<'interner> Interpreter<'interner> {
-    /// Scan through a function, evaluating any CompTime nodes found.
+    /// Scan through a function, evaluating any Comptime nodes found.
     /// These nodes will be modified in place, replaced with the
     /// result of their evaluation.
     pub fn scan_function(&mut self, function: FuncId) -> IResult<()> {
@@ -185,7 +185,7 @@ impl<'interner> Interpreter<'interner> {
             HirStatement::Expression(expression) => self.scan_expression(expression),
             HirStatement::Semi(semi) => self.scan_expression(semi),
             HirStatement::Error => Ok(()),
-            HirStatement::CompTime(comptime) => {
+            HirStatement::Comptime(comptime) => {
                 let location = self.interner.statement_location(comptime);
                 let new_expr =
                     self.evaluate_comptime(comptime)?.into_expression(self.interner, location)?;

compiler/noirc_frontend/src/hir/comptime/value.rs

@@ -16,7 +16,6 @@ use rustc_hash::FxHashMap as HashMap;
 
 use super::errors::{IResult, InterpreterError};
 
-#[allow(unused)]
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum Value {
     Unit,

compiler/noirc_frontend/src/hir/resolution/resolver.rs

@@ -1272,9 +1272,9 @@ impl<'a> Resolver<'a> {
                 HirStatement::Continue
             }
             StatementKind::Error => HirStatement::Error,
-            StatementKind::CompTime(statement) => {
+            StatementKind::Comptime(statement) => {
                 let statement = self.resolve_stmt(*statement, span);
-                HirStatement::CompTime(self.interner.push_stmt(statement))
+                HirStatement::Comptime(self.interner.push_stmt(statement))
             }
         }
     }

compiler/noirc_frontend/src/hir/type_check/stmt.rs

@@ -51,7 +51,7 @@ impl<'interner> TypeChecker<'interner> {
             HirStatement::Constrain(constrain_stmt) => self.check_constrain_stmt(constrain_stmt),
             HirStatement::Assign(assign_stmt) => self.check_assign_stmt(assign_stmt, stmt_id),
             HirStatement::For(for_loop) => self.check_for_loop(for_loop),
-            HirStatement::CompTime(statement) => return self.check_statement(&statement),
+            HirStatement::Comptime(statement) => return self.check_statement(&statement),
             HirStatement::Break | HirStatement::Continue | HirStatement::Error => (),
         }
         Type::Unit

compiler/noirc_frontend/src/hir_def/stmt.rs

@@ -20,7 +20,7 @@ pub enum HirStatement {
     Continue,
     Expression(ExprId),
     Semi(ExprId),
-    CompTime(StmtId),
+    Comptime(StmtId),
     Error,
 }
 

compiler/noirc_frontend/src/lexer/token.rs

@@ -811,7 +811,7 @@ pub enum Keyword {
     Break,
     CallData,
     Char,
-    CompTime,
+    Comptime,
     Constrain,
     Continue,
     Contract,
@@ -856,7 +856,7 @@ impl fmt::Display for Keyword {
             Keyword::Break => write!(f, "break"),
             Keyword::Char => write!(f, "char"),
             Keyword::CallData => write!(f, "call_data"),
-            Keyword::CompTime => write!(f, "comptime"),
+            Keyword::Comptime => write!(f, "comptime"),
             Keyword::Constrain => write!(f, "constrain"),
             Keyword::Continue => write!(f, "continue"),
             Keyword::Contract => write!(f, "contract"),
@@ -904,7 +904,7 @@ impl Keyword {
             "break" => Keyword::Break,
             "call_data" => Keyword::CallData,
             "char" => Keyword::Char,
-            "comptime" => Keyword::CompTime,
+            "comptime" => Keyword::Comptime,
             "constrain" => Keyword::Constrain,
             "continue" => Keyword::Continue,
             "contract" => Keyword::Contract,