feat(experimental): comptime globals (#4918)

# Description

## Problem\*

Resolves #4916 

## Summary\*

Implements immutable `comptime` globals.

I've made a separate issue for mutable comptime globals since those
would require allowing a `mut` modifier when parsing globals in general.

## Additional Context

The scopes of the interpreter were already set up for globals to work.
The main main part that was needed here was evaluating comptime-known
identifiers during the scan pass. Without this monomorphization would
just use the original rhs of each global.

## Documentation\*

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

# PR Checklist\*

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

---------

Co-authored-by: Tom French <15848336+TomAFrench@users.noreply.github.com>

aztec_macros/src/utils/hir_utils.rs

@@ -228,6 +228,7 @@ pub fn inject_global(
         module_id,
         file_id,
         global.attributes.clone(),
+        false,
     );
 
     // Add the statement to the scope so its path can be looked up later

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<Statement>),
     // This is an expression with a trailing semi-colon
     Semi(Expression),
     // This statement is the result of a recovered parse error.
@@ -486,7 +486,8 @@ impl Pattern {
     pub fn name_ident(&self) -> &Ident {
         match self {
             Pattern::Identifier(name_ident) => name_ident,
-            _ => panic!("only the identifier pattern can return a name"),
+            Pattern::Mutable(pattern, ..) => pattern.name_ident(),
+            _ => panic!("Only the Identifier or Mutable patterns can return a name"),
         }
     }
 
@@ -685,7 +686,7 @@ 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.kind),
             StatementKind::Semi(semi) => write!(f, "{semi};"),
             StatementKind::Error => write!(f, "Error"),
         }

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

@@ -67,7 +67,7 @@ impl StmtId {
             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))
+                StatementKind::Comptime(Box::new(statement.to_ast(interner)))
             }
         };
 

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

@@ -134,8 +134,11 @@ impl<'a> Interpreter<'a> {
     /// `exit_function` is called.
     pub(super) fn enter_function(&mut self) -> (bool, Vec<HashMap<DefinitionId, Value>>) {
         // Drain every scope except the global scope
-        let scope = self.scopes.drain(1..).collect();
-        self.push_scope();
+        let mut scope = Vec::new();
+        if self.scopes.len() > 1 {
+            scope = self.scopes.drain(1..).collect();
+            self.push_scope();
+        }
         (std::mem::take(&mut self.in_loop), scope)
     }
 
@@ -160,7 +163,7 @@ impl<'a> Interpreter<'a> {
         self.scopes.last_mut().unwrap()
     }
 
-    fn define_pattern(
+    pub(super) fn define_pattern(
         &mut self,
         pattern: &HirPattern,
         typ: &Type,
@@ -262,7 +265,7 @@ impl<'a> Interpreter<'a> {
         Err(InterpreterError::NonComptimeVarReferenced { name, location })
     }
 
-    fn lookup(&self, ident: &HirIdent) -> IResult<Value> {
+    pub(super) fn lookup(&self, ident: &HirIdent) -> IResult<Value> {
         self.lookup_id(ident.id, ident.location)
     }
 
@@ -291,7 +294,7 @@ impl<'a> Interpreter<'a> {
     }
 
     /// Evaluate an expression and return the result
-    fn evaluate(&mut self, id: ExprId) -> IResult<Value> {
+    pub(super) fn evaluate(&mut self, id: ExprId) -> IResult<Value> {
         match self.interner.expression(&id) {
             HirExpression::Ident(ident) => self.evaluate_ident(ident, id),
             HirExpression::Literal(literal) => self.evaluate_literal(literal, id),
@@ -322,7 +325,7 @@ impl<'a> Interpreter<'a> {
         }
     }
 
-    fn evaluate_ident(&mut self, ident: HirIdent, id: ExprId) -> IResult<Value> {
+    pub(super) fn evaluate_ident(&mut self, ident: HirIdent, id: ExprId) -> IResult<Value> {
         let definition = self.interner.definition(ident.id);
 
         match &definition.kind {
@@ -332,9 +335,15 @@ impl<'a> Interpreter<'a> {
             }
             DefinitionKind::Local(_) => self.lookup(&ident),
             DefinitionKind::Global(global_id) => {
-                let let_ = self.interner.get_global_let_statement(*global_id).unwrap();
-                self.evaluate_let(let_)?;
-                self.lookup(&ident)
+                // Don't need to check let_.comptime, we can evaluate non-comptime globals too.
+                // Avoid resetting the value if it is already known
+                if let Ok(value) = self.lookup(&ident) {
+                    Ok(value)
+                } else {
+                    let let_ = self.interner.get_global_let_statement(*global_id).unwrap();
+                    self.evaluate_let(let_)?;
+                    self.lookup(&ident)
+                }
             }
             DefinitionKind::GenericType(type_variable) => {
                 let value = match &*type_variable.borrow() {
@@ -1027,7 +1036,7 @@ impl<'a> Interpreter<'a> {
         }
     }
 
-    fn evaluate_let(&mut self, let_: HirLetStatement) -> IResult<Value> {
+    pub(super) fn evaluate_let(&mut self, let_: HirLetStatement) -> IResult<Value> {
         let rhs = self.evaluate(let_.expression)?;
         let location = self.interner.expr_location(&let_.expression);
         self.define_pattern(&let_.pattern, &let_.r#type, rhs, location)?;

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

@@ -7,3 +7,4 @@ mod value;
 
 pub use errors::InterpreterError;
 pub use interpreter::Interpreter;
+pub use value::Value;

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

@@ -14,18 +14,19 @@ use crate::{
     hir_def::{
         expr::{
             HirArrayLiteral, HirBlockExpression, HirCallExpression, HirConstructorExpression,
-            HirIfExpression, HirIndexExpression, HirInfixExpression, HirLambda,
+            HirIdent, HirIfExpression, HirIndexExpression, HirInfixExpression, HirLambda,
             HirMethodCallExpression,
         },
         stmt::HirForStatement,
     },
     macros_api::{HirExpression, HirLiteral, HirStatement},
-    node_interner::{ExprId, FuncId, StmtId},
+    node_interner::{DefinitionKind, ExprId, FuncId, GlobalId, StmtId},
 };
 
 use super::{
     errors::{IResult, InterpreterError},
     interpreter::Interpreter,
+    Value,
 };
 
 #[allow(dead_code)]
@@ -48,9 +49,23 @@ impl<'interner> Interpreter<'interner> {
         Ok(())
     }
 
+    /// Evaluate this global if it is a comptime global.
+    /// Otherwise, scan through its expression for any comptime blocks to evaluate.
+    pub fn scan_global(&mut self, global: GlobalId) -> IResult<()> {
+        if let Some(let_) = self.interner.get_global_let_statement(global) {
+            if let_.comptime {
+                self.evaluate_let(let_)?;
+            } else {
+                self.scan_expression(let_.expression)?;
+            }
+        }
+
+        Ok(())
+    }
+
     fn scan_expression(&mut self, expr: ExprId) -> IResult<()> {
         match self.interner.expression(&expr) {
-            HirExpression::Ident(_) => Ok(()),
+            HirExpression::Ident(ident) => self.scan_ident(ident, expr),
             HirExpression::Literal(literal) => self.scan_literal(literal),
             HirExpression::Block(block) => self.scan_block(block),
             HirExpression::Prefix(prefix) => self.scan_expression(prefix.rhs),
@@ -91,6 +106,27 @@ impl<'interner> Interpreter<'interner> {
         }
     }
 
+    // Identifiers have no code to execute but we may need to inline any values
+    // of comptime variables into runtime code.
+    fn scan_ident(&mut self, ident: HirIdent, id: ExprId) -> IResult<()> {
+        let definition = self.interner.definition(ident.id);
+
+        match &definition.kind {
+            DefinitionKind::Function(_) => Ok(()),
+            _ => {
+                // Opportunistically evaluate this identifier to see if it is compile-time known.
+                // If so, inline its value.
+                if let Ok(value) = self.evaluate_ident(ident, id) {
+                    // TODO(#4922): Inlining closures is currently unimplemented
+                    if !matches!(value, Value::Closure(..)) {
+                        self.inline_expression(value, id)?;
+                    }
+                }
+                Ok(())
+            }
+        }
+    }
+
     fn scan_literal(&mut self, literal: HirLiteral) -> IResult<()> {
         match literal {
             HirLiteral::Array(elements) | HirLiteral::Slice(elements) => match elements {
@@ -210,4 +246,12 @@ impl<'interner> Interpreter<'interner> {
         self.pop_scope();
         Ok(())
     }
+
+    fn inline_expression(&mut self, value: Value, expr: ExprId) -> IResult<()> {
+        let location = self.interner.expr_location(&expr);
+        let new_expr = value.into_expression(self.interner, location)?;
+        let new_expr = self.interner.expression(&new_expr);
+        self.interner.replace_expr(&expr, new_expr);
+        Ok(())
+    }
 }

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

@@ -76,6 +76,20 @@ fn mutating_arrays() {
     assert_eq!(result, Value::U8(22));
 }
 
+#[test]
+fn mutate_in_new_scope() {
+    let program = "fn main() -> pub u8 {
+        let mut x = 0;
+        x += 1;
+        {
+            x += 1;
+        }
+        x
+    }";
+    let result = interpret(program, vec!["main".into()]);
+    assert_eq!(result, Value::U8(2));
+}
+
 #[test]
 fn for_loop() {
     let program = "fn main() -> pub u8 {

compiler/noirc_frontend/src/hir/def_collector/dc_crate.rs

@@ -158,6 +158,13 @@ pub enum CompilationError {
     InterpreterError(InterpreterError),
 }
 
+impl CompilationError {
+    fn is_error(&self) -> bool {
+        let diagnostic = CustomDiagnostic::from(self);
+        diagnostic.is_error()
+    }
+}
+
 impl<'a> From<&'a CompilationError> for CustomDiagnostic {
     fn from(value: &'a CompilationError) -> Self {
         match value {
@@ -404,10 +411,15 @@ impl DefCollector {
             );
         }
 
-        resolved_module.errors.extend(context.def_interner.check_for_dependency_cycles());
+        let cycle_errors = context.def_interner.check_for_dependency_cycles();
+        let cycles_present = !cycle_errors.is_empty();
+        resolved_module.errors.extend(cycle_errors);
 
         resolved_module.type_check(context);
-        resolved_module.evaluate_comptime(&mut context.def_interner);
+
+        if !cycles_present {
+            resolved_module.evaluate_comptime(&mut context.def_interner);
+        }
 
         resolved_module.errors
     }
@@ -503,13 +515,21 @@ impl ResolvedModule {
 
     /// Evaluate all `comptime` expressions in this module
     fn evaluate_comptime(&mut self, interner: &mut NodeInterner) {
-        let mut interpreter = Interpreter::new(interner);
+        if self.count_errors() == 0 {
+            let mut interpreter = Interpreter::new(interner);
 
-        for (_file, function) in &self.functions {
-            // The file returned by the error may be different than the file the
-            // function is in so only use the error's file id.
-            if let Err(error) = interpreter.scan_function(*function) {
-                self.errors.push(error.into_compilation_error_pair());
+            for (_file, global) in &self.globals {
+                if let Err(error) = interpreter.scan_global(*global) {
+                    self.errors.push(error.into_compilation_error_pair());
+                }
+            }
+
+            for (_file, function) in &self.functions {
+                // The file returned by the error may be different than the file the
+                // function is in so only use the error's file id.
+                if let Err(error) = interpreter.scan_function(*function) {
+                    self.errors.push(error.into_compilation_error_pair());
+                }
             }
         }
     }
@@ -524,4 +544,9 @@ impl ResolvedModule {
         self.globals.extend(globals.globals);
         self.errors.extend(globals.errors);
     }
+
+    /// Counts the number of errors (minus warnings) this program currently has
+    fn count_errors(&self) -> usize {
+        self.errors.iter().filter(|(error, _)| error.is_error()).count()
+    }
 }

compiler/noirc_frontend/src/hir/def_collector/dc_mod.rs

@@ -6,7 +6,8 @@ use noirc_errors::Location;
 
 use crate::ast::{
     FunctionDefinition, Ident, ItemVisibility, LetStatement, ModuleDeclaration, NoirFunction,
-    NoirStruct, NoirTrait, NoirTraitImpl, NoirTypeAlias, TraitImplItem, TraitItem, TypeImpl,
+    NoirStruct, NoirTrait, NoirTraitImpl, NoirTypeAlias, Pattern, TraitImplItem, TraitItem,
+    TypeImpl,
 };
 use crate::{
     graph::CrateId,
@@ -109,6 +110,7 @@ impl<'a> ModCollector<'a> {
                 self.module_id,
                 self.file_id,
                 global.attributes.clone(),
+                matches!(global.pattern, Pattern::Mutable { .. }),
             );
 
             // Add the statement to the scope so its path can be looked up later
@@ -463,6 +465,7 @@ impl<'a> ModCollector<'a> {
                             trait_id.0.local_id,
                             self.file_id,
                             vec![],
+                            false,
                         );
 
                         if let Err((first_def, second_def)) = self.def_collector.def_map.modules

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

@@ -86,6 +86,8 @@ pub enum ResolverError {
     JumpInConstrainedFn { is_break: bool, span: Span },
     #[error("break/continue are only allowed within loops")]
     JumpOutsideLoop { is_break: bool, span: Span },
+    #[error("Only `comptime` globals can be mutable")]
+    MutableGlobal { span: Span },
     #[error("Self-referential structs are not supported")]
     SelfReferentialStruct { span: Span },
     #[error("#[inline(tag)] attribute is only allowed on constrained functions")]
@@ -346,6 +348,13 @@ impl<'a> From<&'a ResolverError> for Diagnostic {
                     *span,
                 )
             },
+            ResolverError::MutableGlobal { span } => {
+                Diagnostic::simple_error(
+                    "Only `comptime` globals may be mutable".into(),
+                    String::new(),
+                    *span,
+                )
+            },
             ResolverError::SelfReferentialStruct { span } => {
                 Diagnostic::simple_error(
                     "Self-referential structs are not supported".into(),