feat(experimental): comptime globals

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.
@@ -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.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 {
@@ -332,6 +335,7 @@ impl<'a> Interpreter<'a> {
             }
             DefinitionKind::Local(_) => self.lookup(&ident),
             DefinitionKind::Global(global_id) => {
+                // Don't need to check let_.comptime, we can evaluate non-comptime globals too.
                 let let_ = self.interner.get_global_let_statement(*global_id).unwrap();
                 self.evaluate_let(let_)?;
                 self.lookup(&ident)
@@ -1027,7 +1031,7 @@ 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/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/resolution/resolver.rs

@@ -1238,6 +1238,7 @@ impl<'a> Resolver<'a> {
             r#type: self.resolve_type(let_stmt.r#type),
             expression,
             attributes: let_stmt.attributes,
+            comptime: let_stmt.comptime,
         })
     }
 
@@ -1251,6 +1252,7 @@ impl<'a> Resolver<'a> {
                     r#type: self.resolve_type(let_stmt.r#type),
                     expression,
                     attributes: let_stmt.attributes,
+                    comptime: let_stmt.comptime,
                 })
             }
             StatementKind::Constrain(constrain_stmt) => {
@@ -1322,8 +1324,10 @@ impl<'a> Resolver<'a> {
             }
             StatementKind::Error => HirStatement::Error,
             StatementKind::Comptime(statement) => {
-                let statement = self.resolve_stmt(*statement, span);
-                HirStatement::Comptime(self.interner.push_stmt(statement))
+                let hir_statement = self.resolve_stmt(statement.kind, statement.span);
+                let statement_id = self.interner.push_stmt(hir_statement);
+                self.interner.push_statement_location(statement_id, statement.span, self.file);
+                HirStatement::Comptime(statement_id)
             }
         }
     }

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

@@ -508,6 +508,7 @@ pub mod test {
             r#type: Type::FieldElement,
             expression: expr_id,
             attributes: vec![],
+            comptime: false,
         };
         let stmt_id = interner.push_stmt(HirStatement::Let(let_stmt));
         let expr_id = interner

compiler/noirc_frontend/src/hir_def/stmt.rs

@@ -30,6 +30,7 @@ pub struct HirLetStatement {
     pub r#type: Type,
     pub expression: ExprId,
     pub attributes: Vec<SecondaryAttribute>,
+    pub comptime: bool,
 }
 
 impl HirLetStatement {

compiler/noirc_frontend/src/parser/parser.rs

@@ -532,15 +532,16 @@ where
     P2: ExprParser + 'a,
     S: NoirParser<StatementKind> + 'a,
 {
-    keyword(Keyword::Comptime)
-        .ignore_then(choice((
-            declaration(expr),
-            for_loop(expr_no_constructors, statement.clone()),
-            block(statement).map_with_span(|block, span| {
-                StatementKind::Expression(Expression::new(ExpressionKind::Block(block), span))
-            }),
-        )))
-        .map(|statement| StatementKind::Comptime(Box::new(statement)))
+    let comptime_statement = choice((
+        declaration(expr),
+        for_loop(expr_no_constructors, statement.clone()),
+        block(statement).map_with_span(|block, span| {
+            StatementKind::Expression(Expression::new(ExpressionKind::Block(block), span))
+        }),
+    ))
+    .map_with_span(|kind, span| Box::new(Statement { kind, span }));
+
+    keyword(Keyword::Comptime).ignore_then(comptime_statement).map(StatementKind::Comptime)
 }
 
 /// Comptime in an expression position only accepts entire blocks

test_programs/noir_test_success/comptime_globals/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "comptime_globals"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.27.0"
+
+[dependencies]

test_programs/noir_test_success/comptime_globals/src/main.nr

@@ -0,0 +1,21 @@
+// Normal globals can be evaluated in a comptime context too,
+// but comptime globals can only be evaluated in a comptime context.
+comptime global FOO: Field = foo();
+
+// Due to this function's mutability and branching, SSA currently fails
+// to fold this function into a constant before the assert_constant check
+// is evaluated before loop unrolling.
+fn foo() -> Field {
+    let mut three = 3;
+    if three == 3 { 5 } else { 6 }
+}
+
+#[test]
+fn foo_global_constant() {
+    assert_constant(FOO);
+}
+
+#[test(should_fail)]
+fn foo_function_not_constant() {
+    assert_constant(foo());
+}

tooling/nargo_fmt/src/visitor/stmt.rs

@@ -103,7 +103,7 @@ impl super::FmtVisitor<'_> {
             StatementKind::Error => unreachable!(),
             StatementKind::Break => self.push_rewrite("break;".into(), span),
             StatementKind::Continue => self.push_rewrite("continue;".into(), span),
-            StatementKind::Comptime(statement) => self.visit_stmt(*statement, span, is_last),
+            StatementKind::Comptime(statement) => self.visit_stmt(statement.kind, span, is_last),
         }
     }
 }