feat: loop keyword in runtime and comptime code

compiler/noirc_evaluator/src/ssa/opt/unrolling.rs

@@ -310,11 +310,13 @@ impl Loop {
             // simplified to a simple jump.
             return None;
         }
-        assert_eq!(
-            instructions.len(),
-            1,
-            "The header should just compare the induction variable and jump"
-        );
+
+        if instructions.len() != 1 {
+            // The header should just compare the induction variable and jump.
+            // If that's not the case, this might be a `loop` and not a `for` loop.
+            return None;
+        }
+
         match &function.dfg[instructions[0]] {
             Instruction::Binary(Binary { lhs: _, operator: BinaryOp::Lt, rhs }) => {
                 function.dfg.get_numeric_constant(*rhs)
@@ -750,7 +752,13 @@ fn get_induction_variable(function: &Function, block: BasicBlockId) -> Result<Va
             // block parameters. If that becomes the case we'll need to figure out which variable
             // is generally constant and increasing to guess which parameter is the induction
             // variable.
-            assert_eq!(arguments.len(), 1, "It is expected that a loop's induction variable is the only block parameter of the loop header");
+            if arguments.len() != 1 {
+                // It is expected that a loop's induction variable is the only block parameter of the loop header.
+                // If there's no variable this might be a `loop`.
+                let call_stack = function.dfg.get_call_stack(*location);
+                return Err(call_stack);
+            }
+
             let value = arguments[0];
             if function.dfg.get_numeric_constant(value).is_some() {
                 Ok(value)

compiler/noirc_evaluator/src/ssa/ssa_gen/context.rs

@@ -88,7 +88,8 @@ pub(super) struct SharedContext {
 #[derive(Copy, Clone)]
 pub(super) struct Loop {
     pub(super) loop_entry: BasicBlockId,
-    pub(super) loop_index: ValueId,
+    /// The loop index will be `Some` for a `for` and `None` for a `loop`
+    pub(super) loop_index: Option<ValueId>,
     pub(super) loop_end: BasicBlockId,
 }
 
@@ -1010,13 +1011,8 @@ impl<'a> FunctionContext<'a> {
         }
     }
 
-    pub(crate) fn enter_loop(
-        &mut self,
-        loop_entry: BasicBlockId,
-        loop_index: ValueId,
-        loop_end: BasicBlockId,
-    ) {
-        self.loops.push(Loop { loop_entry, loop_index, loop_end });
+    pub(crate) fn enter_loop(&mut self, loop_: Loop) {
+        self.loops.push(loop_);
     }
 
     pub(crate) fn exit_loop(&mut self) {

compiler/noirc_evaluator/src/ssa/ssa_gen/mod.rs

@@ -6,7 +6,7 @@ use acvm::AcirField;
 use noirc_frontend::token::FmtStrFragment;
 pub(crate) use program::Ssa;
 
-use context::SharedContext;
+use context::{Loop, SharedContext};
 use iter_extended::{try_vecmap, vecmap};
 use noirc_errors::Location;
 use noirc_frontend::ast::{UnaryOp, Visibility};
@@ -152,6 +152,7 @@ impl<'a> FunctionContext<'a> {
             Expression::Index(index) => self.codegen_index(index),
             Expression::Cast(cast) => self.codegen_cast(cast),
             Expression::For(for_expr) => self.codegen_for(for_expr),
+            Expression::Loop(block) => self.codegen_loop(block),
             Expression::If(if_expr) => self.codegen_if(if_expr),
             Expression::Tuple(tuple) => self.codegen_tuple(tuple),
             Expression::ExtractTupleField(tuple, index) => {
@@ -557,7 +558,7 @@ impl<'a> FunctionContext<'a> {
 
         // Remember the blocks and variable used in case there are break/continue instructions
         // within the loop which need to jump to them.
-        self.enter_loop(loop_entry, loop_index, loop_end);
+        self.enter_loop(Loop { loop_entry, loop_index: Some(loop_index), loop_end });
 
         // Set the location of the initial jmp instruction to the start range. This is the location
         // used to issue an error if the start range cannot be determined at compile-time.
@@ -587,6 +588,38 @@ impl<'a> FunctionContext<'a> {
         Ok(Self::unit_value())
     }
 
+    /// Codegens a loop, creating three new blocks in the process.
+    /// The return value of a loop is always a unit literal.
+    ///
+    /// For example, the loop `loop { body }` is codegen'd as:
+    ///
+    /// ```text
+    ///   br loop_body()
+    /// loop_body():
+    ///   v3 = ... codegen body ...
+    ///   br loop_body()
+    /// loop_end():
+    ///   ... This is the current insert point after codegen_for finishes ...
+    /// ```
+    fn codegen_loop(&mut self, block: &Expression) -> Result<Values, RuntimeError> {
+        let loop_body = self.builder.insert_block();
+        let loop_end = self.builder.insert_block();
+
+        self.enter_loop(Loop { loop_entry: loop_body, loop_index: None, loop_end });
+
+        self.builder.terminate_with_jmp(loop_body, vec![]);
+
+        // Compile the loop body
+        self.builder.switch_to_block(loop_body);
+        self.codegen_expression(block)?;
+        self.builder.terminate_with_jmp(loop_body, vec![]);
+
+        // Finish by switching to the end of the loop
+        self.builder.switch_to_block(loop_end);
+        self.exit_loop();
+        Ok(Self::unit_value())
+    }
+
     /// Codegens an if expression, handling the case of what to do if there is no 'else'.
     ///
     /// For example, the expression `if cond { a } else { b }` is codegen'd as:
@@ -852,8 +885,12 @@ impl<'a> FunctionContext<'a> {
         let loop_ = self.current_loop();
 
         // Must remember to increment i before jumping
-        let new_loop_index = self.make_offset(loop_.loop_index, 1);
-        self.builder.terminate_with_jmp(loop_.loop_entry, vec![new_loop_index]);
+        if let Some(loop_index) = loop_.loop_index {
+            let new_loop_index = self.make_offset(loop_index, 1);
+            self.builder.terminate_with_jmp(loop_.loop_entry, vec![new_loop_index]);
+        } else {
+            self.builder.terminate_with_jmp(loop_.loop_entry, vec![]);
+        }
         Self::unit_value()
     }
 }

compiler/noirc_frontend/src/elaborator/lints.rs

@@ -282,6 +282,7 @@ fn can_return_without_recursing(interner: &NodeInterner, func_id: FuncId, expr_i
             HirStatement::Semi(e) => check(e),
             // Rust doesn't seem to check the for loop body (it's bounds might mean it's never called).
             HirStatement::For(e) => check(e.start_range) && check(e.end_range),
+            HirStatement::Loop(e) => check(e),
             HirStatement::Constrain(_)
             | HirStatement::Comptime(_)
             | HirStatement::Break

compiler/noirc_frontend/src/elaborator/statements.rs

@@ -271,11 +271,25 @@ impl<'context> Elaborator<'context> {
 
     pub(super) fn elaborate_loop(
         &mut self,
-        _block: Expression,
+        block: Expression,
         span: noirc_errors::Span,
     ) -> (HirStatement, Type) {
-        self.push_err(ResolverError::LoopNotYetSupported { span });
-        (HirStatement::Error, Type::Unit)
+        let in_constrained_function = self.in_constrained_function();
+        if in_constrained_function {
+            self.push_err(ResolverError::LoopInConstrainedFn { span });
+        }
+
+        self.nested_loops += 1;
+        self.push_scope();
+
+        let (block, _block_type) = self.elaborate_expression(block);
+
+        self.pop_scope();
+        self.nested_loops -= 1;
+
+        let statement = HirStatement::Loop(block);
+
+        (statement, Type::Unit)
     }
 
     fn elaborate_jump(&mut self, is_break: bool, span: noirc_errors::Span) -> (HirStatement, Type) {

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

@@ -246,6 +246,9 @@ pub enum InterpreterError {
     GlobalsDependencyCycle {
         location: Location,
     },
+    LoopHaltedForUiResponsiveness {
+        location: Location,
+    },
 
     // These cases are not errors, they are just used to prevent us from running more code
     // until the loop can be resumed properly. These cases will never be displayed to users.
@@ -323,7 +326,8 @@ impl InterpreterError {
             | InterpreterError::CannotSetFunctionBody { location, .. }
             | InterpreterError::UnknownArrayLength { location, .. }
             | InterpreterError::CannotInterpretFormatStringWithErrors { location }
-            | InterpreterError::GlobalsDependencyCycle { location } => *location,
+            | InterpreterError::GlobalsDependencyCycle { location }
+            | InterpreterError::LoopHaltedForUiResponsiveness { location } => *location,
 
             InterpreterError::FailedToParseMacro { error, file, .. } => {
                 Location::new(error.span(), *file)
@@ -683,6 +687,13 @@ impl<'a> From<&'a InterpreterError> for CustomDiagnostic {
                 let secondary = String::new();
                 CustomDiagnostic::simple_error(msg, secondary, location.span)
             }
+            InterpreterError::LoopHaltedForUiResponsiveness { location } => {
+                let msg = "This loop took too much time to execute so it was halted for UI responsiveness"
+                    .to_string();
+                let secondary =
+                    "This error doesn't happen in normal executions of `nargo`".to_string();
+                CustomDiagnostic::simple_warning(msg, secondary, location.span)
+            }
         }
     }
 }

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

@@ -59,6 +59,7 @@ impl HirStatement {
                 block: for_stmt.block.to_display_ast(interner),
                 span,
             }),
+            HirStatement::Loop(block) => StatementKind::Loop(block.to_display_ast(interner)),
             HirStatement::Break => StatementKind::Break,
             HirStatement::Continue => StatementKind::Continue,
             HirStatement::Expression(expr) => {

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

@@ -1550,6 +1550,7 @@ impl<'local, 'interner> Interpreter<'local, 'interner> {
             HirStatement::Constrain(constrain) => self.evaluate_constrain(constrain),
             HirStatement::Assign(assign) => self.evaluate_assign(assign),
             HirStatement::For(for_) => self.evaluate_for(for_),
+            HirStatement::Loop(expression) => self.evaluate_loop(expression),
             HirStatement::Break => self.evaluate_break(statement),
             HirStatement::Continue => self.evaluate_continue(statement),
             HirStatement::Expression(expression) => self.evaluate(expression),
@@ -1741,6 +1742,34 @@ impl<'local, 'interner> Interpreter<'local, 'interner> {
         Ok(Value::Unit)
     }
 
+    fn evaluate_loop(&mut self, expr: ExprId) -> IResult<Value> {
+        let was_in_loop = std::mem::replace(&mut self.in_loop, true);
+        let in_lsp = self.elaborator.interner.is_in_lsp_mode();
+        let mut counter = 0;
+
+        loop {
+            self.push_scope();
+
+            match self.evaluate(expr) {
+                Ok(_) => (),
+                Err(InterpreterError::Break) => break,
+                Err(InterpreterError::Continue) => continue,
+                Err(other) => return Err(other),
+            }
+
+            self.pop_scope();
+
+            counter += 1;
+            if in_lsp && counter == 10_000 {
+                let location = self.elaborator.interner.expr_location(&expr);
+                return Err(InterpreterError::LoopHaltedForUiResponsiveness { location });
+            }
+        }
+
+        self.in_loop = was_in_loop;
+        Ok(Value::Unit)
+    }
+
     fn evaluate_break(&mut self, id: StmtId) -> IResult<Value> {
         if self.in_loop {
             Err(InterpreterError::Break)

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

@@ -98,6 +98,8 @@ pub enum ResolverError {
     DependencyCycle { span: Span, item: String, cycle: String },
     #[error("break/continue are only allowed in unconstrained functions")]
     JumpInConstrainedFn { is_break: bool, span: Span },
+    #[error("loop is only allowed in unconstrained functions")]
+    LoopInConstrainedFn { span: Span },
     #[error("break/continue are only allowed within loops")]
     JumpOutsideLoop { is_break: bool, span: Span },
     #[error("Only `comptime` globals can be mutable")]
@@ -434,6 +436,13 @@ impl<'a> From<&'a ResolverError> for Diagnostic {
                     *span,
                 )
             },
+            ResolverError::LoopInConstrainedFn { span } => {
+                Diagnostic::simple_error(
+                    "loop is only allowed in unconstrained functions".into(),
+                    "Constrained code must always have a known number of loop iterations".into(),
+                    *span,
+                )
+            },
             ResolverError::JumpOutsideLoop { is_break, span } => {
                 let item = if *is_break { "break" } else { "continue" };
                 Diagnostic::simple_error(

compiler/noirc_frontend/src/hir_def/stmt.rs

@@ -16,6 +16,7 @@ pub enum HirStatement {
     Constrain(HirConstrainStatement),
     Assign(HirAssignStatement),
     For(HirForStatement),
+    Loop(ExprId),
     Break,
     Continue,
     Expression(ExprId),

compiler/noirc_frontend/src/monomorphization/ast.rs

@@ -36,6 +36,7 @@ pub enum Expression {
     Index(Index),
     Cast(Cast),
     For(For),
+    Loop(Box<Expression>),
     If(If),
     Tuple(Vec<Expression>),
     ExtractTupleField(Box<Expression>, usize),

compiler/noirc_frontend/src/monomorphization/mod.rs

@@ -695,6 +695,10 @@ impl<'interner> Monomorphizer<'interner> {
                     block,
                 }))
             }
+            HirStatement::Loop(block) => {
+                let block = Box::new(self.expr(block)?);
+                Ok(ast::Expression::Loop(block))
+            }
             HirStatement::Expression(expr) => self.expr(expr),
             HirStatement::Semi(expr) => {
                 self.expr(expr).map(|expr| ast::Expression::Semi(Box::new(expr)))

compiler/noirc_frontend/src/monomorphization/printer.rs

@@ -49,6 +49,7 @@ impl AstPrinter {
                 write!(f, " as {})", cast.r#type)
             }
             Expression::For(for_expr) => self.print_for(for_expr, f),
+            Expression::Loop(block) => self.print_loop(block, f),
             Expression::If(if_expr) => self.print_if(if_expr, f),
             Expression::Tuple(tuple) => self.print_tuple(tuple, f),
             Expression::ExtractTupleField(expr, index) => {
@@ -209,6 +210,15 @@ impl AstPrinter {
         write!(f, "}}")
     }
 
+    fn print_loop(&mut self, block: &Expression, f: &mut Formatter) -> Result<(), std::fmt::Error> {
+        write!(f, "loop {{")?;
+        self.indent_level += 1;
+        self.print_expr_expect_block(block, f)?;
+        self.indent_level -= 1;
+        self.next_line(f)?;
+        write!(f, "}}")
+    }
+
     fn print_if(
         &mut self,
         if_expr: &super::ast::If,

compiler/noirc_frontend/src/parser/parser/statement.rs

@@ -294,10 +294,13 @@ impl<'a> Parser<'a> {
 
     /// LoopStatement = 'loop' Block
     fn parse_loop(&mut self) -> Option<Expression> {
+        let start_span = self.current_token_span;
         if !self.eat_keyword(Keyword::Loop) {
             return None;
         }
 
+        self.push_error(ParserErrorReason::ExperimentalFeature("loops"), start_span);
+
         let block_start_span = self.current_token_span;
         let block = if let Some(block) = self.parse_block() {
             Expression {
@@ -819,7 +822,8 @@ mod tests {
     #[test]
     fn parses_empty_loop() {
         let src = "loop { }";
-        let statement = parse_statement_no_errors(src);
+        let mut parser = Parser::for_str(src);
+        let statement = parser.parse_statement_or_error();
         let StatementKind::Loop(block) = statement.kind else {
             panic!("Expected loop");
         };
@@ -832,7 +836,8 @@ mod tests {
     #[test]
     fn parses_loop_with_statements() {
         let src = "loop { 1; 2 }";
-        let statement = parse_statement_no_errors(src);
+        let mut parser = Parser::for_str(src);
+        let statement = parser.parse_statement_or_error();
         let StatementKind::Loop(block) = statement.kind else {
             panic!("Expected loop");
         };

compiler/noirc_frontend/src/tests.rs

@@ -903,6 +903,7 @@ fn find_lambda_captures(stmts: &[StmtId], interner: &NodeInterner, result: &mut
             HirStatement::Constrain(constr_stmt) => constr_stmt.0,
             HirStatement::Semi(semi_expr) => semi_expr,
             HirStatement::For(for_loop) => for_loop.block,
+            HirStatement::Loop(block) => block,
             HirStatement::Error => panic!("Invalid HirStatement!"),
             HirStatement::Break => panic!("Unexpected break"),
             HirStatement::Continue => panic!("Unexpected continue"),