fix: Error on empty function bodies (#5519)

# Description

## Problem\*

Resolves an issue mentioned in this PR:
#5511 And in @asterite's comptime
prefix operators PR.

## Summary\*

Previously, if a function was every (erroneously) allowed to be called
in a comptime context before it had been elaborated, we'd see just an
empty block as its body. I've changed this so we don't default function
bodies to an empty block anymore. Using an option we can now properly
error in this case as well.

## 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\*

- [x] I have tested the changes locally.
- [x] 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>

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

@@ -129,7 +129,11 @@ impl<'a> Interpreter<'a> {
             self.define_pattern(parameter, typ, argument, arg_location)?;
         }
 
-        let function_body = self.interner.function(&function).as_expr();
+        let function_body = self.interner.function(&function).try_as_expr().ok_or_else(|| {
+            let function = self.interner.function_name(&function).to_owned();
+            InterpreterError::NonComptimeFnCallInSameCrate { function, location }
+        })?;
+
         let result = self.evaluate(function_body)?;
 
         self.exit_function(previous_state);

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

@@ -43,11 +43,12 @@ impl<'interner> Interpreter<'interner> {
             return Ok(());
         }
 
-        let function = self.interner.function(&function);
+        if let Some(function) = self.interner.function(&function).try_as_expr() {
+            let state = self.enter_function();
+            self.scan_expression(function)?;
+            self.exit_function(state);
+        }
 
-        let state = self.enter_function();
-        self.scan_expression(function.as_expr())?;
-        self.exit_function(state);
         Ok(())
     }
 

compiler/noirc_frontend/src/hir_def/expr.rs

@@ -40,13 +40,6 @@ pub enum HirExpression {
     Error,
 }
 
-impl HirExpression {
-    /// Returns an empty block expression
-    pub const fn empty_block() -> HirExpression {
-        HirExpression::Block(HirBlockExpression { statements: vec![] })
-    }
-}
-
 /// Corresponds to a variable in the source code
 #[derive(Debug, Clone)]
 pub struct HirIdent {

compiler/noirc_frontend/src/hir_def/function.rs

@@ -10,26 +10,30 @@ use crate::macros_api::{BlockExpression, StructId};
 use crate::node_interner::{ExprId, NodeInterner, TraitImplId};
 use crate::{ResolvedGeneric, Type};
 
-/// A Hir function is a block expression
-/// with a list of statements
+/// A Hir function is a block expression with a list of statements.
+/// If the function has yet to be resolved, the body starts off empty (None).
 #[derive(Debug, Clone)]
-pub struct HirFunction(ExprId);
+pub struct HirFunction(Option<ExprId>);
 
 impl HirFunction {
     pub fn empty() -> HirFunction {
-        HirFunction(ExprId::empty_block_id())
+        HirFunction(None)
     }
 
     pub const fn unchecked_from_expr(expr_id: ExprId) -> HirFunction {
-        HirFunction(expr_id)
+        HirFunction(Some(expr_id))
     }
 
-    pub const fn as_expr(&self) -> ExprId {
+    pub fn as_expr(&self) -> ExprId {
+        self.0.expect("Function has yet to be elaborated, cannot get an ExprId of its body!")
+    }
+
+    pub fn try_as_expr(&self) -> Option<ExprId> {
         self.0
     }
 
     pub fn block(&self, interner: &NodeInterner) -> HirBlockExpression {
-        match interner.expression(&self.0) {
+        match interner.expression(&self.as_expr()) {
             HirExpression::Block(block_expr) => block_expr,
             _ => unreachable!("ice: functions can only be block expressions"),
         }

compiler/noirc_frontend/src/node_interner.rs

@@ -386,11 +386,6 @@ impl StmtId {
 #[derive(Debug, Eq, PartialEq, Hash, Copy, Clone, PartialOrd, Ord)]
 pub struct ExprId(Index);
 
-impl ExprId {
-    pub fn empty_block_id() -> ExprId {
-        ExprId(Index::unsafe_zeroed())
-    }
-}
 #[derive(Debug, Eq, PartialEq, Hash, Copy, Clone)]
 pub struct FuncId(Index);
 
@@ -558,7 +553,7 @@ pub struct QuotedTypeId(noirc_arena::Index);
 
 impl Default for NodeInterner {
     fn default() -> Self {
-        let mut interner = NodeInterner {
+        NodeInterner {
             nodes: Arena::default(),
             func_meta: HashMap::new(),
             function_definition_ids: HashMap::new(),
@@ -598,12 +593,7 @@ impl Default for NodeInterner {
             reference_graph: petgraph::graph::DiGraph::new(),
             reference_graph_indices: HashMap::new(),
             reference_modules: HashMap::new(),
-        };
-
-        // An empty block expression is used often, we add this into the `node` on startup
-        let expr_id = interner.push_expr(HirExpression::empty_block());
-        assert_eq!(expr_id, ExprId::empty_block_id());
-        interner
+        }
     }
 }