From ad66fbdfa2d39e39c2a7d7411e27cb61ada2b648 Mon Sep 17 00:00:00 2001
From: Laurenz <laurmaedje@gmail.com>
Date: Wed, 14 Dec 2022 09:56:24 +0100
Subject: [PATCH] Detect infinite loops

---
 src/model/eval.rs            | 37 ++++++++++++++++++++++++++++++++++--
 tests/typ/compiler/while.typ | 12 +++++++-----
 2 files changed, 42 insertions(+), 7 deletions(-)

diff --git a/src/model/eval.rs b/src/model/eval.rs
index 53a393c33..ef9ba0a8c 100644
--- a/src/model/eval.rs
+++ b/src/model/eval.rs
@@ -16,7 +16,7 @@ use crate::diag::{
 };
 use crate::geom::{Abs, Angle, Em, Fr, Ratio};
 use crate::syntax::ast::AstNode;
-use crate::syntax::{ast, Source, SourceId, Span, Spanned, Unit};
+use crate::syntax::{ast, Source, SourceId, Span, Spanned, SyntaxKind, SyntaxNode, Unit};
 use crate::util::{format_eco, EcoString, PathExt};
 use crate::World;
 
@@ -994,12 +994,25 @@ impl Eval for ast::WhileLoop {
     type Output = Value;
 
     fn eval(&self, vm: &mut Vm) -> SourceResult<Self::Output> {
+        const MAX_ITERS: usize = 10_000;
+
         let flow = vm.flow.take();
         let mut output = Value::None;
+        let mut i = 0;
 
         let condition = self.condition();
+        let body = self.body();
+
         while condition.eval(vm)?.cast::<bool>().at(condition.span())? {
-            let body = self.body();
+            if i == 0
+                && is_invariant(condition.as_untyped())
+                && !can_diverge(body.as_untyped())
+            {
+                bail!(condition.span(), "condition is always true");
+            } else if i >= MAX_ITERS {
+                bail!(self.span(), "loop seems to be infinite");
+            }
+
             let value = body.eval(vm)?;
             output = ops::join(output, value).at(body.span())?;
 
@@ -1012,6 +1025,8 @@ impl Eval for ast::WhileLoop {
                 Some(Flow::Return(..)) => break,
                 None => {}
             }
+
+            i += 1;
         }
 
         if flow.is_some() {
@@ -1022,6 +1037,24 @@ impl Eval for ast::WhileLoop {
     }
 }
 
+/// Whether the expression always evaluates to the same value.
+fn is_invariant(expr: &SyntaxNode) -> bool {
+    match expr.cast() {
+        Some(ast::Expr::Ident(_)) => false,
+        Some(ast::Expr::MethodCall(call)) => {
+            is_invariant(call.target().as_untyped())
+                && is_invariant(call.args().as_untyped())
+        }
+        _ => expr.children().all(is_invariant),
+    }
+}
+
+/// Whether the expression contains a break or return.
+fn can_diverge(expr: &SyntaxNode) -> bool {
+    matches!(expr.kind(), SyntaxKind::Break | SyntaxKind::Return)
+        || expr.children().any(can_diverge)
+}
+
 impl Eval for ast::ForLoop {
     type Output = Value;
 
diff --git a/tests/typ/compiler/while.typ b/tests/typ/compiler/while.typ
index 5dc5ae417..3c28a32a1 100644
--- a/tests/typ/compiler/while.typ
+++ b/tests/typ/compiler/while.typ
@@ -34,11 +34,13 @@
 #while [nope] [nope]
 
 ---
-// Make sure that we terminate and don't complain multiple times.
-#while true {
-  // Error: 3-7 unknown variable
-  nope
-}
+// Error: 8-25 condition is always true
+#while 2 < "hello".len() {}
+
+---
+// Error: 2:1-2:24 loop seems to be infinite
+#let i = 1
+#while i > 0 { i += 1 }
 
 ---
 // Error: 7 expected expression