Raise an error if recursion depth exceeds a maximum value

This is preferable to having the assembler crash with a stack overflow
error, because the user can now see which invocation caused the
overflow.

3 files changed, 10 insertions, 0 deletions

diff --git a/src/lib.rs b/src/lib.rs
index 9b9b87a..09e2fbb 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,4 +1,5 @@
 const MAX_ITERATIONS_TO_STABILISE: usize = 4;
+const MAX_RECURSION_DEPTH: usize = 1024;
 
 mod stages;
 mod types;
diff --git a/src/stages/intermediate.rs b/src/stages/intermediate.rs
index 07773a0..8cabe26 100644
--- a/src/stages/intermediate.rs
+++ b/src/stages/intermediate.rs
@@ -438,6 +438,12 @@ impl IntermediateParser {
                             unreachable!("Uncaught duplicate macro argument name '{name}'");
                         };
                     }
+                    // Test the current recursion depth.
+                    if self.environment_stack.len() == MAX_RECURSION_DEPTH {
+                        let error = IntermediateError::MaxRecursionDepthExceeded;
+                        self.errors.push(Tracked::from(error, source.clone()));
+                        return None;
+                    }
                     // Invoke the macro once.
                     let env = Environment { arguments: argument_map, id: next_id!() };
                     self.environment_stack.push(env);
diff --git a/src/stages/intermediate_tokens.rs b/src/stages/intermediate_tokens.rs
index 2fb29fa..d796299 100644
--- a/src/stages/intermediate_tokens.rs
+++ b/src/stages/intermediate_tokens.rs
@@ -77,6 +77,7 @@ pub enum IntermediateError {
     /// expected, received
     ValueTooWide(u32, u32),
     LabelNeverStabilised(String),
+    MaxRecursionDepthExceeded,
 }
 
 pub fn report_intermediate_errors(errors: &[Tracked<IntermediateError>], source_code: &str) {
@@ -116,6 +117,8 @@ fn report_intermediate_error(error: &Tracked<IntermediateError>, source_code: &s
 
         IntermediateError::LabelNeverStabilised(name) =>
             &format!("Label '{name}' never stabilised"),
+        IntermediateError::MaxRecursionDepthExceeded =>
+            &format!("Macro invocation exceededs the maximum recursion depth of {MAX_RECURSION_DEPTH}"),
     };
 
     report_source_issue(LogLevel::Error, &context, message);