feat(tvix/eval): add method for emitting runtime warnings r/5049

This lets the VM emit warnings when it encounters situations that
should only be warned about at runtime.

For starters, this is used to pass through compilation warnings that
come up when `import` is used.

Change-Id: I0c4bc8c534d699999887c430d93629fadfa662c4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6868
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

4 files changed, 57 insertions, 8 deletions

diff --git a/corp/tvixbolt/src/main.rs b/corp/tvixbolt/src/main.rs
index f81ae070a2ce..8099a5238d02 100644
--- a/corp/tvixbolt/src/main.rs
+++ b/corp/tvixbolt/src/main.rs
@@ -292,7 +292,18 @@ fn eval(trace: bool, code: &str) -> Output {
     };
 
     match result {
-        Ok(value) => writeln!(&mut out.output, "{}", value).unwrap(),
+        Ok(result) => {
+            for warning in result.warnings {
+                writeln!(
+                    &mut out.warnings,
+                    "{}\n",
+                    warning.fancy_format_str(&source).trim(),
+                )
+                .unwrap();
+            }
+
+            writeln!(&mut out.output, "{}", result.value).unwrap()
+        }
         Err(err) => writeln!(
             &mut out.runtime_errors,
             "{}",
diff --git a/tvix/eval/src/builtins/impure.rs b/tvix/eval/src/builtins/impure.rs
index 7c98fcb4e192..d73e5ccfa8f2 100644
--- a/tvix/eval/src/builtins/impure.rs
+++ b/tvix/eval/src/builtins/impure.rs
@@ -51,7 +51,7 @@ pub fn builtins_import(
     Builtin::new(
         "import",
         &[true],
-        move |mut args: Vec<Value>, _: &mut VM| {
+        move |mut args: Vec<Value>, vm: &mut VM| {
             let path = match args.pop().unwrap() {
                 Value::Path(path) => path,
                 Value::String(_) => {
@@ -102,9 +102,9 @@ pub fn builtins_import(
                 });
             }
 
-            // TODO: deal with runtime *warnings* (most likely through an
-            // emit_warning function on the VM that might return it together with
-            // the result)
+            for warning in result.warnings {
+                vm.push_warning(warning);
+            }
 
             // Compilation succeeded, we can construct a thunk from whatever it spat
             // out and return that.
diff --git a/tvix/eval/src/eval.rs b/tvix/eval/src/eval.rs
index 4e50b7279c38..5af5f9d7588e 100644
--- a/tvix/eval/src/eval.rs
+++ b/tvix/eval/src/eval.rs
@@ -115,5 +115,11 @@ pub fn interpret(code: &str, location: Option<PathBuf>, options: Options) -> Eva
         err.fancy_format_stderr(&source);
     }
 
-    result
+    result.map(|r| {
+        for warning in r.warnings {
+            warning.fancy_format_stderr(&source);
+        }
+
+        r.value
+    })
 }
diff --git a/tvix/eval/src/vm.rs b/tvix/eval/src/vm.rs
index 41f51a987459..d616b9ce1c5c 100644
--- a/tvix/eval/src/vm.rs
+++ b/tvix/eval/src/vm.rs
@@ -10,6 +10,7 @@ use crate::{
     opcode::{CodeIdx, Count, JumpOffset, OpCode, StackIdx, UpvalueIdx},
     upvalues::{UpvalueCarrier, Upvalues},
     value::{Builtin, Closure, CoercionKind, Lambda, NixAttrs, NixList, Thunk, Value},
+    warnings::{EvalWarning, WarningKind},
 };
 
 struct CallFrame {
@@ -43,9 +44,18 @@ pub struct VM<'o> {
     /// dynamically resolved (`with`).
     with_stack: Vec<usize>,
 
+    /// Runtime warnings collected during evaluation.
+    warnings: Vec<EvalWarning>,
+
     observer: &'o mut dyn RuntimeObserver,
 }
 
+/// The result of a VM's runtime evaluation.
+pub struct RuntimeResult {
+    pub value: Value,
+    pub warnings: Vec<EvalWarning>,
+}
+
 /// This macro wraps a computation that returns an ErrorKind or a
 /// result, and wraps the ErrorKind in an Error struct if present.
 ///
@@ -133,6 +143,7 @@ impl<'o> VM<'o> {
             frames: vec![],
             stack: vec![],
             with_stack: vec![],
+            warnings: vec![],
         }
     }
 
@@ -182,6 +193,20 @@ impl<'o> VM<'o> {
         }
     }
 
+    /// Push an already constructed warning.
+    pub fn push_warning(&mut self, warning: EvalWarning) {
+        self.warnings.push(warning);
+    }
+
+    /// Emit a warning with the given WarningKind and the source span
+    /// of the current instruction.
+    pub fn emit_warning(&mut self, kind: WarningKind) {
+        self.push_warning(EvalWarning {
+            kind,
+            span: self.current_span(),
+        });
+    }
+
     /// Execute the given value in this VM's context, if it is a
     /// callable.
     ///
@@ -780,9 +805,16 @@ fn unwrap_or_clone_rc<T: Clone>(rc: Rc<T>) -> T {
     Rc::try_unwrap(rc).unwrap_or_else(|rc| (*rc).clone())
 }
 
-pub fn run_lambda(observer: &mut dyn RuntimeObserver, lambda: Rc<Lambda>) -> EvalResult<Value> {
+pub fn run_lambda(
+    observer: &mut dyn RuntimeObserver,
+    lambda: Rc<Lambda>,
+) -> EvalResult<RuntimeResult> {
     let mut vm = VM::new(observer);
     let value = vm.call(lambda, Upvalues::with_capacity(0), 0)?;
     vm.force_for_output(&value)?;
-    Ok(value)
+
+    Ok(RuntimeResult {
+        value,
+        warnings: vm.warnings,
+    })
 }