fix(tvix/eval): coerce string interpolation parts to string r/4860

With this puzzle piece of string compilation in place, `compile_str`
becomes less redundant, as every part now needs to be compiled the same.
The thunking logic becomes a bit trickier, since we need to thunk even
in the case of `count == 1` if the single part is interpolating.
Splitting the inner (shared) code in a separate function turned out to
be easier for making rustc content.

Change-Id: I6a554ca599926ae5907d7acffce349c9616f568f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6582
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

1 files changed, 11 insertions, 1 deletions

diff --git a/tvix/eval/src/vm.rs b/tvix/eval/src/vm.rs
index 6fe94184b8a2..980a3ed0f5dd 100644
--- a/tvix/eval/src/vm.rs
+++ b/tvix/eval/src/vm.rs
@@ -9,7 +9,7 @@ use crate::{
     observer::Observer,
     opcode::{CodeIdx, Count, JumpOffset, OpCode, StackIdx, UpvalueIdx},
     upvalues::{UpvalueCarrier, Upvalues},
-    value::{Builtin, Closure, Lambda, NixAttrs, NixList, Thunk, Value},
+    value::{Builtin, Closure, CoercionKind, Lambda, NixAttrs, NixList, Thunk, Value},
 };
 
 struct CallFrame {
@@ -344,6 +344,16 @@ impl<'o> VM<'o> {
 
                 OpCode::OpInterpolate(Count(count)) => self.run_interpolate(count)?,
 
+                OpCode::OpCoerceToString => {
+                    // TODO: handle string context, copying to store
+                    let string = fallible!(
+                        self,
+                        // note that coerce_to_string also forces
+                        self.pop().coerce_to_string(CoercionKind::Weak, self)
+                    );
+                    self.push(Value::String(string));
+                }
+
                 OpCode::OpJump(JumpOffset(offset)) => {
                     debug_assert!(offset != 0);
                     self.frame_mut().ip += offset;