feat(tvix/eval): impl unsafeDiscardStringContext

Change-Id: I7f0cc42cbebfe5cd27bf6d4f58a4af927b83646a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10423
Autosubmit: raitobezarius <tvl@lahfa.xyz>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/docs/builtins.md

@@ -121,7 +121,7 @@ The `impl` column indicates implementation status in tvix:
 | tryEval                       | false  |       |       |         |
 | typeOf                        | false  |       |       |         |
 | unsafeDiscardOutputDependency | false  |       |       | context |
-| unsafeDiscardStringContext    | false  |       |       | context |
+| unsafeDiscardStringContext    | false  |       |       |         |
 | unsafeGetAttrPos              | false  |       |       | todo    |
 | valueSize                     | false  |       |       | todo    |
 

tvix/eval/src/builtins/mod.rs

@@ -1348,11 +1348,26 @@ mod placeholder_builtins {
 
     #[builtin("unsafeDiscardStringContext")]
     async fn builtin_unsafe_discard_string_context(
-        _: GenCo,
-        #[lazy] s: Value,
+        co: GenCo,
+        s: Value,
     ) -> Result<Value, ErrorKind> {
-        // Tvix does not manually track contexts, and this is a no-op for us.
-        Ok(s)
+        let span = generators::request_span(&co).await;
+        let mut v = s
+            .coerce_to_string(
+                co,
+                // It's weak because
+                // lists, integers, floats and null are not
+                // accepted as parameters.
+                CoercionKind {
+                    strong: false,
+                    import_paths: true,
+                },
+                span,
+            )
+            .await?
+            .to_contextful_str()?;
+        v.clear_context();
+        Ok(Value::String(v))
     }
 
     #[builtin("addErrorContext")]

tvix/glue/src/builtins/mod.rs

@@ -143,6 +143,51 @@ mod tests {
         );
     }
 
+    /// Construct two derivations with the same parameters except
+    /// one of them lost a context string for a dependency, causing
+    /// the loss of an element in the `inputDrvs` derivation.
+    /// Therefore, making `outPath` different.
+    #[test]
+    fn test_unsafe_discard_string_context() {
+        let code = r#"
+        let
+            dep = builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; };
+        in
+          (builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; env = "${dep}"; }).outPath !=
+          (builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; env = "${builtins.unsafeDiscardStringContext dep}"; }).outPath
+        "#;
+
+        let value = eval(code).value.expect("must succeed");
+        match value {
+            tvix_eval::Value::Bool(v) => {
+                assert!(v);
+            }
+            _ => panic!("unexpected value type: {:?}", value),
+        }
+    }
+
+    /// Construct an attribute set
+    /// that coerces to a derivation
+    /// and verify that the return type is a string.
+    #[test]
+    fn test_unsafe_discard_string_context_of_coercible() {
+        let code = r#"
+        let
+            dep = builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; };
+            attr = { __toString = _: dep; };
+        in
+            builtins.typeOf (builtins.unsafeDiscardStringContext attr) == "string"
+        "#;
+
+        let value = eval(code).value.expect("must succeed");
+        match value {
+            tvix_eval::Value::Bool(v) => {
+                assert!(v);
+            }
+            _ => panic!("unexpected value type: {:?}", value),
+        }
+    }
+
     #[test]
     fn builtins_placeholder_hashes() {
         assert_eq!(