fix(tvix/eval): remove incorrect imports when coercing r/7218

The default behavior of string coercion in C++ Nix is to weakly coerce
and import to store if necessary. There is a flag to make it strongly
coerce (coerceMore) and a flag that controls whether path values have
the corresponding file/directory imported into the store before
returning the (store) path as a string (copyToStore). We need to
implement our equivalent to the copyToStore (import_paths) flag for the
benefit of weak coercions that don't import into the store (dirOf,
baseNameOf, readFile, ...) and strong coercions that don't import into
the store (toString).

This makes coerce_to_string as well as CoercionKind weirder and more
versatile, but prevents us from reimplementing parts of the coercion
logic constantly as can be seen in the case of baseNameOf.

Note that it is not possible to test this properly in //tvix/eval tests
due to the lack of an appropriate EvalIO implementation being available.
Tests should be added to //tvix/glue down the line.

Change-Id: I8fb8ab99c7fe08e311d2ba1c36960746bf22f566
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10361
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: Adam Joseph <adam@westernsemico.com>

8 files changed, 207 insertions, 54 deletions

diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index 27b7d5648f48..6cdb434f42b4 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -41,6 +41,8 @@ pub const CURRENT_PLATFORM: &str = env!("TVIX_CURRENT_SYSTEM");
 /// builtin. This coercion can _never_ be performed in a Nix program
 /// without using builtins (i.e. the trick `path: /. + path` to
 /// convert from a string to a path wouldn't hit this code).
+///
+/// This operation doesn't import a Nix path value into the store.
 pub async fn coerce_value_to_path(
     co: &GenCo,
     v: Value,
@@ -50,7 +52,16 @@ pub async fn coerce_value_to_path(
         return Ok(Ok(*p));
     }
 
-    match generators::request_string_coerce(co, value, CoercionKind::Weak).await {
+    match generators::request_string_coerce(
+        co,
+        value,
+        CoercionKind {
+            strong: false,
+            import_paths: false,
+        },
+    )
+    .await
+    {
         Ok(vs) => {
             let path = PathBuf::from(vs.as_str());
             if path.is_absolute() {
@@ -71,6 +82,7 @@ mod pure_builtins {
 
     #[builtin("abort")]
     async fn builtin_abort(co: GenCo, message: Value) -> Result<Value, ErrorKind> {
+        // TODO(sterni): coerces to string
         Err(ErrorKind::Abort(message.to_str()?.to_string()))
     }
 
@@ -136,14 +148,15 @@ mod pure_builtins {
         let span = generators::request_span(&co).await;
         let s = match s {
             val @ Value::Catchable(_) => return Ok(val),
-            // it is important that builtins.baseNameOf does not
-            // coerce paths into strings, since this will turn them
-            // into store paths, and `builtins.baseNameOf
-            // ./config.nix` will return a hash-prefixed value like
-            // "hpmyf3vlqg5aadri97xglcvvjbk8xw3g-config.nix"
-            Value::Path(p) => NixString::from(p.to_string_lossy().to_string()),
             _ => s
-                .coerce_to_string(co, CoercionKind::Weak, span)
+                .coerce_to_string(
+                    co,
+                    CoercionKind {
+                        strong: false,
+                        import_paths: false,
+                    },
+                    span,
+                )
                 .await?
                 .to_str()?,
         };
@@ -239,7 +252,16 @@ mod pure_builtins {
             if i != 0 {
                 res.push_str(&separator);
             }
-            match generators::request_string_coerce(&co, val, CoercionKind::Weak).await {
+            match generators::request_string_coerce(
+                &co,
+                val,
+                CoercionKind {
+                    strong: false,
+                    import_paths: true,
+                },
+            )
+            .await
+            {
                 Ok(s) => res.push_str(s.as_str()),
                 Err(c) => return Ok(Value::Catchable(c)),
             }
@@ -263,7 +285,14 @@ mod pure_builtins {
         let is_path = s.is_path();
         let span = generators::request_span(&co).await;
         let str = s
-            .coerce_to_string(co, CoercionKind::Weak, span)
+            .coerce_to_string(
+                co,
+                CoercionKind {
+                    strong: false,
+                    import_paths: false,
+                },
+                span,
+            )
             .await?
             .to_str()?;
         let result = str
@@ -983,7 +1012,16 @@ mod pure_builtins {
     async fn builtin_string_length(co: GenCo, #[lazy] s: Value) -> Result<Value, ErrorKind> {
         // also forces the value
         let span = generators::request_span(&co).await;
-        let s = s.coerce_to_string(co, CoercionKind::Weak, span).await?;
+        let s = s
+            .coerce_to_string(
+                co,
+                CoercionKind {
+                    strong: false,
+                    import_paths: true,
+                },
+                span,
+            )
+            .await?;
         Ok(Value::Integer(s.to_str()?.as_str().len() as i64))
     }
 
@@ -1002,7 +1040,16 @@ mod pure_builtins {
         let beg = start.as_int()?;
         let len = len.as_int()?;
         let span = generators::request_span(&co).await;
-        let x = s.coerce_to_string(co, CoercionKind::Weak, span).await?;
+        let x = s
+            .coerce_to_string(
+                co,
+                CoercionKind {
+                    strong: false,
+                    import_paths: true,
+                },
+                span,
+            )
+            .await?;
         if x.is_catchable() {
             return Ok(x);
         }
@@ -1043,6 +1090,7 @@ mod pure_builtins {
 
     #[builtin("throw")]
     async fn builtin_throw(co: GenCo, message: Value) -> Result<Value, ErrorKind> {
+        // TODO(sterni): coerces to string
         Ok(Value::Catchable(CatchableErrorKind::Throw(
             message.to_str()?.to_string(),
         )))
@@ -1052,7 +1100,15 @@ mod pure_builtins {
     async fn builtin_to_string(co: GenCo, #[lazy] x: Value) -> Result<Value, ErrorKind> {
         // coerce_to_string forces for us
         let span = generators::request_span(&co).await;
-        x.coerce_to_string(co, CoercionKind::Strong, span).await
+        x.coerce_to_string(
+            co,
+            CoercionKind {
+                strong: true,
+                import_paths: false,
+            },
+            span,
+        )
+        .await
     }
 
     #[builtin("toXML")]
@@ -1085,7 +1141,16 @@ mod pure_builtins {
             Ok(path) => {
                 let path: Value = crate::value::canon_path(path).into();
                 let span = generators::request_span(&co).await;
-                Ok(path.coerce_to_string(co, CoercionKind::Weak, span).await?)
+                Ok(path
+                    .coerce_to_string(
+                        co,
+                        CoercionKind {
+                            strong: false,
+                            import_paths: false,
+                        },
+                        span,
+                    )
+                    .await?)
             }
         }
     }
diff --git a/tvix/eval/src/errors.rs b/tvix/eval/src/errors.rs
index 23905e438edb..6ac29492dfc3 100644
--- a/tvix/eval/src/errors.rs
+++ b/tvix/eval/src/errors.rs
@@ -378,10 +378,7 @@ to a missing value in the attribute set(s) included via `with`."#,
             ErrorKind::BytecodeError(_) => write!(f, "while evaluating this Nix code"),
 
             ErrorKind::NotCoercibleToString { kind, from } => {
-                let kindly = match kind {
-                    CoercionKind::Strong => "strongly",
-                    CoercionKind::Weak => "weakly",
-                };
+                let kindly = if kind.strong { "strongly" } else { "weakly" };
 
                 let hint = if *from == "set" {
                     ", missing a `__toString` or `outPath` attribute"
diff --git a/tvix/eval/src/opcode.rs b/tvix/eval/src/opcode.rs
index 467798177550..b57a79a3baec 100644
--- a/tvix/eval/src/opcode.rs
+++ b/tvix/eval/src/opcode.rs
@@ -202,7 +202,8 @@ pub enum OpCode {
     OpInterpolate(Count),
 
     /// Force the Value on the stack and coerce it to a string, always using
-    /// `CoercionKind::Weak`.
+    /// `CoercionKind::Weak { import_paths: true }`. This is the behavior
+    /// necessary for path interpolation.
     OpCoerceToString,
 
     // Paths
diff --git a/tvix/eval/src/value/json.rs b/tvix/eval/src/value/json.rs
index f7ecf121de5e..54b29131109a 100644
--- a/tvix/eval/src/value/json.rs
+++ b/tvix/eval/src/value/json.rs
@@ -47,7 +47,14 @@ impl Value {
                 if attrs.select("__toString").is_some() {
                     let span = generators::request_span(co).await;
                     match Value::Attrs(attrs)
-                        .coerce_to_string_(co, CoercionKind::Weak, span)
+                        .coerce_to_string_(
+                            co,
+                            CoercionKind {
+                                strong: false,
+                                import_paths: false,
+                            },
+                            span,
+                        )
                         .await?
                     {
                         Value::Catchable(cek) => return Ok(Err(cek)),
diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs
index 300824b88ff7..0007735cc096 100644
--- a/tvix/eval/src/value/mod.rs
+++ b/tvix/eval/src/value/mod.rs
@@ -162,15 +162,22 @@ macro_rules! gen_is {
 
 /// Describes what input types are allowed when coercing a `Value` to a string
 #[derive(Clone, Copy, PartialEq, Debug)]
-pub enum CoercionKind {
-    /// Only coerce already "stringly" types like strings and paths, but also
-    /// coerce sets that have a `__toString` attribute. Equivalent to
-    /// `!coerceMore` in C++ Nix.
-    Weak,
-    /// Coerce all value types included by `Weak`, but also coerce `null`,
-    /// booleans, integers, floats and lists of coercible types. Equivalent to
-    /// `coerceMore` in C++ Nix.
-    Strong,
+pub struct CoercionKind {
+    /// If false only coerce already "stringly" types like strings and paths, but
+    /// also coerce sets that have a `__toString` attribute. In Tvix, this is
+    /// usually called a weak coercion. Equivalent to passing `false` as the
+    /// `coerceMore` argument of `EvalState::coerceToString` in C++ Nix.
+    ///
+    /// If true coerce all value types included by a weak coercion, but also
+    /// coerce `null`, booleans, integers, floats and lists of coercible types.
+    /// Consequently, we call this a strong coercion. Equivalent to passing
+    /// `true` as `coerceMore` in C++ Nix.
+    pub strong: bool,
+
+    /// If `import_paths` is `true`, paths are imported into the store and their
+    /// store path is the result of the coercion (equivalent to the
+    /// `copyToStore` argument of `EvalState::coerceToString` in C++ Nix).
+    pub import_paths: bool,
 }
 
 impl<T> From<T> for Value
@@ -323,11 +330,22 @@ impl Value {
                 // C++ Nix and Tvix is that the former's strings are arbitrary byte
                 // sequences without NUL bytes, whereas Tvix only allows valid
                 // Unicode. See also b/189.
-                (Value::Path(p), _) => {
-                    // TODO(tazjin): there are cases where coerce_to_string does not import
+                (
+                    Value::Path(p),
+                    CoercionKind {
+                        import_paths: true, ..
+                    },
+                ) => {
                     let imported = generators::request_path_import(co, *p).await;
                     Ok(imported.to_string_lossy().into_owned())
                 }
+                (
+                    Value::Path(p),
+                    CoercionKind {
+                        import_paths: false,
+                        ..
+                    },
+                ) => Ok(p.to_string_lossy().into_owned()),
 
                 // Attribute sets can be converted to strings if they either have an
                 // `__toString` attribute which holds a function that receives the
@@ -358,19 +376,19 @@ impl Value {
                 }
 
                 // strong coercions
-                (Value::Null, CoercionKind::Strong)
-                | (Value::Bool(false), CoercionKind::Strong) => Ok("".to_owned()),
-                (Value::Bool(true), CoercionKind::Strong) => Ok("1".to_owned()),
+                (Value::Null, CoercionKind { strong: true, .. })
+                | (Value::Bool(false), CoercionKind { strong: true, .. }) => Ok("".to_owned()),
+                (Value::Bool(true), CoercionKind { strong: true, .. }) => Ok("1".to_owned()),
 
-                (Value::Integer(i), CoercionKind::Strong) => Ok(format!("{i}")),
-                (Value::Float(f), CoercionKind::Strong) => {
+                (Value::Integer(i), CoercionKind { strong: true, .. }) => Ok(format!("{i}")),
+                (Value::Float(f), CoercionKind { strong: true, .. }) => {
                     // contrary to normal Display, coercing a float to a string will
                     // result in unconditional 6 decimal places
                     Ok(format!("{:.6}", f))
                 }
 
                 // Lists are coerced by coercing their elements and interspersing spaces
-                (Value::List(list), CoercionKind::Strong) => {
+                (Value::List(list), CoercionKind { strong: true, .. }) => {
                     for elem in list.into_iter().rev() {
                         vals.push(elem);
                     }
diff --git a/tvix/eval/src/vm/generators.rs b/tvix/eval/src/vm/generators.rs
index 6563a82790a6..9686e6542f13 100644
--- a/tvix/eval/src/vm/generators.rs
+++ b/tvix/eval/src/vm/generators.rs
@@ -144,10 +144,19 @@ impl Display for VMRequest {
             }
             VMRequest::StackPush(v) => write!(f, "stack_push({})", v.type_of()),
             VMRequest::StackPop => write!(f, "stack_pop"),
-            VMRequest::StringCoerce(v, kind) => match kind {
-                CoercionKind::Weak => write!(f, "weak_string_coerce({})", v.type_of()),
-                CoercionKind::Strong => write!(f, "strong_string_coerce({})", v.type_of()),
-            },
+            VMRequest::StringCoerce(
+                v,
+                CoercionKind {
+                    strong,
+                    import_paths,
+                },
+            ) => write!(
+                f,
+                "{}_{}importing_string_coerce({})",
+                if *strong { "strong" } else { "weak" },
+                if *import_paths { "" } else { "non_" },
+                v.type_of()
+            ),
             VMRequest::Call(v) => write!(f, "call({})", v),
             VMRequest::EnterLambda { lambda, .. } => {
                 write!(f, "enter_lambda({:p})", *lambda)
diff --git a/tvix/eval/src/vm/mod.rs b/tvix/eval/src/vm/mod.rs
index 902121ebc727..0ac5024d6b11 100644
--- a/tvix/eval/src/vm/mod.rs
+++ b/tvix/eval/src/vm/mod.rs
@@ -780,7 +780,14 @@ impl<'o> VM<'o> {
                     self.push_call_frame(span, frame);
 
                     self.enqueue_generator("coerce_to_string", gen_span.clone(), |co| {
-                        value.coerce_to_string(co, CoercionKind::Weak, gen_span)
+                        value.coerce_to_string(
+                            co,
+                            CoercionKind {
+                                strong: false,
+                                import_paths: true,
+                            },
+                            gen_span,
+                        )
                     });
 
                     return Ok(false);
@@ -1206,7 +1213,23 @@ async fn add_values(co: GenCo, a: Value, b: Value) -> Result<Value, ErrorKind> {
     let result = match (a, b) {
         (Value::Path(p), v) => {
             let mut path = p.to_string_lossy().into_owned();
-            match generators::request_string_coerce(&co, v, CoercionKind::Weak).await {
+            match generators::request_string_coerce(
+                &co,
+                v,
+                CoercionKind {
+                    strong: false,
+
+                    // Concatenating a Path with something else results in a
+                    // Path, so we don't need to import any paths (paths
+                    // imported by Nix always exist as a string, unless
+                    // converted by the user). In C++ Nix they even may not
+                    // contain any string context, the resulting error of such a
+                    // case can not be replicated by us.
+                    import_paths: false,
+                },
+            )
+            .await
+            {
                 Ok(vs) => {
                     path.push_str(vs.as_str());
                     crate::value::canon_path(PathBuf::from(path)).into()
@@ -1215,14 +1238,38 @@ async fn add_values(co: GenCo, a: Value, b: Value) -> Result<Value, ErrorKind> {
             }
         }
         (Value::String(s1), Value::String(s2)) => Value::String(s1.concat(&s2)),
-        (Value::String(s1), v) => generators::request_string_coerce(&co, v, CoercionKind::Weak)
-            .await
-            .map(|s2| Value::String(s1.concat(&s2)))
-            .into(),
+        (Value::String(s1), v) => generators::request_string_coerce(
+            &co,
+            v,
+            CoercionKind {
+                strong: false,
+                // Behaves the same as string interpolation
+                import_paths: true,
+            },
+        )
+        .await
+        .map(|s2| Value::String(s1.concat(&s2)))
+        .into(),
         (a @ Value::Integer(_), b) | (a @ Value::Float(_), b) => arithmetic_op!(&a, &b, +)?,
         (a, b) => {
-            let r1 = generators::request_string_coerce(&co, a, CoercionKind::Weak).await;
-            let r2 = generators::request_string_coerce(&co, b, CoercionKind::Weak).await;
+            let r1 = generators::request_string_coerce(
+                &co,
+                a,
+                CoercionKind {
+                    strong: false,
+                    import_paths: false,
+                },
+            )
+            .await;
+            let r2 = generators::request_string_coerce(
+                &co,
+                b,
+                CoercionKind {
+                    strong: false,
+                    import_paths: false,
+                },
+            )
+            .await;
             match (r1, r2) {
                 (Ok(s1), Ok(s2)) => Value::String(s1.concat(&s2)),
                 (Err(c), _) => return Ok(Value::Catchable(c)),
diff --git a/tvix/glue/src/builtins/derivation.rs b/tvix/glue/src/builtins/derivation.rs
index 6eedbc13e7e3..993e7612aed4 100644
--- a/tvix/glue/src/builtins/derivation.rs
+++ b/tvix/glue/src/builtins/derivation.rs
@@ -161,7 +161,7 @@ async fn handle_derivation_parameters(
         "args" => {
             let args = value.to_list()?;
             for arg in args {
-                match strong_coerce_to_string(co, arg).await? {
+                match strong_importing_coerce_to_string(co, arg).await? {
                     Err(cek) => return Ok(Err(cek)),
                     Ok(s) => drv.arguments.push(s),
                 }
@@ -194,12 +194,21 @@ async fn handle_derivation_parameters(
     Ok(Ok(true))
 }
 
-async fn strong_coerce_to_string(
+async fn strong_importing_coerce_to_string(
     co: &GenCo,
     val: Value,
 ) -> Result<Result<String, CatchableErrorKind>, ErrorKind> {
     let val = generators::request_force(co, val).await;
-    match generators::request_string_coerce(co, val, CoercionKind::Strong).await {
+    match generators::request_string_coerce(
+        co,
+        val,
+        CoercionKind {
+            strong: true,
+            import_paths: true,
+        },
+    )
+    .await
+    {
         Err(cek) => Ok(Err(cek)),
         Ok(val_str) => Ok(Ok(val_str.as_str().to_string())),
     }
@@ -268,7 +277,7 @@ pub(crate) mod derivation_builtins {
             key: &str,
         ) -> Result<Result<Option<String>, CatchableErrorKind>, ErrorKind> {
             if let Some(attr) = attrs.select(key) {
-                match strong_coerce_to_string(co, attr.clone()).await? {
+                match strong_importing_coerce_to_string(co, attr.clone()).await? {
                     Err(cek) => return Ok(Err(cek)),
                     Ok(str) => return Ok(Ok(Some(str))),
                 }
@@ -283,7 +292,7 @@ pub(crate) mod derivation_builtins {
                 continue;
             }
 
-            match strong_coerce_to_string(&co, value.clone()).await? {
+            match strong_importing_coerce_to_string(&co, value.clone()).await? {
                 Err(cek) => return Ok(Value::Catchable(cek)),
                 Ok(val_str) => {
                     // handle_derivation_parameters tells us whether the