feat(tvix/eval): introduce generators module r/5899

This module contains the request/response types for generators
requesting actions from the VM.

For most of these, an async helper function is added that will be used
inside of generator functions to make use of these requests/responses
instead of constructing them directly.

Change-Id: I1e085f88adaf784a34867957a0e82532d3a83d7c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8148
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

6 files changed, 821 insertions, 9 deletions

diff --git a/tvix/Cargo.lock b/tvix/Cargo.lock
index fa3ac2269b5b..913d88b9fc12 100644
--- a/tvix/Cargo.lock
+++ b/tvix/Cargo.lock
@@ -364,7 +364,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "684a277d672e91966334af371f1a7b5833f9aa00b07c84e92fbce95e00208ce8"
 dependencies = [
  "heck",
- "proc-macro-error",
+ "proc-macro-error 1.0.4",
  "proc-macro2 1.0.50",
  "quote 1.0.23",
  "syn 1.0.107",
@@ -820,6 +820,36 @@ dependencies = [
 ]
 
 [[package]]
+name = "genawaiter"
+version = "0.99.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c86bd0361bcbde39b13475e6e36cb24c329964aa2611be285289d1e4b751c1a0"
+dependencies = [
+ "genawaiter-macro",
+ "genawaiter-proc-macro",
+ "proc-macro-hack",
+]
+
+[[package]]
+name = "genawaiter-macro"
+version = "0.99.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0b32dfe1fdfc0bbde1f22a5da25355514b5e450c33a6af6770884c8750aedfbc"
+
+[[package]]
+name = "genawaiter-proc-macro"
+version = "0.99.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "784f84eebc366e15251c4a8c3acee82a6a6f427949776ecb88377362a9621738"
+dependencies = [
+ "proc-macro-error 0.4.12",
+ "proc-macro-hack",
+ "proc-macro2 1.0.50",
+ "quote 1.0.23",
+ "syn 1.0.107",
+]
+
+[[package]]
 name = "generic-array"
 version = "0.12.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -1546,14 +1576,40 @@ dependencies = [
 
 [[package]]
 name = "proc-macro-error"
+version = "0.4.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "18f33027081eba0a6d8aba6d1b1c3a3be58cbb12106341c2d5759fcd9b5277e7"
+dependencies = [
+ "proc-macro-error-attr 0.4.12",
+ "proc-macro2 1.0.50",
+ "quote 1.0.23",
+ "syn 1.0.107",
+ "version_check",
+]
+
+[[package]]
+name = "proc-macro-error"
 version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "da25490ff9892aab3fcf7c36f08cfb902dd3e71ca0f9f9517bea02a73a5ce38c"
 dependencies = [
- "proc-macro-error-attr",
+ "proc-macro-error-attr 1.0.4",
+ "proc-macro2 1.0.50",
+ "quote 1.0.23",
+ "syn 1.0.107",
+ "version_check",
+]
+
+[[package]]
+name = "proc-macro-error-attr"
+version = "0.4.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8a5b4b77fdb63c1eca72173d68d24501c54ab1269409f6b672c85deb18af69de"
+dependencies = [
  "proc-macro2 1.0.50",
  "quote 1.0.23",
  "syn 1.0.107",
+ "syn-mid",
  "version_check",
 ]
 
@@ -1569,6 +1625,12 @@ dependencies = [
 ]
 
 [[package]]
+name = "proc-macro-hack"
+version = "0.5.20+deprecated"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dc375e1527247fe1a97d8b7156678dfe7c1af2fc075c9a4db3690ecd2a148068"
+
+[[package]]
 name = "proc-macro2"
 version = "0.4.30"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -2184,6 +2246,17 @@ dependencies = [
 ]
 
 [[package]]
+name = "syn-mid"
+version = "0.5.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "baa8e7560a164edb1621a55d18a0c59abf49d360f47aa7b821061dd7eea7fac9"
+dependencies = [
+ "proc-macro2 1.0.50",
+ "quote 1.0.23",
+ "syn 1.0.107",
+]
+
+[[package]]
 name = "sync_wrapper"
 version = "0.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -2247,7 +2320,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e45b7bf6e19353ddd832745c8fcf77a17a93171df7151187f26623f2b75b5b26"
 dependencies = [
  "cfg-if",
- "proc-macro-error",
+ "proc-macro-error 1.0.4",
  "proc-macro2 1.0.50",
  "quote 1.0.23",
  "syn 1.0.107",
@@ -2649,6 +2722,7 @@ dependencies = [
  "codemap-diagnostic",
  "criterion",
  "dirs",
+ "genawaiter",
  "imbl",
  "itertools",
  "lazy_static",
diff --git a/tvix/Cargo.nix b/tvix/Cargo.nix
index 9f172e1b9efb..126edecdff17 100644
--- a/tvix/Cargo.nix
+++ b/tvix/Cargo.nix
@@ -1118,7 +1118,7 @@ rec {
           }
           {
             name = "proc-macro-error";
-            packageId = "proc-macro-error";
+            packageId = "proc-macro-error 1.0.4";
           }
           {
             name = "proc-macro2";
@@ -2308,6 +2308,85 @@ rec {
         ];
 
       };
+      "genawaiter" = rec {
+        crateName = "genawaiter";
+        version = "0.99.1";
+        edition = "2018";
+        sha256 = "1861a6vy9lc9a8lbw496m9j9jcjcn9nf7rkm6jqkkpnb3cvd0sy8";
+        authors = [
+          "John Simon <john@whatisaph.one>"
+        ];
+        dependencies = [
+          {
+            name = "genawaiter-macro";
+            packageId = "genawaiter-macro";
+          }
+          {
+            name = "genawaiter-proc-macro";
+            packageId = "genawaiter-proc-macro";
+            optional = true;
+          }
+          {
+            name = "proc-macro-hack";
+            packageId = "proc-macro-hack";
+            optional = true;
+          }
+        ];
+        features = {
+          "default" = [ "proc_macro" ];
+          "futures-core" = [ "dep:futures-core" ];
+          "futures03" = [ "futures-core" ];
+          "genawaiter-proc-macro" = [ "dep:genawaiter-proc-macro" ];
+          "proc-macro-hack" = [ "dep:proc-macro-hack" ];
+          "proc_macro" = [ "genawaiter-proc-macro" "proc-macro-hack" "genawaiter-macro/proc_macro" ];
+        };
+        resolvedDefaultFeatures = [ "default" "genawaiter-proc-macro" "proc-macro-hack" "proc_macro" ];
+      };
+      "genawaiter-macro" = rec {
+        crateName = "genawaiter-macro";
+        version = "0.99.1";
+        edition = "2018";
+        sha256 = "1g6zmr88fk48f1ksz9ik1i2mwjsiam9s4p9aybhvs2zwzphxychb";
+        authors = [
+          "Devin R <devin.ragotzy@gmail.com>"
+        ];
+        features = { };
+        resolvedDefaultFeatures = [ "proc_macro" ];
+      };
+      "genawaiter-proc-macro" = rec {
+        crateName = "genawaiter-proc-macro";
+        version = "0.99.1";
+        edition = "2018";
+        sha256 = "0f0pcaln4wrpi35nwxs9g516ysiax373m32a3hjiavinpkp88kvq";
+        procMacro = true;
+        authors = [
+          "Devin R <devin.ragotzy@gmail.com>"
+        ];
+        dependencies = [
+          {
+            name = "proc-macro-error";
+            packageId = "proc-macro-error 0.4.12";
+          }
+          {
+            name = "proc-macro-hack";
+            packageId = "proc-macro-hack";
+          }
+          {
+            name = "proc-macro2";
+            packageId = "proc-macro2 1.0.50";
+          }
+          {
+            name = "quote";
+            packageId = "quote 1.0.23";
+          }
+          {
+            name = "syn";
+            packageId = "syn 1.0.107";
+            features = [ "visit-mut" "full" ];
+          }
+        ];
+        features = { };
+      };
       "generic-array 0.12.4" = rec {
         crateName = "generic-array";
         version = "0.12.4";
@@ -4357,7 +4436,43 @@ rec {
           "verbatim" = [ "syn/parsing" ];
         };
       };
-      "proc-macro-error" = rec {
+      "proc-macro-error 0.4.12" = rec {
+        crateName = "proc-macro-error";
+        version = "0.4.12";
+        edition = "2018";
+        sha256 = "1rvpaadwv7vmsp142qqh2axqrr9v78f1nvdsi9nhmfhy10kk1wqq";
+        authors = [
+          "CreepySkeleton <creepy-skeleton@yandex.ru>"
+        ];
+        dependencies = [
+          {
+            name = "proc-macro-error-attr";
+            packageId = "proc-macro-error-attr 0.4.12";
+          }
+          {
+            name = "proc-macro2";
+            packageId = "proc-macro2 1.0.50";
+          }
+          {
+            name = "quote";
+            packageId = "quote 1.0.23";
+          }
+          {
+            name = "syn";
+            packageId = "syn 1.0.107";
+            usesDefaultFeatures = false;
+            features = [ "derive" "parsing" "proc-macro" "printing" ];
+          }
+        ];
+        buildDependencies = [
+          {
+            name = "version_check";
+            packageId = "version_check";
+          }
+        ];
+
+      };
+      "proc-macro-error 1.0.4" = rec {
         crateName = "proc-macro-error";
         version = "1.0.4";
         edition = "2018";
@@ -4368,7 +4483,7 @@ rec {
         dependencies = [
           {
             name = "proc-macro-error-attr";
-            packageId = "proc-macro-error-attr";
+            packageId = "proc-macro-error-attr 1.0.4";
           }
           {
             name = "proc-macro2";
@@ -4398,7 +4513,44 @@ rec {
         };
         resolvedDefaultFeatures = [ "default" "syn" "syn-error" ];
       };
-      "proc-macro-error-attr" = rec {
+      "proc-macro-error-attr 0.4.12" = rec {
+        crateName = "proc-macro-error-attr";
+        version = "0.4.12";
+        edition = "2018";
+        sha256 = "1pk9mwcfnpf8favgc2cl4sqlmi818p96hg8pfb51wg5nzmvlnnwa";
+        procMacro = true;
+        authors = [
+          "CreepySkeleton <creepy-skeleton@yandex.ru>"
+        ];
+        dependencies = [
+          {
+            name = "proc-macro2";
+            packageId = "proc-macro2 1.0.50";
+          }
+          {
+            name = "quote";
+            packageId = "quote 1.0.23";
+          }
+          {
+            name = "syn";
+            packageId = "syn 1.0.107";
+            usesDefaultFeatures = false;
+            features = [ "derive" "parsing" "proc-macro" "printing" ];
+          }
+          {
+            name = "syn-mid";
+            packageId = "syn-mid";
+          }
+        ];
+        buildDependencies = [
+          {
+            name = "version_check";
+            packageId = "version_check";
+          }
+        ];
+
+      };
+      "proc-macro-error-attr 1.0.4" = rec {
         crateName = "proc-macro-error-attr";
         version = "1.0.4";
         edition = "2018";
@@ -4425,6 +4577,17 @@ rec {
         ];
 
       };
+      "proc-macro-hack" = rec {
+        crateName = "proc-macro-hack";
+        version = "0.5.20+deprecated";
+        edition = "2018";
+        sha256 = "0s402hmcs3k9nd6rlp07zkr1lz7yimkmcwcbgnly2zr44wamwdyw";
+        procMacro = true;
+        authors = [
+          "David Tolnay <dtolnay@gmail.com>"
+        ];
+
+      };
       "proc-macro2 0.4.30" = rec {
         crateName = "proc-macro2";
         version = "0.4.30";
@@ -6247,6 +6410,36 @@ rec {
         };
         resolvedDefaultFeatures = [ "clone-impls" "default" "derive" "extra-traits" "full" "parsing" "printing" "proc-macro" "quote" "visit" "visit-mut" ];
       };
+      "syn-mid" = rec {
+        crateName = "syn-mid";
+        version = "0.5.3";
+        edition = "2018";
+        sha256 = "1jgslzpdf78646wafyplc39lkgwsqnh1hpd544bdnkhn19bfga5s";
+        authors = [
+          "Taiki Endo <te316e89@gmail.com>"
+        ];
+        dependencies = [
+          {
+            name = "proc-macro2";
+            packageId = "proc-macro2 1.0.50";
+            usesDefaultFeatures = false;
+          }
+          {
+            name = "quote";
+            packageId = "quote 1.0.23";
+            usesDefaultFeatures = false;
+          }
+          {
+            name = "syn";
+            packageId = "syn 1.0.107";
+            usesDefaultFeatures = false;
+            features = [ "parsing" "printing" "derive" ];
+          }
+        ];
+        features = {
+          "clone-impls" = [ "syn/clone-impls" ];
+        };
+      };
       "sync_wrapper" = rec {
         crateName = "sync_wrapper";
         version = "0.1.2";
@@ -6401,7 +6594,7 @@ rec {
           }
           {
             name = "proc-macro-error";
-            packageId = "proc-macro-error";
+            packageId = "proc-macro-error 1.0.4";
           }
           {
             name = "proc-macro2";
@@ -7858,6 +8051,10 @@ rec {
             packageId = "dirs";
           }
           {
+            name = "genawaiter";
+            packageId = "genawaiter";
+          }
+          {
             name = "imbl";
             packageId = "imbl";
             features = [ "serde" ];
diff --git a/tvix/eval/Cargo.toml b/tvix/eval/Cargo.toml
index cf46d4bbd464..6bb68834ab93 100644
--- a/tvix/eval/Cargo.toml
+++ b/tvix/eval/Cargo.toml
@@ -14,6 +14,7 @@ builtin-macros = { path = "./builtin-macros", package = "tvix-eval-builtin-macro
 codemap = "0.1.3"
 codemap-diagnostic = "0.1.1"
 dirs = "4.0.0"
+genawaiter = "0.99.1"
 imbl = { version = "2.0", features = [ "serde" ] }
 lazy_static = "1.4.0"
 lexical-core = { version = "0.8.5", features = ["format", "parse-floats"] }
diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs
index ede93e7dddd2..07ce83020613 100644
--- a/tvix/eval/src/value/mod.rs
+++ b/tvix/eval/src/value/mod.rs
@@ -32,7 +32,7 @@ pub use path::canon_path;
 pub use string::NixString;
 pub use thunk::Thunk;
 
-use self::thunk::ThunkSet;
+pub use self::thunk::{SharedThunkSet, ThunkSet};
 
 use lazy_static::lazy_static;
 
diff --git a/tvix/eval/src/vm.rs b/tvix/eval/src/vm.rs
index 10966aca811b..3e3052555639 100644
--- a/tvix/eval/src/vm.rs
+++ b/tvix/eval/src/vm.rs
@@ -1,6 +1,8 @@
 //! This module implements the virtual (or abstract) machine that runs
 //! Tvix bytecode.
 
+pub mod generators;
+
 use serde_json::json;
 use std::{cmp::Ordering, collections::HashMap, ops::DerefMut, path::PathBuf, rc::Rc};
 
diff --git a/tvix/eval/src/vm/generators.rs b/tvix/eval/src/vm/generators.rs
new file mode 100644
index 000000000000..3b822b086346
--- /dev/null
+++ b/tvix/eval/src/vm/generators.rs
@@ -0,0 +1,538 @@
+//! This module implements generator logic for the VM. Generators are functions
+//! used during evaluation which can suspend their execution during their
+//! control flow, and request that the VM do something.
+//!
+//! This is used to keep the VM's stack size constant even when evaluating
+//! deeply nested recursive data structures.
+//!
+//! We implement generators using the [`genawaiter`] crate.
+
+use core::pin::Pin;
+use genawaiter::rc::Co;
+pub use genawaiter::rc::Gen;
+use smol_str::SmolStr;
+use std::fmt::Display;
+use std::future::Future;
+
+use crate::value::SharedThunkSet;
+use crate::warnings::WarningKind;
+use crate::FileType;
+use crate::NixString;
+
+use super::*;
+
+/// Dummy type, before the actual implementation is in place.
+#[derive(Debug)]
+pub struct PointerEquality {}
+
+// -- Implementation of generic generator logic.
+
+/// States that a generator can be in while being driven by the VM.
+pub(crate) enum GeneratorState {
+    /// Normal execution of the generator.
+    Running,
+
+    /// Generator is awaiting the result of a forced value.
+    AwaitingValue,
+}
+
+/// Messages that can be sent from generators to the VM. In most
+/// cases, the VM will suspend the generator when receiving a message
+/// and enter some other frame to process the request.
+///
+/// Responses are returned to generators via the [`GeneratorResponse`] type.
+pub enum GeneratorRequest {
+    /// Request that the VM forces this value. This message is first sent to the
+    /// VM with the unforced value, then returned to the generator with the
+    /// forced result.
+    ForceValue(Value),
+
+    /// Request that the VM deep-forces the value.
+    DeepForceValue(Value, SharedThunkSet),
+
+    /// Request the value at the given index from the VM's with-stack, in forced
+    /// state.
+    ///
+    /// The value is returned in the `ForceValue` message.
+    WithValue(usize),
+
+    /// Request the value at the given index from the *captured* with-stack, in
+    /// forced state.
+    CapturedWithValue(usize),
+
+    /// Request that the two values be compared for Nix equality. The result is
+    /// returned in the `ForceValue` message.
+    NixEquality(Box<(Value, Value)>, PointerEquality),
+
+    /// Push the given value to the VM's stack. This is used to prepare the
+    /// stack for requesting a function call from the VM.
+    ///
+    /// The VM does not respond to this request, so the next message received is
+    /// a `NoOp`.
+    StackPush(Value),
+
+    /// Pop a value from the stack and return it to the generator.
+    StackPop,
+
+    /// Request that the VM coerces this value to a string.
+    StringCoerce(Value, CoercionKind),
+
+    /// Request that the VM calls the given value, with arguments already
+    /// prepared on the stack. Value must already be forced.
+    Call(Value),
+
+    /// Request a call frame entering the given lambda immediately. This can be
+    /// used to force thunks.
+    EnterLambda {
+        lambda: Rc<Lambda>,
+        upvalues: Rc<Upvalues>,
+        light_span: LightSpan,
+    },
+
+    /// Emit a runtime warning through the VM. Receives a NoOp-response.
+    EmitWarning(WarningKind),
+
+    /// Request a lookup in the VM's import cache, which tracks the
+    /// thunks yielded by previously imported files.
+    ImportCacheLookup(PathBuf),
+
+    /// Provide the VM with an imported value for a given path, which
+    /// it can populate its input cache with.
+    ImportCachePut(PathBuf, Value),
+
+    /// Request that the VM imports the given path through its I/O interface.
+    PathImport(PathBuf),
+
+    /// Request that the VM reads the given path to a string.
+    ReadToString(PathBuf),
+
+    /// Request that the VM checks whether the given path exists.
+    PathExists(PathBuf),
+
+    /// Request that the VM reads the given path.
+    ReadDir(PathBuf),
+
+    /// Request a reasonable span from the VM.
+    Span,
+
+    /// Request evaluation of `builtins.tryEval` from the VM. See
+    /// [`VM::catch_result`] for an explanation of how this works.
+    TryForce(Value),
+}
+
+/// Human-readable representation of a generator message, used by observers.
+impl Display for GeneratorRequest {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            GeneratorRequest::ForceValue(v) => write!(f, "force_value({})", v),
+            GeneratorRequest::DeepForceValue(v, _) => write!(f, "deep_force_value({})", v),
+            GeneratorRequest::WithValue(_) => write!(f, "with_value"),
+            GeneratorRequest::CapturedWithValue(_) => write!(f, "captured_with_value"),
+            GeneratorRequest::NixEquality(values, ptr_eq) => {
+                write!(
+                    f,
+                    "nix_eq({}, {}, PointerEquality::{:?})",
+                    values.0, values.1, ptr_eq
+                )
+            }
+            GeneratorRequest::StackPush(v) => write!(f, "stack_push({})", v),
+            GeneratorRequest::StackPop => write!(f, "stack_pop"),
+            GeneratorRequest::StringCoerce(v, kind) => match kind {
+                CoercionKind::Weak => write!(f, "weak_string_coerce({})", v),
+                CoercionKind::Strong => write!(f, "strong_string_coerce({})", v),
+                CoercionKind::ThunksOnly => todo!("remove this branch (not live)"),
+            },
+            GeneratorRequest::Call(v) => write!(f, "call({})", v),
+            GeneratorRequest::EnterLambda { lambda, .. } => {
+                write!(f, "enter_lambda({:p})", *lambda)
+            }
+            GeneratorRequest::EmitWarning(_) => write!(f, "emit_warning"),
+            GeneratorRequest::ImportCacheLookup(p) => {
+                write!(f, "import_cache_lookup({})", p.to_string_lossy())
+            }
+            GeneratorRequest::ImportCachePut(p, _) => {
+                write!(f, "import_cache_put({})", p.to_string_lossy())
+            }
+            GeneratorRequest::PathImport(p) => write!(f, "path_import({})", p.to_string_lossy()),
+            GeneratorRequest::ReadToString(p) => {
+                write!(f, "read_to_string({})", p.to_string_lossy())
+            }
+            GeneratorRequest::PathExists(p) => write!(f, "path_exists({})", p.to_string_lossy()),
+            GeneratorRequest::ReadDir(p) => write!(f, "read_dir({})", p.to_string_lossy()),
+            GeneratorRequest::Span => write!(f, "span"),
+            GeneratorRequest::TryForce(v) => write!(f, "try_force({})", v),
+        }
+    }
+}
+
+/// Responses returned to generators from the VM.
+pub enum GeneratorResponse {
+    /// Empty message. Passed to the generator as the first message,
+    /// or when return values were optional.
+    Empty,
+
+    /// Value produced by the VM and returned to the generator.
+    Value(Value),
+
+    /// Path produced by the VM in response to some IO operation.
+    Path(PathBuf),
+
+    /// VM response with the contents of a directory.
+    Directory(Vec<(SmolStr, FileType)>),
+
+    /// VM response with a span to use at the current point.
+    Span(LightSpan),
+
+    /// Message returned by the VM when a catchable error is encountered during
+    /// the evaluation of `builtins.tryEval`.
+    ForceError,
+}
+
+impl Display for GeneratorResponse {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            GeneratorResponse::Empty => write!(f, "empty"),
+            GeneratorResponse::Value(v) => write!(f, "value({})", v),
+            GeneratorResponse::Path(p) => write!(f, "path({})", p.to_string_lossy()),
+            GeneratorResponse::Directory(d) => write!(f, "dir(len = {})", d.len()),
+            GeneratorResponse::Span(_) => write!(f, "span"),
+            GeneratorResponse::ForceError => write!(f, "force_error"),
+        }
+    }
+}
+
+pub(crate) type Generator = Gen<
+    GeneratorRequest,
+    GeneratorResponse,
+    Pin<Box<dyn Future<Output = Result<Value, ErrorKind>>>>,
+>;
+
+/// Helper function to provide type annotations which are otherwise difficult to
+/// infer.
+pub fn pin_generator(
+    f: impl Future<Output = Result<Value, ErrorKind>> + 'static,
+) -> Pin<Box<dyn Future<Output = Result<Value, ErrorKind>>>> {
+    Box::pin(f)
+}
+
+pub type GenCo = Co<GeneratorRequest, GeneratorResponse>;
+
+// -- Implementation of concrete generator use-cases.
+
+/// Request that the VM place the given value on its stack.
+pub async fn request_stack_push(co: &GenCo, val: Value) {
+    match co.yield_(GeneratorRequest::StackPush(val)).await {
+        GeneratorResponse::Empty => {}
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Request that the VM pop a value from the stack and return it to the
+/// generator.
+pub async fn request_stack_pop(co: &GenCo) -> Value {
+    match co.yield_(GeneratorRequest::StackPop).await {
+        GeneratorResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Force any value and return the evaluated result from the VM.
+pub async fn request_force(co: &GenCo, val: Value) -> Value {
+    if let Value::Thunk(_) = val {
+        match co.yield_(GeneratorRequest::ForceValue(val)).await {
+            GeneratorResponse::Value(value) => value,
+            msg => panic!(
+                "Tvix bug: VM responded with incorrect generator message: {}",
+                msg
+            ),
+        }
+    } else {
+        val
+    }
+}
+
+/// Force a value, but inform the caller (by returning `None`) if a catchable
+/// error occured.
+pub(crate) async fn request_try_force(co: &GenCo, val: Value) -> Option<Value> {
+    if let Value::Thunk(_) = val {
+        match co.yield_(GeneratorRequest::TryForce(val)).await {
+            GeneratorResponse::Value(value) => Some(value),
+            GeneratorResponse::ForceError => None,
+            msg => panic!(
+                "Tvix bug: VM responded with incorrect generator message: {}",
+                msg
+            ),
+        }
+    } else {
+        Some(val)
+    }
+}
+
+/// Call the given value as a callable. The argument(s) must already be prepared
+/// on the stack.
+pub async fn request_call(co: &GenCo, val: Value) -> Value {
+    let val = request_force(co, val).await;
+    match co.yield_(GeneratorRequest::Call(val)).await {
+        GeneratorResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Helper function to call the given value with the provided list of arguments.
+/// This uses the StackPush and Call messages under the hood.
+pub async fn request_call_with<I>(co: &GenCo, mut callable: Value, args: I) -> Value
+where
+    I: IntoIterator<Item = Value>,
+    I::IntoIter: DoubleEndedIterator,
+{
+    let mut num_args = 0_usize;
+    for arg in args.into_iter().rev() {
+        num_args += 1;
+        request_stack_push(co, arg).await;
+    }
+
+    debug_assert!(num_args > 0, "call_with called with an empty list of args");
+
+    while num_args > 0 {
+        callable = request_call(co, callable).await;
+        num_args -= 1;
+    }
+
+    callable
+}
+
+pub async fn request_string_coerce(co: &GenCo, val: Value, kind: CoercionKind) -> NixString {
+    match val {
+        Value::String(s) => s,
+        _ => match co.yield_(GeneratorRequest::StringCoerce(val, kind)).await {
+            GeneratorResponse::Value(value) => value
+                .to_str()
+                .expect("coerce_to_string always returns a string"),
+            msg => panic!(
+                "Tvix bug: VM responded with incorrect generator message: {}",
+                msg
+            ),
+        },
+    }
+}
+
+/// Deep-force any value and return the evaluated result from the VM.
+pub async fn request_deep_force(co: &GenCo, val: Value, thunk_set: SharedThunkSet) -> Value {
+    match co
+        .yield_(GeneratorRequest::DeepForceValue(val, thunk_set))
+        .await
+    {
+        GeneratorResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Fetch and force a value on the with-stack from the VM.
+async fn fetch_forced_with(co: &GenCo, idx: usize) -> Value {
+    match co.yield_(GeneratorRequest::WithValue(idx)).await {
+        GeneratorResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Fetch and force a value on the *captured* with-stack from the VM.
+async fn fetch_captured_with(co: &GenCo, idx: usize) -> Value {
+    match co.yield_(GeneratorRequest::CapturedWithValue(idx)).await {
+        GeneratorResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Ask the VM to compare two values for equality.
+pub(crate) async fn check_equality(
+    co: &GenCo,
+    a: Value,
+    b: Value,
+    ptr_eq: PointerEquality,
+) -> Result<bool, ErrorKind> {
+    match co
+        .yield_(GeneratorRequest::NixEquality(Box::new((a, b)), ptr_eq))
+        .await
+    {
+        GeneratorResponse::Value(value) => value.as_bool(),
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Emit a runtime warning.
+pub(crate) async fn emit_warning(co: &GenCo, kind: WarningKind) {
+    match co.yield_(GeneratorRequest::EmitWarning(kind)).await {
+        GeneratorResponse::Empty => {}
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Request that the VM enter the given lambda.
+pub(crate) async fn request_enter_lambda(
+    co: &GenCo,
+    lambda: Rc<Lambda>,
+    upvalues: Rc<Upvalues>,
+    light_span: LightSpan,
+) -> Value {
+    let msg = GeneratorRequest::EnterLambda {
+        lambda,
+        upvalues,
+        light_span,
+    };
+
+    match co.yield_(msg).await {
+        GeneratorResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Request a lookup in the VM's import cache.
+pub(crate) async fn request_import_cache_lookup(co: &GenCo, path: PathBuf) -> Option<Value> {
+    match co.yield_(GeneratorRequest::ImportCacheLookup(path)).await {
+        GeneratorResponse::Value(value) => Some(value),
+        GeneratorResponse::Empty => None,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Request that the VM populate its input cache for the given path.
+pub(crate) async fn request_import_cache_put(co: &GenCo, path: PathBuf, value: Value) {
+    match co
+        .yield_(GeneratorRequest::ImportCachePut(path, value))
+        .await
+    {
+        GeneratorResponse::Empty => {}
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Request that the VM import the given path.
+pub(crate) async fn request_path_import(co: &GenCo, path: PathBuf) -> PathBuf {
+    match co.yield_(GeneratorRequest::PathImport(path)).await {
+        GeneratorResponse::Path(path) => path,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+pub(crate) async fn request_read_to_string(co: &GenCo, path: PathBuf) -> Value {
+    match co.yield_(GeneratorRequest::ReadToString(path)).await {
+        GeneratorResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+pub(crate) async fn request_path_exists(co: &GenCo, path: PathBuf) -> Value {
+    match co.yield_(GeneratorRequest::PathExists(path)).await {
+        GeneratorResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+pub(crate) async fn request_read_dir(co: &GenCo, path: PathBuf) -> Vec<(SmolStr, FileType)> {
+    match co.yield_(GeneratorRequest::ReadDir(path)).await {
+        GeneratorResponse::Directory(dir) => dir,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+pub(crate) async fn request_span(co: &GenCo) -> LightSpan {
+    match co.yield_(GeneratorRequest::Span).await {
+        GeneratorResponse::Span(span) => span,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+pub(crate) async fn neo_resolve_with(
+    co: GenCo,
+    ident: String,
+    vm_with_len: usize,
+    upvalue_with_len: usize,
+) -> Result<Value, ErrorKind> {
+    for with_stack_idx in (0..vm_with_len).rev() {
+        // TODO(tazjin): is this branch still live with the current with-thunking?
+        let with = fetch_forced_with(&co, with_stack_idx).await;
+
+        match with.to_attrs()?.select(&ident) {
+            None => continue,
+            Some(val) => return Ok(val.clone()),
+        }
+    }
+
+    for upvalue_with_idx in (0..upvalue_with_len).rev() {
+        let with = fetch_captured_with(&co, upvalue_with_idx).await;
+
+        match with.to_attrs()?.select(&ident) {
+            None => continue,
+            Some(val) => return Ok(val.clone()),
+        }
+    }
+
+    Err(ErrorKind::UnknownDynamicVariable(ident))
+}
+
+/// Call the given value as if it was an attribute set containing a functor. The
+/// arguments must already be prepared on the stack when a generator frame from
+/// this function is invoked.
+///
+pub(crate) async fn call_functor(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+    let attrs = value.to_attrs()?;
+
+    match attrs.select("__functor") {
+        None => Err(ErrorKind::NotCallable("set without `__functor_` attribute")),
+        Some(functor) => {
+            // The functor receives the set itself as its first argument and
+            // needs to be called with it.
+            let functor = request_force(&co, functor.clone()).await;
+            let primed = request_call_with(&co, functor, [value]).await;
+            Ok(request_call(&co, primed).await)
+        }
+    }
+}