refactor(tvix/eval): streamline construction of globals/builtins r/5581

Previously the construction of globals (a compiler-only concept) and
builtins (a (now) user-facing API) was intermingled between multiple
different modules, and kind of difficult to understand.

The complexity of this had grown in large part due to the
implementation of `builtins.import`, which required the notorious
"knot-tying" trick using Rc::new_cyclic (see cl/7097) for constructing
the set of globals.

As part of the new `Evaluation` API users should have the ability to
bring their own builtins, and control explicitly whether or not impure
builtins are available (regardless of whether they're compiled in or
not).

To streamline the construction and allow the new API features to work,
this commit restructures things by making these changes:

1. The `tvix_eval::builtins` module is now only responsible for
   exporting sets of builtins. It no longer has any knowledge of
   whether or not certain sets (e.g. only pure, or pure+impure) are
   enabled, and it has no control over which builtins are globally
   available (this is now handled in the compiler).

2. The compiler module is now responsible for both constructing the
   final attribute set of builtins from the set of builtins supplied
   by a user, as well as for populating its globals (that is
   identifiers which are available at the top-level scope).

3. The `Evaluation` API now carries a `builtins` field which is
   populated with the pure builtins by default, and can be extended by
   users.

4. The `import` feature has been moved into the compiler, as a
   special case. In general, builtins no longer have the ability to
   reference the "fix point" of the globals set.

This should not change any functionality, and in fact preserves minor
differences between Tvix/Nix that we already had (such as
`builtins.builtins` not existing).

Change-Id: Icdf5dd50eb81eb9260d89269d6e08b1e67811a2c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7738
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

1 files changed, 82 insertions, 21 deletions

diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index 12fd269c2f..4a44f95691 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -14,6 +14,7 @@
 //! mistakes early during development.
 
 mod bindings;
+mod import;
 mod scope;
 
 use codemap::Span;
@@ -30,8 +31,9 @@ use crate::observer::CompilerObserver;
 use crate::opcode::{CodeIdx, Count, JumpOffset, OpCode, UpvalueIdx};
 use crate::spans::LightSpan;
 use crate::spans::ToSpan;
-use crate::value::{Closure, Formals, Lambda, Thunk, Value};
+use crate::value::{Closure, Formals, Lambda, NixAttrs, Thunk, Value};
 use crate::warnings::{EvalWarning, WarningKind};
+use crate::SourceCode;
 
 use self::scope::{LocalIdx, LocalPosition, Scope, Upvalue, UpvalueKind};
 
@@ -73,17 +75,38 @@ impl LambdaCtx {
     }
 }
 
+/// The type of a global as used inside of the compiler. Differs from
+/// Nix's own notion of "builtins" in that it can emit arbitrary code.
+/// Nix's builtins are wrapped inside of this type.
+pub type Global = Rc<dyn Fn(&mut Compiler, Span)>;
+
 /// The map of globally available functions that should implicitly
 /// be resolvable in the global scope.
-pub type GlobalsMap = HashMap<&'static str, Rc<dyn Fn(&mut Compiler, Span)>>;
-
-/// Functions with this type are used to construct a
-/// self-referential `builtins` object; it takes a weak reference to
-/// its own result, similar to how nixpkgs' overlays work.
-/// Rc::new_cyclic() is what "ties the knot".  The heap allocation
-/// (Box) and vtable (dyn) do not impair runtime or compile-time
-/// performance; they exist only during compiler startup.
-pub type GlobalsMapFunc = Box<dyn FnOnce(&Weak<GlobalsMap>) -> GlobalsMap>;
+type GlobalsMap = HashMap<&'static str, Rc<dyn Fn(&mut Compiler, Span)>>;
+
+/// Set of builtins that (if they exist) should be made available in
+/// the global scope, meaning that they can be accessed not just
+/// through `builtins.<name>`, but directly as `<name>`. This is not
+/// configurable, it is based on what Nix 2.3 exposed.
+const GLOBAL_BUILTINS: &'static [&'static str] = &[
+    "abort",
+    "baseNameOf",
+    "derivation",
+    "derivationStrict",
+    "dirOf",
+    "fetchGit",
+    "fetchMercurial",
+    "fetchTarball",
+    "fromTOML",
+    "import",
+    "isNull",
+    "map",
+    "placeholder",
+    "removeAttrs",
+    "scopedImport",
+    "throw",
+    "toString",
+];
 
 pub struct Compiler<'observer> {
     contexts: Vec<LambdaCtx>,
@@ -1183,19 +1206,57 @@ fn optimise_tail_call(chunk: &mut Chunk) {
     }
 }
 
-/// Prepare the full set of globals from additional globals supplied
-/// by the caller of the compiler, as well as the built-in globals
-/// that are always part of the language.  This also "ties the knot"
-/// required in order for import to have a reference cycle back to
-/// the globals.
+/// Prepare the full set of globals available in evaluated code. These
+/// are constructed from the set of builtins supplied by the caller,
+/// which are made available globally under the `builtins` identifier.
+///
+/// A subset of builtins (specified by [`GLOBAL_BUILTINS`]) is
+/// available globally *iff* they are set.
 ///
-/// Note that all builtin functions are *not* considered part of the
-/// language in this sense and MUST be supplied as additional global
-/// values, including the `builtins` set itself.
-pub fn prepare_globals(additional: GlobalsMapFunc) -> Rc<GlobalsMap> {
-    Rc::new_cyclic(Box::new(|weak: &Weak<GlobalsMap>| {
-        let mut globals = additional(weak);
+/// Optionally adds the `import` feature if desired by the caller.
+pub fn prepare_globals(
+    builtins: Vec<(&'static str, Value)>,
+    source: SourceCode,
+    enable_import: bool,
+) -> Rc<GlobalsMap> {
+    Rc::new_cyclic(Box::new(move |weak: &Weak<GlobalsMap>| {
+        // First step is to construct the builtins themselves as
+        // `NixAttrs`.
+        let mut builtins_under_construction: HashMap<&'static str, Value> =
+            HashMap::from_iter(builtins.into_iter());
+
+        // At this point, optionally insert `import` if enabled. To
+        // "tie the knot" of `import` needing the full set of globals
+        // to instantiate its compiler, the `Weak` reference is passed
+        // here.
+        if enable_import {
+            let import = Value::Builtin(import::builtins_import(weak, source));
+            builtins_under_construction.insert("import", import);
+        }
+
+        // Next, the actual map of globals is constructed and
+        // populated with (copies) of the values that should be
+        // available in the global scope (see [`GLOBAL_BUILTINS`]).
+        let mut globals: GlobalsMap = HashMap::new();
+
+        for global in GLOBAL_BUILTINS {
+            if let Some(builtin) = builtins_under_construction.get(global).cloned() {
+                let global_builtin: Global =
+                    Rc::new(move |c, s| c.emit_constant(builtin.clone(), &s));
+                globals.insert(global, global_builtin);
+            }
+        }
+
+        // This is followed by the actual `builtins` attribute set
+        // being constructed and inserted in the global scope.
+        let builtins_set =
+            Value::attrs(NixAttrs::from_iter(builtins_under_construction.into_iter()));
+        globals.insert(
+            "builtins",
+            Rc::new(move |c, s| c.emit_constant(builtins_set.clone(), &s)),
+        );
 
+        // Finally insert the compiler-internal "magic" builtins for top-level values.
         globals.insert(
             "true",
             Rc::new(|compiler, span| {