feat(tvix/eval): initial implementation of `builtins.import` r/5041

This adds an initial working version of builtins.import which
encapsulates the entire functionality of `import` within the builtin
itself, without requiring any changes in the compiler or VM.

The key insight that enables this is that we can simply return a Thunk
from `import` that is constructed from the output of running the
compiler and - ta-da! - no other component needs to know about it.

A couple of notes:

* builtins.import needs to capture variables like the SourceCode
  structure. This means it can not currently be constructed the same
  way as other builtins and has special handling, which leaks out to
  `eval.rs`. I have postponed dealing with that until we have this
  working a bit more.

* the `globals` are not yet passed through

* the error representation for the new variants is absolutely not done
  yet, we probably want to switch to something that supports
  cause-chaining now (like miette)

* there is no mechanism for emitting warnings at runtime; we need to
  add that

Change-Id: I3117a7ae3ff2432bf44f5ff05ad35f47faca31d5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6857
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

1 files changed, 52 insertions, 0 deletions

diff --git a/tvix/eval/src/errors.rs b/tvix/eval/src/errors.rs
index 75fac9c926..b374696171 100644
--- a/tvix/eval/src/errors.rs
+++ b/tvix/eval/src/errors.rs
@@ -1,5 +1,6 @@
 use crate::value::CoercionKind;
 use std::path::PathBuf;
+use std::rc::Rc;
 use std::{fmt::Display, num::ParseIntError};
 
 use codemap::Span;
@@ -99,6 +100,24 @@ pub enum ErrorKind {
     /// literal attribute sets.
     UnmergeableValue,
 
+    /// Tvix failed to read a file from disk for some reason.
+    ReadFileError {
+        path: PathBuf,
+        error: Rc<std::io::Error>,
+    },
+
+    /// Parse errors occured while importing a file.
+    ImportParseError {
+        path: PathBuf,
+        errors: Vec<rnix::parser::ParseError>,
+    },
+
+    /// Compilation errors occured while importing a file.
+    ImportCompilerError {
+        path: PathBuf,
+        errors: Vec<Error>,
+    },
+
     /// Tvix internal warning for features triggered by users that are
     /// not actually implemented yet, and without which eval can not
     /// proceed.
@@ -271,6 +290,34 @@ to a missing value in the attribute set(s) included via `with`."#,
                     .into()
             }
 
+            ErrorKind::ReadFileError { path, error } => {
+                format!(
+                    "failed to read file '{}': {}",
+                    path.to_string_lossy(),
+                    error
+                )
+            }
+
+            ErrorKind::ImportParseError { errors, path } => {
+                format!(
+                    "{} parse errors occured while importing '{}'",
+                    errors.len(),
+                    path.to_string_lossy()
+                )
+            }
+
+            ErrorKind::ImportCompilerError { errors, path } => {
+                // TODO: chain display of these errors, though this is
+                // probably not the right place for that (should
+                // branch into a more elaborate diagnostic() call
+                // below).
+                format!(
+                    "{} errors occured while importing '{}'",
+                    errors.len(),
+                    path.to_string_lossy()
+                )
+            }
+
             ErrorKind::NotImplemented(feature) => {
                 format!("feature not yet implemented in Tvix: {}", feature)
             }
@@ -305,6 +352,11 @@ to a missing value in the attribute set(s) included via `with`."#,
             ErrorKind::TailEmptyList { .. } => "E023",
             ErrorKind::UnmergeableInherit { .. } => "E024",
             ErrorKind::UnmergeableValue => "E025",
+            ErrorKind::ReadFileError { .. } => "E026",
+            ErrorKind::ImportParseError { .. } => "E027",
+            ErrorKind::ImportCompilerError { .. } => "E028",
+
+            // Placeholder error while Tvix is under construction.
             ErrorKind::NotImplemented(_) => "E999",
 
             // TODO: thunk force errors should yield a chained