refactor(tvix/eval): collect vector of errors in compiler r/4572

Instead of exiting the compiler at the first sight of an error,
skip any erroneous nodes and continue compiling, collecting more
errors along the way.

This paves the way for nicer error reporting in which multiple errors
can be reported at once, avoiding situations in which users are
hunting a fault error-by-error and possibly getting distracted by
less useful output.

Change-Id: I80c9a87272e33a31297167ae2eb2706a46adf15a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6236
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

3 files changed, 111 insertions, 102 deletions

diff --git a/tvix/eval/src/compiler.rs b/tvix/eval/src/compiler.rs
index 92f04c84dc..577578a936 100644
--- a/tvix/eval/src/compiler.rs
+++ b/tvix/eval/src/compiler.rs
@@ -19,7 +19,7 @@ use rowan::ast::AstNode;
 use std::path::{Path, PathBuf};
 
 use crate::chunk::Chunk;
-use crate::errors::{ErrorKind, EvalResult};
+use crate::errors::{Error, ErrorKind, EvalResult};
 use crate::opcode::{CodeIdx, OpCode};
 use crate::value::Value;
 use crate::warnings::{EvalWarning, WarningKind};
@@ -30,6 +30,7 @@ use crate::warnings::{EvalWarning, WarningKind};
 pub struct CompilationResult {
     pub chunk: Chunk,
     pub warnings: Vec<EvalWarning>,
+    pub errors: Vec<Error>,
 }
 
 // Represents a single local already known to the compiler.
@@ -87,11 +88,12 @@ struct Compiler {
     scope: Scope,
 
     warnings: Vec<EvalWarning>,
+    errors: Vec<Error>,
     root_dir: PathBuf,
 }
 
 impl Compiler {
-    fn compile(&mut self, expr: ast::Expr) -> EvalResult<()> {
+    fn compile(&mut self, expr: ast::Expr) {
         match expr {
             ast::Expr::Literal(literal) => self.compile_literal(literal),
             ast::Expr::Path(path) => self.compile_path(path),
@@ -121,18 +123,16 @@ impl Compiler {
         }
     }
 
-    fn compile_literal(&mut self, node: ast::Literal) -> EvalResult<()> {
+    fn compile_literal(&mut self, node: ast::Literal) {
         match node.kind() {
             ast::LiteralKind::Float(f) => {
                 let idx = self.chunk.push_constant(Value::Float(f.value().unwrap()));
                 self.chunk.push_op(OpCode::OpConstant(idx));
-                Ok(())
             }
 
             ast::LiteralKind::Integer(i) => {
                 let idx = self.chunk.push_constant(Value::Integer(i.value().unwrap()));
                 self.chunk.push_op(OpCode::OpConstant(idx));
-                Ok(())
             }
             ast::LiteralKind::Uri(u) => {
                 self.emit_warning(node.syntax().clone(), WarningKind::DeprecatedLiteralURL);
@@ -141,12 +141,11 @@ impl Compiler {
                     .chunk
                     .push_constant(Value::String(u.syntax().text().into()));
                 self.chunk.push_op(OpCode::OpConstant(idx));
-                Ok(())
             }
         }
     }
 
-    fn compile_path(&mut self, node: ast::Path) -> EvalResult<()> {
+    fn compile_path(&mut self, node: ast::Path) {
         // TODO(tazjin): placeholder implementation while waiting for
         // https://github.com/nix-community/rnix-parser/pull/96
 
@@ -154,9 +153,16 @@ impl Compiler {
         let path = if raw_path.starts_with('/') {
             Path::new(&raw_path).to_owned()
         } else if raw_path.starts_with('~') {
-            let mut buf = dirs::home_dir().ok_or_else(|| {
-                ErrorKind::PathResolution("failed to determine home directory".into())
-            })?;
+            let mut buf = match dirs::home_dir() {
+                Some(buf) => buf,
+                None => {
+                    self.emit_error(
+                        node.syntax().clone(),
+                        ErrorKind::PathResolution("failed to determine home directory".into()),
+                    );
+                    return;
+                }
+            };
 
             buf.push(&raw_path);
             buf
@@ -174,11 +180,9 @@ impl Compiler {
         let value = Value::Path(path.clean());
         let idx = self.chunk.push_constant(value);
         self.chunk.push_op(OpCode::OpConstant(idx));
-
-        Ok(())
     }
 
-    fn compile_str(&mut self, node: ast::Str) -> EvalResult<()> {
+    fn compile_str(&mut self, node: ast::Str) {
         let mut count = 0;
 
         // The string parts are produced in literal order, however
@@ -192,7 +196,7 @@ impl Compiler {
                 // Interpolated expressions are compiled as normal and
                 // dealt with by the VM before being assembled into
                 // the final string.
-                ast::InterpolPart::Interpolation(node) => self.compile(node.expr().unwrap())?,
+                ast::InterpolPart::Interpolation(node) => self.compile(node.expr().unwrap()),
 
                 ast::InterpolPart::Literal(lit) => {
                     let idx = self.chunk.push_constant(Value::String(lit.into()));
@@ -204,12 +208,10 @@ impl Compiler {
         if count != 1 {
             self.chunk.push_op(OpCode::OpInterpolate(count));
         }
-
-        Ok(())
     }
 
-    fn compile_unary_op(&mut self, op: ast::UnaryOp) -> EvalResult<()> {
-        self.compile(op.expr().unwrap())?;
+    fn compile_unary_op(&mut self, op: ast::UnaryOp) {
+        self.compile(op.expr().unwrap());
 
         let opcode = match op.operator().unwrap() {
             ast::UnaryOpKind::Invert => OpCode::OpInvert,
@@ -217,10 +219,9 @@ impl Compiler {
         };
 
         self.chunk.push_op(opcode);
-        Ok(())
     }
 
-    fn compile_binop(&mut self, op: ast::BinOp) -> EvalResult<()> {
+    fn compile_binop(&mut self, op: ast::BinOp) {
         use ast::BinOpKind;
 
         // Short-circuiting and other strange operators, which are
@@ -238,8 +239,8 @@ impl Compiler {
         // For all other operators, the two values need to be left on
         // the stack in the correct order before pushing the
         // instruction for the operation itself.
-        self.compile(op.lhs().unwrap())?;
-        self.compile(op.rhs().unwrap())?;
+        self.compile(op.lhs().unwrap());
+        self.compile(op.rhs().unwrap());
 
         match op.operator().unwrap() {
             BinOpKind::Add => self.chunk.push_op(OpCode::OpAdd),
@@ -264,11 +265,9 @@ impl Compiler {
                 unreachable!()
             }
         };
-
-        Ok(())
     }
 
-    fn compile_and(&mut self, node: ast::BinOp) -> EvalResult<()> {
+    fn compile_and(&mut self, node: ast::BinOp) {
         debug_assert!(
             matches!(node.operator(), Some(ast::BinOpKind::And)),
             "compile_and called with wrong operator kind: {:?}",
@@ -276,7 +275,7 @@ impl Compiler {
         );
 
         // Leave left-hand side value on the stack.
-        self.compile(node.lhs().unwrap())?;
+        self.compile(node.lhs().unwrap());
 
         // If this value is false, jump over the right-hand side - the
         // whole expression is false.
@@ -286,15 +285,13 @@ impl Compiler {
         // right-hand side on the stack. Its result is now the value
         // of the whole expression.
         self.chunk.push_op(OpCode::OpPop);
-        self.compile(node.rhs().unwrap())?;
+        self.compile(node.rhs().unwrap());
 
         self.patch_jump(end_idx);
         self.chunk.push_op(OpCode::OpAssertBool);
-
-        Ok(())
     }
 
-    fn compile_or(&mut self, node: ast::BinOp) -> EvalResult<()> {
+    fn compile_or(&mut self, node: ast::BinOp) {
         debug_assert!(
             matches!(node.operator(), Some(ast::BinOpKind::Or)),
             "compile_or called with wrong operator kind: {:?}",
@@ -302,20 +299,18 @@ impl Compiler {
         );
 
         // Leave left-hand side value on the stack
-        self.compile(node.lhs().unwrap())?;
+        self.compile(node.lhs().unwrap());
 
         // Opposite of above: If this value is **true**, we can
         // short-circuit the right-hand side.
         let end_idx = self.chunk.push_op(OpCode::OpJumpIfTrue(0));
         self.chunk.push_op(OpCode::OpPop);
-        self.compile(node.rhs().unwrap())?;
+        self.compile(node.rhs().unwrap());
         self.patch_jump(end_idx);
         self.chunk.push_op(OpCode::OpAssertBool);
-
-        Ok(())
     }
 
-    fn compile_implication(&mut self, node: ast::BinOp) -> EvalResult<()> {
+    fn compile_implication(&mut self, node: ast::BinOp) {
         debug_assert!(
             matches!(node.operator(), Some(ast::BinOpKind::Implication)),
             "compile_implication called with wrong operator kind: {:?}",
@@ -323,22 +318,20 @@ impl Compiler {
         );
 
         // Leave left-hand side value on the stack and invert it.
-        self.compile(node.lhs().unwrap())?;
+        self.compile(node.lhs().unwrap());
         self.chunk.push_op(OpCode::OpInvert);
 
         // Exactly as `||` (because `a -> b` = `!a || b`).
         let end_idx = self.chunk.push_op(OpCode::OpJumpIfTrue(0));
         self.chunk.push_op(OpCode::OpPop);
-        self.compile(node.rhs().unwrap())?;
+        self.compile(node.rhs().unwrap());
         self.patch_jump(end_idx);
         self.chunk.push_op(OpCode::OpAssertBool);
-
-        Ok(())
     }
 
-    fn compile_has_attr(&mut self, node: ast::HasAttr) -> EvalResult<()> {
+    fn compile_has_attr(&mut self, node: ast::HasAttr) {
         // Put the attribute set on the stack.
-        self.compile(node.expr().unwrap())?;
+        self.compile(node.expr().unwrap());
         let mut count = 0;
 
         // Push all path fragments with an operation for fetching the
@@ -348,24 +341,19 @@ impl Compiler {
                 self.chunk.push_op(OpCode::OpAttrOrNotFound);
             }
             count += 1;
-            self.compile_attr(fragment)?;
+            self.compile_attr(fragment);
         }
 
         // After the last fragment, emit the actual instruction that
         // leaves a boolean on the stack.
         self.chunk.push_op(OpCode::OpAttrsIsSet);
-
-        Ok(())
     }
 
-    fn compile_attr(&mut self, node: ast::Attr) -> EvalResult<()> {
+    fn compile_attr(&mut self, node: ast::Attr) {
         match node {
             ast::Attr::Dynamic(dynamic) => self.compile(dynamic.expr().unwrap()),
             ast::Attr::Str(s) => self.compile_str(s),
-            ast::Attr::Ident(ident) => {
-                self.emit_literal_ident(&ident);
-                Ok(())
-            }
+            ast::Attr::Ident(ident) => self.emit_literal_ident(&ident),
         }
     }
 
@@ -375,16 +363,15 @@ impl Compiler {
     //
     // The VM, after evaluating the code for each element, simply
     // constructs the list from the given number of elements.
-    fn compile_list(&mut self, node: ast::List) -> EvalResult<()> {
+    fn compile_list(&mut self, node: ast::List) {
         let mut count = 0;
 
         for item in node.items() {
             count += 1;
-            self.compile(item)?;
+            self.compile(item);
         }
 
         self.chunk.push_op(OpCode::OpList(count));
-        Ok(())
     }
 
     // Compile attribute set literals into equivalent bytecode.
@@ -395,7 +382,7 @@ impl Compiler {
     // 1. Keys can be dynamically constructed through interpolation.
     // 2. Keys can refer to nested attribute sets.
     // 3. Attribute sets can (optionally) be recursive.
-    fn compile_attr_set(&mut self, node: ast::AttrSet) -> EvalResult<()> {
+    fn compile_attr_set(&mut self, node: ast::AttrSet) {
         if node.rec_token().is_some() {
             todo!("recursive attribute sets are not yet implemented")
         }
@@ -422,7 +409,7 @@ impl Compiler {
                         // instruction followed by a merge, rather
                         // than pushing/popping the same attrs
                         // potentially a lot of times.
-                        self.compile(from.expr().unwrap())?;
+                        self.compile(from.expr().unwrap());
                         self.emit_literal_ident(&ident);
                         self.chunk.push_op(OpCode::OpAttrsSelect);
                     }
@@ -435,7 +422,13 @@ impl Compiler {
 
                         match self.resolve_local(ident.ident_token().unwrap().text()) {
                             Some(idx) => self.chunk.push_op(OpCode::OpGetLocal(idx)),
-                            None => return Err(ErrorKind::UnknownStaticVariable(ident).into()),
+                            None => {
+                                self.emit_error(
+                                    ident.syntax().clone(),
+                                    ErrorKind::UnknownStaticVariable,
+                                );
+                                continue;
+                            }
                         };
                     }
                 }
@@ -453,7 +446,7 @@ impl Compiler {
             let mut key_count = 0;
             for fragment in kv.attrpath().unwrap().attrs() {
                 key_count += 1;
-                self.compile_attr(fragment)?;
+                self.compile_attr(fragment);
             }
 
             // We're done with the key if there was only one fragment,
@@ -466,34 +459,32 @@ impl Compiler {
             // The value is just compiled as normal so that its
             // resulting value is on the stack when the attribute set
             // is constructed at runtime.
-            self.compile(kv.value().unwrap())?;
+            self.compile(kv.value().unwrap());
         }
 
         self.chunk.push_op(OpCode::OpAttrs(count));
-        Ok(())
     }
 
-    fn compile_select(&mut self, node: ast::Select) -> EvalResult<()> {
+    fn compile_select(&mut self, node: ast::Select) {
         let set = node.expr().unwrap();
         let path = node.attrpath().unwrap();
 
         if node.or_token().is_some() {
-            return self.compile_select_or(set, path, node.default_expr().unwrap());
+            self.compile_select_or(set, path, node.default_expr().unwrap());
+            return;
         }
 
         // Push the set onto the stack
-        self.compile(set)?;
+        self.compile(set);
 
         // Compile each key fragment and emit access instructions.
         //
         // TODO: multi-select instruction to avoid re-pushing attrs on
         // nested selects.
         for fragment in path.attrs() {
-            self.compile_attr(fragment)?;
+            self.compile_attr(fragment);
             self.chunk.push_op(OpCode::OpAttrsSelect);
         }
-
-        Ok(())
     }
 
     /// Compile an `or` expression into a chunk of conditional jumps.
@@ -525,17 +516,12 @@ impl Compiler {
     ///  │ 14 ...                     │   │ ..   ....               │
     ///  └────────────────────────────┘   └─────────────────────────┘
     /// ```
-    fn compile_select_or(
-        &mut self,
-        set: ast::Expr,
-        path: ast::Attrpath,
-        default: ast::Expr,
-    ) -> EvalResult<()> {
-        self.compile(set)?;
+    fn compile_select_or(&mut self, set: ast::Expr, path: ast::Attrpath, default: ast::Expr) {
+        self.compile(set);
         let mut jumps = vec![];
 
         for fragment in path.attrs() {
-            self.compile_attr(fragment)?;
+            self.compile_attr(fragment);
             self.chunk.push_op(OpCode::OpAttrOrNotFound);
             jumps.push(self.chunk.push_op(OpCode::OpJumpIfNotFound(0)));
         }
@@ -548,21 +534,19 @@ impl Compiler {
 
         // Compile the default value expression and patch the final
         // jump to point *beyond* it.
-        self.compile(default)?;
+        self.compile(default);
         self.patch_jump(final_jump);
-
-        Ok(())
     }
 
-    fn compile_assert(&mut self, node: ast::Assert) -> EvalResult<()> {
+    fn compile_assert(&mut self, node: ast::Assert) {
         // Compile the assertion condition to leave its value on the stack.
-        self.compile(node.condition().unwrap())?;
+        self.compile(node.condition().unwrap());
         self.chunk.push_op(OpCode::OpAssert);
 
         // The runtime will abort evaluation at this point if the
         // assertion failed, if not the body simply continues on like
         // normal.
-        self.compile(node.body().unwrap())
+        self.compile(node.body().unwrap());
     }
 
     // Compile conditional expressions using jumping instructions in the VM.
@@ -575,23 +559,21 @@ impl Compiler {
     //  Jump over else body  ││ 4  [  else body  ]←┼─┘
     //  if condition is true.└┼─5─→     ...        │
     //                        └────────────────────┘
-    fn compile_if_else(&mut self, node: ast::IfElse) -> EvalResult<()> {
-        self.compile(node.condition().unwrap())?;
+    fn compile_if_else(&mut self, node: ast::IfElse) {
+        self.compile(node.condition().unwrap());
 
         let then_idx = self.chunk.push_op(OpCode::OpJumpIfFalse(0));
 
         self.chunk.push_op(OpCode::OpPop); // discard condition value
-        self.compile(node.body().unwrap())?;
+        self.compile(node.body().unwrap());
 
         let else_idx = self.chunk.push_op(OpCode::OpJump(0));
 
         self.patch_jump(then_idx); // patch jump *to* else_body
         self.chunk.push_op(OpCode::OpPop); // discard condition value
-        self.compile(node.else_body().unwrap())?;
+        self.compile(node.else_body().unwrap());
 
         self.patch_jump(else_idx); // patch jump *over* else body
-
-        Ok(())
     }
 
     // Compile a standard `let ...; in ...` statement.
@@ -599,7 +581,7 @@ impl Compiler {
     // Unless in a non-standard scope, the encountered values are
     // simply pushed on the stack and their indices noted in the
     // entries vector.
-    fn compile_let_in(&mut self, node: ast::LetIn) -> EvalResult<()> {
+    fn compile_let_in(&mut self, node: ast::LetIn) {
         self.begin_scope();
 
         for inherit in node.inherits() {
@@ -614,7 +596,7 @@ impl Compiler {
 
                 Some(from) => {
                     for ident in inherit.idents() {
-                        self.compile(from.expr().unwrap())?;
+                        self.compile(from.expr().unwrap());
                         self.emit_literal_ident(&ident);
                         self.chunk.push_op(OpCode::OpAttrsSelect);
                         self.declare_local(ident.ident_token().unwrap().text());
@@ -624,23 +606,28 @@ impl Compiler {
         }
 
         for entry in node.attrpath_values() {
-            let mut path = normalise_ident_path(entry.attrpath().unwrap().attrs())?;
+            let mut path = match normalise_ident_path(entry.attrpath().unwrap().attrs()) {
+                Ok(p) => p,
+                Err(err) => {
+                    self.errors.push(err);
+                    continue;
+                }
+            };
 
             if path.len() != 1 {
                 todo!("nested bindings in let expressions :(")
             }
 
-            self.compile(entry.value().unwrap())?;
+            self.compile(entry.value().unwrap());
             self.declare_local(path.pop().unwrap());
         }
 
         // Deal with the body, then clean up the locals afterwards.
-        self.compile(node.body().unwrap())?;
+        self.compile(node.body().unwrap());
         self.end_scope();
-        Ok(())
     }
 
-    fn compile_ident(&mut self, node: ast::Ident) -> EvalResult<()> {
+    fn compile_ident(&mut self, node: ast::Ident) {
         match node.ident_token().unwrap().text() {
             // TODO(tazjin): Nix technically allows code like
             //
@@ -663,7 +650,11 @@ impl Compiler {
                     Some(idx) => self.chunk.push_op(OpCode::OpGetLocal(idx)),
                     None => {
                         if self.scope.with_stack.is_empty() {
-                            return Err(ErrorKind::UnknownStaticVariable(node).into());
+                            self.emit_error(
+                                node.syntax().clone(),
+                                ErrorKind::UnknownStaticVariable,
+                            );
+                            return;
                         }
 
                         // Variable needs to be dynamically resolved
@@ -675,18 +666,16 @@ impl Compiler {
                 }
             }
         };
-
-        Ok(())
     }
 
     // Compile `with` expressions by emitting instructions that
     // pop/remove the indices of attribute sets that are implicitly in
     // scope through `with` on the "with-stack".
-    fn compile_with(&mut self, node: ast::With) -> EvalResult<()> {
+    fn compile_with(&mut self, node: ast::With) {
         // TODO: Detect if the namespace is just an identifier, and
         // resolve that directly (thus avoiding duplication on the
         // stack).
-        self.compile(node.namespace().unwrap())?;
+        self.compile(node.namespace().unwrap());
 
         self.declare_phantom();
         self.scope.with_stack.push(With {
@@ -696,7 +685,7 @@ impl Compiler {
         self.chunk
             .push_op(OpCode::OpPushWith(self.scope.locals.len() - 1));
 
-        self.compile(node.body().unwrap())
+        self.compile(node.body().unwrap());
     }
 
     /// Emit the literal string value of an identifier. Required for
@@ -825,6 +814,13 @@ impl Compiler {
     fn emit_warning(&mut self, node: rnix::SyntaxNode, kind: WarningKind) {
         self.warnings.push(EvalWarning { node, kind })
     }
+
+    fn emit_error(&mut self, node: rnix::SyntaxNode, kind: ErrorKind) {
+        self.errors.push(Error {
+            node: Some(node),
+            kind,
+        })
+    }
 }
 
 /// Convert a non-dynamic string expression to a string if possible,
@@ -836,7 +832,10 @@ fn expr_str_to_string(expr: ast::Str) -> EvalResult<String> {
         }
     }
 
-    return Err(ErrorKind::DynamicKeyInLet(expr.syntax().clone()).into());
+    return Err(Error {
+        node: Some(expr.syntax().clone()),
+        kind: ErrorKind::DynamicKeyInLet(expr.syntax().clone()),
+    });
 }
 
 /// Convert a single identifier path fragment to a string if possible,
@@ -883,13 +882,15 @@ pub fn compile(expr: ast::Expr, location: Option<PathBuf>) -> EvalResult<Compila
         root_dir,
         chunk: Chunk::default(),
         warnings: vec![],
+        errors: vec![],
         scope: Default::default(),
     };
 
-    c.compile(expr)?;
+    c.compile(expr);
 
     Ok(CompilationResult {
         chunk: c.chunk,
         warnings: c.warnings,
+        errors: c.errors,
     })
 }
diff --git a/tvix/eval/src/errors.rs b/tvix/eval/src/errors.rs
index 01a5e81449..fd3fddc326 100644
--- a/tvix/eval/src/errors.rs
+++ b/tvix/eval/src/errors.rs
@@ -1,6 +1,6 @@
 use std::fmt::Display;
 
-#[derive(Debug)]
+#[derive(Clone, Debug)]
 pub enum ErrorKind {
     DuplicateAttrsKey {
         key: String,
@@ -27,7 +27,7 @@ pub enum ErrorKind {
     DynamicKeyInLet(rnix::SyntaxNode),
 
     // Unknown variable in statically known scope.
-    UnknownStaticVariable(rnix::ast::Ident),
+    UnknownStaticVariable,
 
     // Unknown variable in dynamic scope (with, rec, ...).
     UnknownDynamicVariable(String),
@@ -37,7 +37,7 @@ pub enum ErrorKind {
     AssertionFailed,
 }
 
-#[derive(Debug)]
+#[derive(Clone, Debug)]
 pub struct Error {
     pub node: Option<rnix::SyntaxNode>,
     pub kind: ErrorKind,
diff --git a/tvix/eval/src/eval.rs b/tvix/eval/src/eval.rs
index 8f2946e6cc..10d36904e4 100644
--- a/tvix/eval/src/eval.rs
+++ b/tvix/eval/src/eval.rs
@@ -41,5 +41,13 @@ pub fn interpret(code: &str, location: Option<PathBuf>) -> EvalResult<Value> {
         )
     }
 
+    for error in &result.errors {
+        eprintln!("compiler error: {:?} at {:?}", error.kind, error.node,);
+    }
+
+    if let Some(err) = result.errors.last() {
+        return Err(err.clone());
+    }
+
     crate::vm::run_chunk(result.chunk)
 }