feat(tvix/eval): add an AST pretty-printing module r/5143

This implements serde::Serialize for the rnix AST through a wrapper
type, and exposes a function for serialising the AST into
a (pretty-printed JSON) string representation.

This can be used to debug issues with the AST, and to display an AST
reprsentation in tools like tvixbolt.

Serialize is implemented manually because we don't own any of the
structs and the way to traverse them is not easily derived
automatically, and this is quite verbose. We might be able to condense
it a little bit, but at the same time it's also fairly straightforward.

Change-Id: I922df43cfc25636f3c8baee7944c75ade516055c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6943
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: Adam Joseph <adam@westernsemico.com>
Reviewed-by: tazjin <tazjin@tvl.su>

3 files changed, 499 insertions, 1 deletions

diff --git a/tvix/eval/src/eval.rs b/tvix/eval/src/eval.rs
index 4787b55fe475..df2c562cfa33 100644
--- a/tvix/eval/src/eval.rs
+++ b/tvix/eval/src/eval.rs
@@ -5,6 +5,7 @@ use crate::{
     errors::{Error, ErrorKind, EvalResult},
     nix_search_path::NixSearchPath,
     observer::{DisassemblingObserver, NoOpObserver, TracingObserver},
+    pretty_ast::pretty_print_expr,
     value::Value,
     SourceCode,
 };
@@ -76,7 +77,7 @@ pub fn interpret(code: &str, location: Option<PathBuf>, options: Options) -> Eva
         .expect("expression should exist if no errors occured");
 
     if options.display_ast {
-        println!("{:?}", root_expr);
+        println!("{}", pretty_print_expr(&root_expr));
     }
 
     // TODO: encapsulate this import weirdness in builtins
diff --git a/tvix/eval/src/lib.rs b/tvix/eval/src/lib.rs
index a7505eaf3acc..04b5a3290b8d 100644
--- a/tvix/eval/src/lib.rs
+++ b/tvix/eval/src/lib.rs
@@ -5,6 +5,7 @@ mod errors;
 mod eval;
 pub mod observer;
 mod opcode;
+mod pretty_ast;
 mod source;
 mod spans;
 mod upvalues;
@@ -25,6 +26,7 @@ pub use crate::builtins::global_builtins;
 pub use crate::compiler::compile;
 pub use crate::errors::EvalResult;
 pub use crate::eval::{interpret, Options};
+pub use crate::pretty_ast::pretty_print_expr;
 pub use crate::source::SourceCode;
 pub use crate::value::Value;
 pub use crate::vm::run_lambda;
diff --git a/tvix/eval/src/pretty_ast.rs b/tvix/eval/src/pretty_ast.rs
new file mode 100644
index 000000000000..e8e3b214c497
--- /dev/null
+++ b/tvix/eval/src/pretty_ast.rs
@@ -0,0 +1,495 @@
+//! Pretty-printed format for the rnix AST representation.
+//!
+//! The AST is serialised into a JSON structure that can then be
+//! printed in either minimised or well-formatted style.
+
+use rnix::ast::{self, AstToken, HasEntry};
+use serde::{ser::SerializeMap, Serialize, Serializer};
+
+pub fn pretty_print_expr(expr: &ast::Expr) -> String {
+    serde_json::ser::to_string_pretty(&SerializeAST(expr))
+        .expect("serializing AST should always succeed")
+}
+
+#[repr(transparent)]
+struct SerializeAST<S>(S);
+
+impl<'a> Serialize for SerializeAST<&'a ast::Apply> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(3))?;
+        map.serialize_entry("kind", "apply")?;
+        map.serialize_entry("fn", &SerializeAST(&self.0.lambda().unwrap()))?;
+        map.serialize_entry("arg", &SerializeAST(&self.0.argument().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Assert> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(3))?;
+        map.serialize_entry("kind", "assert")?;
+        map.serialize_entry("condition", &SerializeAST(&self.0.condition().unwrap()))?;
+        map.serialize_entry("body", &SerializeAST(&self.0.body().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Error> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "error")?;
+        map.serialize_entry("node", &self.0.to_string())?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::IfElse> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(4))?;
+        map.serialize_entry("kind", "if_else")?;
+        map.serialize_entry("condition", &SerializeAST(&self.0.condition().unwrap()))?;
+        map.serialize_entry("then_body", &SerializeAST(&self.0.body().unwrap()))?;
+        map.serialize_entry("else_body", &SerializeAST(&self.0.else_body().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Select> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let size = match self.0.default_expr() {
+            Some(_) => 4,
+            None => 3,
+        };
+
+        let mut map = serializer.serialize_map(Some(size))?;
+        map.serialize_entry("kind", "select")?;
+        map.serialize_entry("set", &SerializeAST(&self.0.expr().unwrap()))?;
+        map.serialize_entry("path", &SerializeAST(self.0.attrpath().unwrap()))?;
+
+        if let Some(default) = self.0.default_expr() {
+            map.serialize_entry("default", &SerializeAST(&default))?;
+        }
+
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<ast::InterpolPart<String>> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        match &self.0 {
+            ast::InterpolPart::Literal(s) => Serialize::serialize(s, serializer),
+            ast::InterpolPart::Interpolation(node) => {
+                Serialize::serialize(&SerializeAST(&node.expr().unwrap()), serializer)
+            }
+        }
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Str> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "string")?;
+
+        map.serialize_entry(
+            "parts",
+            &self
+                .0
+                .normalized_parts()
+                .into_iter()
+                .map(|part| SerializeAST(part))
+                .collect::<Vec<_>>(),
+        )?;
+
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<ast::InterpolPart<ast::PathContent>> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        match &self.0 {
+            ast::InterpolPart::Literal(p) => Serialize::serialize(p.syntax().text(), serializer),
+            ast::InterpolPart::Interpolation(node) => {
+                Serialize::serialize(&SerializeAST(&node.expr().unwrap()), serializer)
+            }
+        }
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Path> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "path")?;
+
+        map.serialize_entry(
+            "parts",
+            &self
+                .0
+                .parts()
+                .map(|part| SerializeAST(part))
+                .collect::<Vec<_>>(),
+        )?;
+
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Literal> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        match self.0.kind() {
+            ast::LiteralKind::Float(val) => serializer.serialize_f64(val.value().unwrap()),
+            ast::LiteralKind::Integer(val) => serializer.serialize_i64(val.value().unwrap()),
+            ast::LiteralKind::Uri(val) => {
+                let url = val.syntax().text();
+                let mut map = serializer.serialize_map(Some(2))?;
+                map.serialize_entry("kind", "url")?;
+                map.serialize_entry("url", url)?;
+                map.end()
+            }
+        }
+    }
+}
+
+impl<'a> Serialize for SerializeAST<ast::PatEntry> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(None)?;
+        map.serialize_entry("ident", &SerializeAST(&self.0.ident().unwrap()))?;
+
+        if let Some(default) = self.0.default() {
+            map.serialize_entry("default", &SerializeAST(&default))?;
+        }
+
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<ast::Param> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        match &self.0 {
+            ast::Param::Pattern(pat) => {
+                let mut map = serializer.serialize_map(None)?;
+                map.serialize_entry("kind", "formals")?;
+
+                map.serialize_entry(
+                    "entries",
+                    &pat.pat_entries()
+                        .map(|entry| SerializeAST(entry))
+                        .collect::<Vec<_>>(),
+                )?;
+
+                if let Some(bind) = pat.pat_bind() {
+                    map.serialize_entry("bind", &SerializeAST(&bind.ident().unwrap()))?;
+                }
+
+                map.serialize_entry("ellipsis", &pat.ellipsis_token().is_some())?;
+
+                map.end()
+            }
+
+            ast::Param::IdentParam(node) => {
+                Serialize::serialize(&SerializeAST(&node.ident().unwrap()), serializer)
+            }
+        }
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Lambda> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(3))?;
+        map.serialize_entry("kind", "lambda")?;
+        map.serialize_entry("param", &SerializeAST(self.0.param().unwrap()))?;
+        map.serialize_entry("body", &SerializeAST(self.0.body().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::LegacyLet> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(3))?;
+        map.serialize_entry("kind", "legacy_let")?;
+
+        map.serialize_entry(
+            "entries",
+            &self
+                .0
+                .attrpath_values()
+                .map(|val| SerializeAST(val))
+                .collect::<Vec<_>>(),
+        )?;
+
+        map.serialize_entry(
+            "inherits",
+            &self
+                .0
+                .inherits()
+                .map(|val| SerializeAST(val))
+                .collect::<Vec<_>>(),
+        )?;
+
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::LetIn> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(3))?;
+        map.serialize_entry("kind", "let")?;
+
+        map.serialize_entry(
+            "entries",
+            &self
+                .0
+                .attrpath_values()
+                .map(|val| SerializeAST(val))
+                .collect::<Vec<_>>(),
+        )?;
+
+        map.serialize_entry(
+            "inherits",
+            &self
+                .0
+                .inherits()
+                .map(|val| SerializeAST(val))
+                .collect::<Vec<_>>(),
+        )?;
+
+        map.serialize_entry("body", &SerializeAST(&self.0.body().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::List> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let list = self
+            .0
+            .items()
+            .map(|elem| SerializeAST(elem))
+            .collect::<Vec<_>>();
+
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "list")?;
+        map.serialize_entry("items", &list)?;
+
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::BinOp> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(4))?;
+        map.serialize_entry("kind", "binary_op")?;
+
+        map.serialize_entry(
+            "operator",
+            match self.0.operator().unwrap() {
+                ast::BinOpKind::Concat => "concat",
+                ast::BinOpKind::Update => "update",
+                ast::BinOpKind::Add => "add",
+                ast::BinOpKind::Sub => "sub",
+                ast::BinOpKind::Mul => "mul",
+                ast::BinOpKind::Div => "div",
+                ast::BinOpKind::And => "and",
+                ast::BinOpKind::Equal => "equal",
+                ast::BinOpKind::Implication => "implication",
+                ast::BinOpKind::Less => "less",
+                ast::BinOpKind::LessOrEq => "less_or_eq",
+                ast::BinOpKind::More => "more",
+                ast::BinOpKind::MoreOrEq => "more_or_eq",
+                ast::BinOpKind::NotEqual => "not_equal",
+                ast::BinOpKind::Or => "or",
+            },
+        )?;
+
+        map.serialize_entry("lhs", &SerializeAST(&self.0.lhs().unwrap()))?;
+        map.serialize_entry("rhs", &SerializeAST(&self.0.rhs().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Paren> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "paren")?;
+        map.serialize_entry("expr", &SerializeAST(&self.0.expr().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Root> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "root")?;
+        map.serialize_entry("expr", &SerializeAST(&self.0.expr().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<ast::AttrpathValue> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("name", &SerializeAST(self.0.attrpath().unwrap()))?;
+        map.serialize_entry("value", &SerializeAST(self.0.value().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<ast::Inherit> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(None)?;
+
+        if let Some(from) = self.0.from() {
+            map.serialize_entry("namespace", &SerializeAST(&from.expr().unwrap()))?;
+        }
+
+        map.serialize_entry(
+            "names",
+            &self.0.attrs().map(|a| SerializeAST(a)).collect::<Vec<_>>(),
+        )?;
+
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::AttrSet> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(None)?;
+        map.serialize_entry("kind", "attrset")?;
+        map.serialize_entry("recursive", &self.0.rec_token().is_some())?;
+
+        map.serialize_entry(
+            "entries",
+            &self
+                .0
+                .attrpath_values()
+                .map(|val| SerializeAST(val))
+                .collect::<Vec<_>>(),
+        )?;
+
+        map.serialize_entry(
+            "inherits",
+            &self
+                .0
+                .inherits()
+                .map(|val| SerializeAST(val))
+                .collect::<Vec<_>>(),
+        )?;
+
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::UnaryOp> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(3))?;
+        map.serialize_entry("kind", "unary_op")?;
+
+        map.serialize_entry(
+            "operator",
+            match self.0.operator().unwrap() {
+                ast::UnaryOpKind::Invert => "invert",
+                ast::UnaryOpKind::Negate => "negate",
+            },
+        )?;
+
+        map.serialize_entry("expr", &SerializeAST(&self.0.expr().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Ident> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "ident")?;
+        map.serialize_entry("ident", self.0.ident_token().unwrap().text())?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::With> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(3))?;
+        map.serialize_entry("kind", "with")?;
+        map.serialize_entry("with", &SerializeAST(&self.0.namespace().unwrap()))?;
+        map.serialize_entry("body", &SerializeAST(&self.0.body().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Dynamic> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "dynamic")?;
+        map.serialize_entry("expr", &SerializeAST(&self.0.expr().unwrap()))?;
+        map.end()
+    }
+}
+
+impl Serialize for SerializeAST<ast::Attr> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        match &self.0 {
+            ast::Attr::Ident(ident) => Serialize::serialize(&SerializeAST(ident), serializer),
+            ast::Attr::Dynamic(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Attr::Str(node) => Serialize::serialize(&SerializeAST(node), serializer),
+        }
+    }
+}
+
+impl Serialize for SerializeAST<ast::Attrpath> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "attrpath")?;
+
+        map.serialize_entry(
+            "path",
+            &self
+                .0
+                .attrs()
+                .map(|attr| SerializeAST(attr))
+                .collect::<Vec<_>>(),
+        )?;
+
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::HasAttr> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(3))?;
+        map.serialize_entry("kind", "has_attr")?;
+        map.serialize_entry("expr", &SerializeAST(&self.0.expr().unwrap()))?;
+        map.serialize_entry("attrpath", &SerializeAST(self.0.attrpath().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Expr> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        match self.0 {
+            ast::Expr::Apply(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Assert(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Error(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::IfElse(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Select(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Str(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Path(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Literal(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Lambda(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::LegacyLet(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::LetIn(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::List(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::BinOp(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Paren(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Root(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::AttrSet(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::UnaryOp(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Ident(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::With(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::HasAttr(node) => Serialize::serialize(&SerializeAST(node), serializer),
+        }
+    }
+}
+
+impl Serialize for SerializeAST<ast::Expr> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        SerializeAST(&self.0).serialize(serializer)
+    }
+}