feat(tvix/eval): introduce `ToSpan` trait in compiler module r/4883

This trait can be used to convert most structures from rnix-parser
into a codemap::Span. It uses a macro to implement the trait for the
various expression types in the rnix AST, as Rust's silly semantic
versioning restriction stops us from doing a blanket implementation.

This will be used in the next commit to clean up the span handling in
the compiler a bit.

Change-Id: I0a437034e5fa203b5a49c6f25c45932a9f3b2bca
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6615
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

2 files changed, 96 insertions, 18 deletions

diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index 713ae2fdb2..e497e3ce8d 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -15,10 +15,11 @@
 
 mod attrs;
 mod scope;
+mod spans;
 
 use path_clean::PathClean;
 use rnix::ast::{self, AstToken, HasEntry};
-use rowan::ast::{AstChildren, AstNode};
+use rowan::ast::AstChildren;
 use smol_str::SmolStr;
 use std::collections::HashMap;
 use std::path::{Path, PathBuf};
@@ -32,6 +33,7 @@ use crate::value::{Closure, Lambda, Thunk, Value};
 use crate::warnings::{EvalWarning, WarningKind};
 
 use self::scope::{LocalIdx, LocalPosition, Scope, Upvalue, UpvalueKind};
+use self::spans::ToSpan;
 
 /// Represents the result of compiling a piece of Nix code. If
 /// compilation was successful, the resulting bytecode can be passed
@@ -119,24 +121,16 @@ impl Compiler<'_, '_> {
         &mut self.context_mut().scope
     }
 
-    fn span_for<N: AstNode>(&self, node: &N) -> codemap::Span {
-        let rowan_span = node.syntax().text_range();
-        self.file.span.subspan(
-            u32::from(rowan_span.start()) as u64,
-            u32::from(rowan_span.end()) as u64,
-        )
-    }
-
     /// Push a single instruction to the current bytecode chunk and
     /// track the source span from which it was compiled.
-    fn push_op<T: AstNode>(&mut self, data: OpCode, node: &T) -> CodeIdx {
+    fn push_op<T: ToSpan>(&mut self, data: OpCode, node: &T) -> CodeIdx {
         let span = self.span_for(node);
         self.chunk().push_op(data, span)
     }
 
     /// Emit a single constant to the current bytecode chunk and track
     /// the source span from which it was compiled.
-    fn emit_constant<T: AstNode>(&mut self, value: Value, node: &T) {
+    fn emit_constant<T: ToSpan>(&mut self, value: Value, node: &T) {
         let idx = self.chunk().push_constant(value);
         self.push_op(OpCode::OpConstant(idx), node);
     }
@@ -520,7 +514,7 @@ impl Compiler<'_, '_> {
 
     fn compile_recursive_scope<N>(&mut self, slot: LocalIdx, rec_attrs: bool, node: &N)
     where
-        N: AstNode + ast::HasEntry,
+        N: ToSpan + ast::HasEntry,
     {
         self.scope_mut().begin_scope();
 
@@ -1010,7 +1004,7 @@ impl Compiler<'_, '_> {
     // TODO: almost the same as Compiler::compile_lambda; unify?
     fn thunk<N, F>(&mut self, outer_slot: LocalIdx, node: &N, content: F)
     where
-        N: AstNode + Clone,
+        N: ToSpan + Clone,
         F: FnOnce(&mut Compiler, &N, LocalIdx),
     {
         self.new_context();
@@ -1053,7 +1047,7 @@ impl Compiler<'_, '_> {
 
     /// Emit the data instructions that the runtime needs to correctly
     /// assemble the upvalues struct.
-    fn emit_upvalue_data<T: AstNode>(
+    fn emit_upvalue_data<T: ToSpan>(
         &mut self,
         slot: LocalIdx,
         node: &T,
@@ -1126,7 +1120,7 @@ impl Compiler<'_, '_> {
 
     /// Decrease scope depth of the current function and emit
     /// instructions to clean up the stack at runtime.
-    fn cleanup_scope<N: AstNode>(&mut self, node: &N) {
+    fn cleanup_scope<N: ToSpan>(&mut self, node: &N) {
         // When ending a scope, all corresponding locals need to be
         // removed, but the value of the body needs to remain on the
         // stack. This is implemented by a separate instruction.
@@ -1153,7 +1147,7 @@ impl Compiler<'_, '_> {
     /// Declare a local variable known in the scope that is being
     /// compiled by pushing it to the locals. This is used to
     /// determine the stack offset of variables.
-    fn declare_local<S: Into<String>, N: AstNode>(&mut self, node: &N, name: S) -> LocalIdx {
+    fn declare_local<S: Into<String>, N: ToSpan>(&mut self, node: &N, name: S) -> LocalIdx {
         let name = name.into();
         let depth = self.scope().scope_depth();
 
@@ -1266,7 +1260,7 @@ impl Compiler<'_, '_> {
         idx
     }
 
-    fn emit_force<N: AstNode>(&mut self, node: &N) {
+    fn emit_force<N: ToSpan>(&mut self, node: &N) {
         self.push_op(OpCode::OpForce, node);
     }
 
@@ -1316,7 +1310,7 @@ impl Compiler<'_, '_> {
     /// in a `let`-expression.
     fn dynamic_key_error<N>(&self, node: &N) -> Error
     where
-        N: AstNode<Language = rnix::NixLanguage>,
+        N: ToSpan,
     {
         Error {
             kind: ErrorKind::DynamicKeyInLet,
diff --git a/tvix/eval/src/compiler/spans.rs b/tvix/eval/src/compiler/spans.rs
new file mode 100644
index 0000000000..a972a17edd
--- /dev/null
+++ b/tvix/eval/src/compiler/spans.rs
@@ -0,0 +1,84 @@
+//! Utilities for dealing with span tracking in the compiler.
+
+use super::*;
+use codemap::{File, Span};
+use rowan::ast::AstNode;
+
+/// Trait implemented by all types from which we can retrieve a span.
+pub(super) trait ToSpan {
+    fn span_for(&self, file: &File) -> Span;
+}
+
+impl ToSpan for Span {
+    fn span_for(&self, _: &File) -> Span {
+        *self
+    }
+}
+
+impl ToSpan for rnix::SyntaxToken {
+    fn span_for(&self, file: &File) -> Span {
+        let rowan_span = self.text_range();
+        file.span.subspan(
+            u32::from(rowan_span.start()) as u64,
+            u32::from(rowan_span.end()) as u64,
+        )
+    }
+}
+
+impl ToSpan for rnix::SyntaxNode {
+    fn span_for(&self, file: &File) -> Span {
+        let rowan_span = self.text_range();
+        file.span.subspan(
+            u32::from(rowan_span.start()) as u64,
+            u32::from(rowan_span.end()) as u64,
+        )
+    }
+}
+
+/// Generates a `ToSpan` implementation for a type implementing
+/// `rowan::AstNode`. This is impossible to do as a blanket
+/// implementation because `rustc` forbids these implementations for
+/// traits from third-party crates due to a belief that semantic
+/// versioning truly could work (it doesn't).
+macro_rules! expr_to_span {
+    ( $type:path ) => {
+        impl ToSpan for $type {
+            fn span_for(&self, file: &File) -> Span {
+                self.syntax().span_for(file)
+            }
+        }
+    };
+}
+
+expr_to_span!(ast::Expr);
+expr_to_span!(ast::Apply);
+expr_to_span!(ast::Assert);
+expr_to_span!(ast::Attr);
+expr_to_span!(ast::AttrSet);
+expr_to_span!(ast::Attrpath);
+expr_to_span!(ast::AttrpathValue);
+expr_to_span!(ast::BinOp);
+expr_to_span!(ast::HasAttr);
+expr_to_span!(ast::Ident);
+expr_to_span!(ast::IdentParam);
+expr_to_span!(ast::IfElse);
+expr_to_span!(ast::Inherit);
+expr_to_span!(ast::Interpol);
+expr_to_span!(ast::Lambda);
+expr_to_span!(ast::LegacyLet);
+expr_to_span!(ast::LetIn);
+expr_to_span!(ast::List);
+expr_to_span!(ast::Literal);
+expr_to_span!(ast::PatBind);
+expr_to_span!(ast::Path);
+expr_to_span!(ast::Pattern);
+expr_to_span!(ast::Select);
+expr_to_span!(ast::Str);
+expr_to_span!(ast::UnaryOp);
+expr_to_span!(ast::With);
+
+impl Compiler<'_, '_> {
+    pub(super) fn span_for<S: ToSpan>(&self, to_span: &S) -> Span {
+        to_span.span_for(self.file)
+    }
+}