refactor(tvix/eval): introduce source::SourceCode type

This type hides away the lower-level handling of most codemap data
structures, especially to library consumers (see corresponding changes
in tvixbolt).

This will help with implement `import` by giving us central control
over how the codemap works.

Change-Id: Ifcea36776879725871b30c518aeb96ab5fda035a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6855
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>

tvix/eval/src/source.rs

@@ -0,0 +1,57 @@
+//! This module contains utilities for dealing with the codemap that
+//! needs to be carried across different compiler instantiations in an
+//! evaluation.
+//!
+//! The data type `SourceCode` should be carried through all relevant
+//! places instead of copying the codemap structures directly.
+
+use std::{
+    cell::{Ref, RefCell, RefMut},
+    rc::Rc,
+    sync::Arc,
+};
+
+use codemap::{CodeMap, Span};
+
+/// Tracks all source code in a Tvix evaluation for accurate error
+/// reporting.
+#[derive(Clone)]
+pub struct SourceCode(Rc<RefCell<CodeMap>>);
+
+impl SourceCode {
+    /// Create a new SourceCode instance.
+    pub fn new() -> Self {
+        SourceCode(Rc::new(RefCell::new(CodeMap::new())))
+    }
+
+    /// Access a read-only reference to the codemap.
+    pub fn codemap(&self) -> Ref<CodeMap> {
+        self.0.borrow()
+    }
+
+    /// Access a writable reference to the codemap.
+    fn codemap_mut(&self) -> RefMut<CodeMap> {
+        self.0.borrow_mut()
+    }
+
+    /// Add a file to the codemap. The returned Arc is managed by the
+    /// codemap internally and can be used like a normal reference.
+    pub fn add_file(&self, name: String, code: String) -> Arc<codemap::File> {
+        self.codemap_mut().add_file(name, code)
+    }
+
+    /// Retrieve the line number of the given span. If it spans
+    /// multiple lines, the first line will be returned.
+    pub fn get_line(&self, span: Span) -> usize {
+        // lines are 0-indexed in the codemap, but users probably want
+        // real line numbers
+        self.codemap().look_up_span(span).begin.line + 1
+    }
+
+    /// Returns the literal source slice of the given span.
+    pub fn source_slice(&self, span: Span) -> Ref<str> {
+        Ref::map(self.codemap(), |c| {
+            c.find_file(span.low()).source_slice(span)
+        })
+    }
+}