fix(tvix/eval): src/compiler: ensure root_dir is absolute

Cppnix immediately absolutizes pathnames at parse time; if you write
`./foo`, it is immediately converted to `$(pwd)/foo` and manipulated
as an absolute path at all times.

To avoid having to introduce filesystem access operations in the
implementation of otherwise-pure builtins, let's guarantee that the
`root_dir` of the VM is always an absolute path.

Signed-off-by: Adam Joseph <adam@westernsemico.com>
Change-Id: I7cbbae2cba4b2716ff3f5ff7c9ce0ad529358c8a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6995
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/compiler/mod.rs

@@ -112,14 +112,21 @@ impl<'observer> Compiler<'observer> {
         observer: &'observer mut dyn CompilerObserver,
     ) -> EvalResult<Self> {
         let mut root_dir = match location {
-            Some(dir) => Ok(dir),
+            Some(dir) if dir.is_absolute() => Ok(dir),
-            None => std::env::current_dir().map_err(|e| Error {
+            _ => {
+                let current_dir = std::env::current_dir().map_err(|e| Error {
                     kind: ErrorKind::PathResolution(format!(
                         "could not determine current directory: {}",
                         e
                     )),
                     span: file.span,
-            }),
+                })?;
+                if let Some(dir) = location {
+                    Ok(current_dir.join(dir))
+                } else {
+                    Ok(current_dir)
+                }
+            }
         }?;
 
         // If the path passed from the caller points to a file, the
@@ -131,6 +138,7 @@ impl<'observer> Compiler<'observer> {
 
         let globals = globals.borrow();
 
+        debug_assert!(root_dir.is_absolute());
         Ok(Self {
             root_dir,
             file,