feat(tvix/eval): coerce values to paths when importing

This enables the use of string paths (and, in the future,
derivations), as long as their string values represent an absolute
path.

Change-Id: I4b198efeb70415ed52f58bd1da6fa79a24dad14c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6866
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/impure.rs

@@ -52,18 +52,7 @@ pub fn builtins_import(
         "import",
         &[true],
         move |mut args: Vec<Value>, vm: &mut VM| {
-            let path = match args.pop().unwrap() {
+            let path = super::coerce_value_to_path(&args.pop().unwrap(), vm)?;
-                Value::Path(path) => path,
-                Value::String(_) => {
-                    return Err(ErrorKind::NotImplemented("importing from string-paths"))
-                }
-                other => {
-                    return Err(ErrorKind::TypeError {
-                        expected: "path or string",
-                        actual: other.type_of(),
-                    })
-                }
-            };
 
             let contents =
                 std::fs::read_to_string(&path).map_err(|err| ErrorKind::ReadFileError {

tvix/eval/src/errors.rs

@@ -262,7 +262,7 @@ to a missing value in the attribute set(s) included via `with`."#,
 
             ErrorKind::NotAnAbsolutePath(given) => {
                 format!(
-                    "string {} doesn't represent an absolute path",
+                    "string '{}' does not represent an absolute path",
                     given.to_string_lossy()
                 )
             }