feat(tvix/eval): implement Value::coerce_to_path() r/4865

This function is necessary for all builtins that expect some form of
path as an argument. It is merely a wrapper around coerce_to_string that
can shortcut if we already have a path. The absolute path check is done
in the same way as in C++ Nix for compatibility, although it should
probably be revised in the long term (think about Windows, for example).

Since coercing to a path is not an operation possible in the language
directly, this function can live in the builtins module as the only
place it is required.

Change-Id: I69ed5455c00d193fea88b8fa83e28907a761cab5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6574
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

1 files changed, 12 insertions, 0 deletions

diff --git a/tvix/eval/src/errors.rs b/tvix/eval/src/errors.rs
index a84c931d0a4e..23ac6abbf4de 100644
--- a/tvix/eval/src/errors.rs
+++ b/tvix/eval/src/errors.rs
@@ -1,5 +1,6 @@
 use crate::value::CoercionKind;
 use std::fmt::Display;
+use std::path::PathBuf;
 
 use codemap::{CodeMap, Span};
 use codemap_diagnostic::{Diagnostic, Emitter, Level, SpanLabel, SpanStyle};
@@ -73,6 +74,9 @@ pub enum ErrorKind {
         kind: CoercionKind,
     },
 
+    /// The given string doesn't represent an absolute path
+    NotAnAbsolutePath(PathBuf),
+
     /// Tvix internal warning for features triggered by users that are
     /// not actually implemented yet, and without which eval can not
     /// proceed.
@@ -189,6 +193,13 @@ to a missing value in the attribute set(s) included via `with`."#,
                 format!("cannot ({kindly}) coerce {from} to a string{hint}")
             }
 
+            ErrorKind::NotAnAbsolutePath(given) => {
+                format!(
+                    "string {} doesn't represent an absolute path",
+                    given.to_string_lossy()
+                )
+            }
+
             ErrorKind::NotImplemented(feature) => {
                 format!("feature not yet implemented in Tvix: {}", feature)
             }
@@ -218,6 +229,7 @@ to a missing value in the attribute set(s) included via `with`."#,
             ErrorKind::ThunkForce(_) => "E017",
             ErrorKind::NotCoercibleToString { .. } => "E018",
             ErrorKind::IndexOutOfBounds { .. } => "E019",
+            ErrorKind::NotAnAbsolutePath(_) => "E020",
             ErrorKind::NotImplemented(_) => "E999",
         }
     }