feat(tvix/eval): Implement builtins.readFile r/5095

Change-Id: If3fd0b087009a2bfbad8bb7aca0aa20de906eb12
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6921
Tested-by: BuildkiteCI
Reviewed-by: Adam Joseph <adam@westernsemico.com>
Reviewed-by: kanepyork <rikingcoding@gmail.com>
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>

4 files changed, 16 insertions, 1 deletions

diff --git a/tvix/eval/src/builtins/impure.rs b/tvix/eval/src/builtins/impure.rs
index 5cf3ca4287..c8423b6ac3 100644
--- a/tvix/eval/src/builtins/impure.rs
+++ b/tvix/eval/src/builtins/impure.rs
@@ -1,7 +1,8 @@
 use std::{
     cell::RefCell,
     collections::{BTreeMap, HashMap},
-    io,
+    fs::File,
+    io::{self, Read},
     rc::Rc,
     time::{SystemTime, UNIX_EPOCH},
 };
@@ -52,6 +53,11 @@ fn impure_builtins() -> Vec<Builtin> {
             }
             Ok(Value::attrs(NixAttrs::from_map(res)))
         }),
+        Builtin::new("readFile", &[true], |args: Vec<Value>, vm: &mut VM| {
+            let mut buf = String::new();
+            File::open(&super::coerce_value_to_path(&args[0], vm)?)?.read_to_string(&mut buf)?;
+            Ok(buf.into())
+        }),
     ]
 }
 
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-readfile.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-readfile.exp
new file mode 100644
index 0000000000..a2c87d0c43
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-readfile.exp
@@ -0,0 +1 @@
+"builtins.readFile ./eval-okay-readfile.nix\n"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-readfile.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-readfile.nix
new file mode 100644
index 0000000000..82f7cb1743
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-readfile.nix
@@ -0,0 +1 @@
+builtins.readFile ./eval-okay-readfile.nix
diff --git a/tvix/eval/src/value/string.rs b/tvix/eval/src/value/string.rs
index 876b1abe6b..d146ee4cec 100644
--- a/tvix/eval/src/value/string.rs
+++ b/tvix/eval/src/value/string.rs
@@ -1,6 +1,7 @@
 //! This module implements Nix language strings and their different
 //! backing implementations.
 use smol_str::SmolStr;
+use std::ffi::OsStr;
 use std::hash::Hash;
 use std::ops::Deref;
 use std::path::Path;
@@ -186,6 +187,12 @@ impl AsRef<str> for NixString {
     }
 }
 
+impl AsRef<OsStr> for NixString {
+    fn as_ref(&self) -> &OsStr {
+        self.as_str().as_ref()
+    }
+}
+
 impl AsRef<Path> for NixString {
     fn as_ref(&self) -> &Path {
         self.as_str().as_ref()