fix(nix-compat/nar/reader): require BufRead

We rely on being able to make small reads cheaply, so this was already
an implicit practical requirement. Requiring it explicitly removes a
performance footgun, and makes further optimisations possible.

Change-Id: I7f65880a41b1d6b5e6bf2e52dfe47d4c49b34bcd
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10088
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/nix-compat/src/nar/reader/mod.rs

@@ -5,7 +5,7 @@
 //! and transferring store paths between Nix stores.
 
 use std::io::{
-    self,
+    self, BufRead,
     ErrorKind::{InvalidData, UnexpectedEof},
     Read,
 };
@@ -17,7 +17,7 @@ mod read;
 #[cfg(test)]
 mod test;
 
-pub type Reader<'a> = dyn Read + Send + 'a;
+pub type Reader<'a> = dyn BufRead + Send + 'a;
 
 /// Start reading a NAR file from `reader`.
 pub fn open<'a, 'r>(reader: &'a mut Reader<'r>) -> io::Result<Node<'a, 'r>> {