feat(tvix/eval): initial attempt at setting lambda names

When compiling a lambda, take the name of the outer slot (if
available) and store it as the name on the lambda.

These names are then shown in the observer, and nowhere else (so far).

It is of course common for these things to thread through many
different context levels (e.g. `f = a: b: c: ...`), in this setup only
the outermost closure or thunk gains the name, but it's better than
nothing.

Change-Id: I681ba74e624f2b9e7a147144a27acf364fe6ccc7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7065
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/value/function.rs

@@ -2,6 +2,7 @@
 use std::{collections::HashMap, hash::Hash, rc::Rc};
 
 use codemap::Span;
+use smol_str::SmolStr;
 
 use crate::{chunk::Chunk, upvalues::Upvalues};
 
@@ -38,10 +39,15 @@ impl Formals {
 /// OpThunkSuspended referencing it.  At runtime `Lambda` is usually wrapped
 /// in `Rc` to avoid copying the `Chunk` it holds (which can be
 /// quite large).
-#[derive(Debug, PartialEq)]
+#[derive(Debug, Default, PartialEq)]
 pub struct Lambda {
     pub(crate) chunk: Chunk,
 
+    /// Name of the function (equivalent to the name of the
+    /// identifier (e.g. a value in a let-expression or an attribute
+    /// set entry) it is located in).
+    pub(crate) name: Option<SmolStr>,
+
     /// Number of upvalues which the code in this Lambda closes
     /// over, and which need to be initialised at
     /// runtime.  Information about the variables is emitted using
@@ -51,14 +57,6 @@ pub struct Lambda {
 }
 
 impl Lambda {
-    pub fn new_anonymous() -> Self {
-        Lambda {
-            chunk: Default::default(),
-            upvalue_count: 0,
-            formals: None,
-        }
-    }
-
     pub fn chunk(&mut self) -> &mut Chunk {
         &mut self.chunk
     }