feat(tvix/eval): don't warn twice about dead code r/5954

We currently send two warnings in case of detecting dead code - W008
inside compile_dead_code, and a more detailed warning in all places that
invoke compile_dead_code:

```
warning[W007]: useless operation on boolean: this expression is always false
     --> /nix/store/qz3gjn95gazab4fkb7s8lm6hz17rdzzy-414z9nnj1wy66ymq6vgb693x9xjz6hf2-nixpkgs-src/pkgs/top-level/perl-packages.nix:12079:15
      |
12079 |     doCheck = false && !stdenv.isDarwin;
      |               ^^^^^^^^^^^^^^^^^^^^^^^^^

warning[W008]: this code will never be executed
     --> /nix/store/qz3gjn95gazab4fkb7s8lm6hz17rdzzy-414z9nnj1wy66ymq6vgb693x9xjz6hf2-nixpkgs-src/pkgs/top-level/perl-packages.nix:12079:24
      |
12079 |     doCheck = false && !stdenv.isDarwin;
      |                        ^^^^^^^^^^^^^^^^
```

The place invoking `compile_dead_code` has more context to why the code
is unused, so it's error message is much more useful.

Stop emitting the less informative warning inside compile_dead_code
(W008), and update the comment that we expect the caller to emit a
warning.

I kept W008 itself still around, in case we end up having places this
will get used again.

Change-Id: I2c5d84fc0cb4035872cd4b71cc3e9e34e120eb37
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8024
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

1 files changed, 3 insertions, 1 deletions

diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index a59f935b04..69c232926b 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -302,9 +302,11 @@ impl Compiler<'_> {
     /// Compiles an expression, but does not emit any code for it as
     /// it is considered dead. This will still catch errors and
     /// warnings in that expression.
+    ///
+    /// A warning about the that code being dead is assumed to already be
+    /// emitted by the caller of [compile_dead_code].
     fn compile_dead_code(&mut self, slot: LocalIdx, node: ast::Expr) {
         self.dead_scope += 1;
-        self.emit_warning(&node, WarningKind::DeadCode);
         self.compile(slot, node);
         self.dead_scope -= 1;
     }