How to view code coverage

Commits:
* Code coverage

* flake lock update

* update nix rev

* flake update

* llvm-cov does not work on macos

* back to old rev

devtools/debug_tips.md

@@ -48,3 +48,23 @@ Note that the addresses shown in objdump may use a different offset compared to
 
 gdb scripting is very useful, [for example](https://roc.zulipchat.com/#narrow/stream/395097-compiler-development/topic/gdb.20script/near/424422545).
 ChatGPT and Claude are good at writing those scripts as well.
+
+## Code Coverage
+
+When investigating a bug, it can be nice to instantly see if a line of rust code was executed during for example `roc build yourFile.roc`. We can use [`cargo-llvm-cov`](https://github.com/taiki-e/cargo-llvm-cov) for this, on linux, it comes pre-installed with our flake.nix. On macos you'll need to install it with `cargo +stable install cargo-llvm-cov --locked`.
+
+To generate the code coverage file:
+
+```shell
+$ cd roc
+$ source <(cargo llvm-cov show-env --export-prefix)
+$ cargo llvm-cov clean --workspace
+$ cargo build --bin roc
+# Replace with the command you want to generate coverage for:
+$ ./target/debug/roc build ./examples/platform-switching/rocLovesRust.roc
+# To view in editor
+$ cargo llvm-cov report --lcov  --output-path lcov.info
+# To view in browser
+$ cargo llvm-cov report --html
+```
+Viewing lcov.info will depend on your editor. For vscode, you can use the [coverage gutters](https://marketplace.visualstudio.com/items?itemName=ryanluker.vscode-coverage-gutters) extension. After installing, click `Watch` in the bottom bar and go to a file for which you want to see the coverage, for example `crates/compiler/build/src/link.rs`. `Watch` in the bottom bar will now be replaced with `x% Coverage`.

devtools/flake.nix

@@ -32,7 +32,7 @@
               vscodeWithExtensions = pkgs.vscode-with-extensions.override {
                 vscodeExtensions = with pkgs.vscode-extensions;
                   [
-                    matklad.rust-analyzer
+                    rust-lang.rust-analyzer
                     # eamodio.gitlens
                     bbenoist.nix
                     tamasfe.even-better-toml

flake.nix

@@ -58,6 +58,7 @@
             xorg.libXrandr
             xorg.libXi
             xorg.libxcb
+            cargo-llvm-cov # to visualize code coverage
           ];
 
         # DevInputs are not necessary to build roc as a user

nix/default.nix

@@ -14,7 +14,9 @@ let
     inherit rustPlatform;
     compile-deps = callPackage ./compile-deps.nix { };
     rust-shell =
-      (rustVersion.override { extensions = [ "rust-src" "rust-analyzer" ]; });
+      # llvm-tools-preview for code coverage with cargo-llvm-cov
+      (rustVersion.override { extensions = [ "rust-src" "rust-analyzer" "llvm-tools-preview"]; });
+
 
     # contains all rust crates in workspace.members of Cargo.toml
     roc-full = (callPackage ./builder.nix { }).roc-release;