mkDummySrc: drop dummy lib.rs by default, not main.rs

* There are some edge cases where cargo will allow a lib.rs file to be
  present but not a main.rs (e.g. a binary cannot be called "examples",
  which will happen if the crate is called that and a main.rs file is
  present)

checks/default.nix

@@ -47,6 +47,12 @@ onlyDrvs (lib.makeScope myLib.newScope (self:
 
     features = callPackage ./features { };
 
+    illegalBin = myLib.buildPackage {
+      pname = "illegalBin";
+      version = "0.0.1";
+      src = ./illegal-bin;
+    };
+
     manyLibs = myLib.buildPackage {
       src = ./with-libs;
       pname = "my-libs";
@@ -72,6 +78,11 @@ onlyDrvs (lib.makeScope myLib.newScope (self:
       src = ./simple;
     };
 
+    # Test building a real world example
+    ripgrep = myLib.buildPackage {
+      inherit (pkgs.ripgrep) pname src version;
+    };
+
     smoke = callPackage ./smoke.nix { };
     smokeSimple = self.smoke [ "simple" ] self.simple;
 

checks/illegal-bin/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "examples"
+version = "0.0.0"

checks/illegal-bin/Cargo.toml

@@ -0,0 +1,15 @@
+# This cargo workspace contains a member called "examples"
+# If we drop a dummy file in `src/main.rs` cargo will complain with:
+#
+# error: failed to load manifest for workspace member `/build/dummy-src/examples`
+# Caused by:
+#   failed to parse manifest at `/build/dummy-src/examples/Cargo.toml`
+# Caused by:
+#   the binary target name `examples` is forbidden, it conflicts with with cargo's build directory names
+#
+# Instead we need to drop a dummy file at `src/lib.rs`, and this is a
+# regression test for that case.
+[workspace]
+members = [
+  "examples",
+]

checks/illegal-bin/examples/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "examples"
+version = "0.0.0"
+publish = false
+edition = "2021"
+
+[[example]]
+name = "foo"
+path = "foo.rs"

checks/illegal-bin/examples/foo.rs

@@ -0,0 +1,2 @@
+fn main() {
+}

checks/mkDummySrcTests/single/expected/src/main.rs

@@ -1,2 +0,0 @@
-#![allow(dead_code)]
-pub fn main() {}

checks/mkDummySrcTests/workspace/expected/member/foo/bar/baz/src/main.rs

@@ -1,2 +0,0 @@
-#![allow(dead_code)]
-pub fn main() {}

checks/mkDummySrcTests/workspace/expected/member/qux/src/main.rs

@@ -1,2 +0,0 @@
-#![allow(dead_code)]
-pub fn main() {}

lib/mkDummySrc.nix

@@ -96,7 +96,7 @@ let
         # To build build-dependencies
         ${cpDummy parentDir "build.rs"}
         # To build regular and dev dependencies (cargo build + cargo test)
-        ${cpDummy parentDir "src/main.rs"}
+        ${cpDummy parentDir "src/lib.rs"}
 
         # Stub all other targets in case they have particular feature combinations
         ${safeStubLib}