feat: add an error message when trying to build library only crates

Only binaries are supported, so users should have a message reminding
them that when trying to build a crate that do not contain a binary
package.

See #12

Cargo.toml

@@ -52,6 +52,7 @@ exclude = [
     "tests/test-argv",
     "tests/test-nothing",
     "tests/test-workspace",
+    "tests/test-nobin",
 ]
 
 [workspace.lints.rust]

src/multivers.rs

@@ -277,6 +277,15 @@ impl Multivers {
     pub fn build(&self) -> anyhow::Result<()> {
         let (selected_packages, _) = self.workspace.partition_packages(&self.metadata);
 
+        let has_bins = selected_packages
+            .iter()
+            .any(|&package| package.targets.iter().any(|target| target.is_bin()));
+        if !has_bins {
+            anyhow::bail!(
+                "No binary package detected. Only binaries can be built using cargo multivers."
+            );
+        }
+
         let profile_dir = if self.profile == "dev" {
             "debug"
         } else {

tests/test-nobin/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "test-nobin"
+edition = "2021"
+publish = false
+
+[dependencies]

tests/test-nobin/src/lib.rs

@@ -0,0 +1 @@
+

tests/test.rs

@@ -178,6 +178,7 @@ fn rebuild_std_env() {
     )));
 }
 
+/// Checks that we can build using the dev profile
 #[test]
 fn profile_dev() {
     let expected_args = ["z", "foo2", "''"];
@@ -193,3 +194,16 @@ fn profile_dev() {
         expected_args.join(" ")
     )));
 }
+
+/// Checks that users receive a message explaining that library only crates are not supported
+///
+/// See #12
+#[test]
+fn no_bin() {
+    build_crate("test-nobin", |_| ())
+        .0
+        .failure()
+        .stderr(predicate::eq(
+        "Error: No binary package detected. Only binaries can be built using cargo multivers.\n",
+    ));
+}