Make libtest optional.

Author: ehuss

src/cargo/core/compiler/standard_lib.rs

@@ -19,7 +19,6 @@ pub fn parse_unstable_flag(value: Option<&str>) -> Vec<String> {
         crates.insert("core");
         crates.insert("panic_unwind");
         crates.insert("compiler_builtins");
-        crates.insert("test");
     } else if crates.contains("core") {
         crates.insert("compiler_builtins");
     }
@@ -83,7 +82,11 @@ pub fn resolve_std<'cfg>(
     // TODO: Consider doing something to enforce --locked? Or to prevent the
     // lock file from being written, such as setting ephemeral.
     let std_ws = Workspace::new_virtual(src_path, current_manifest, virtual_manifest, config)?;
-    let spec = Packages::Packages(Vec::from(crates));
+    // `test` is not in the default set because it is optional, but it needs
+    // to be part of the resolve in case we do need it.
+    let mut spec_pkgs = Vec::from(crates);
+    spec_pkgs.push("test".to_string());
+    let spec = Packages::Packages(spec_pkgs);
     let specs = spec.to_package_id_specs(&std_ws)?;
     let features = vec!["panic-unwind".to_string(), "backtrace".to_string()];
     // dev_deps setting shouldn't really matter here.

src/cargo/ops/cargo_compile.rs

@@ -390,7 +390,16 @@ pub fn compile_ws<'a>(
     )?;
 
     let std_roots = if let Some(crates) = &config.cli_unstable().build_std {
-        standard_lib::generate_std_roots(&bcx, crates, std_resolve.as_ref().unwrap())?
+        // Only build libtest if it looks like it is needed.
+        let mut crates = crates.clone();
+        if !crates.iter().any(|c| c == "test")
+            && units
+                .iter()
+                .any(|unit| unit.mode.is_rustc_test() && unit.target.harness())
+        {
+            crates.push("test".to_string());
+        }
+        standard_lib::generate_std_roots(&bcx, &crates, std_resolve.as_ref().unwrap())?
     } else {
         Vec::new()
     };