test: -Zbuild-std proc_macro test case without --target requirement

Add a new test case for building a crate with -Zbuild-std, without the
requirement for the --target flag.

Author: harmou01

tests/build-std/main.rs

@@ -131,6 +131,65 @@ fn basic() {
     assert_eq!(p.glob(deps_dir.join("*.dylib")).count(), 0);
 }
 
+#[cargo_test(build_std_real)]
+fn target_proc_macro() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2021"
+
+                [dependencies]
+                macro_test = { path = "macro_test" }
+            "#,
+        )
+        .file(
+            "src/main.rs",
+            r#"
+            extern crate macro_test;
+            use macro_test::make_answer;
+
+            make_answer!();
+
+            fn main() {
+                println!("Hello, World: {}", answer());
+            }
+            "#,
+        )
+        .file(
+            "macro_test/Cargo.toml",
+            r#"
+            [package]
+            name = "macro_test"
+            version = "0.1.0"
+            edition = "2021"
+
+            [lib]
+            proc-macro = true
+            "#,
+        )
+        .file(
+            "macro_test/src/lib.rs",
+            r#"
+            extern crate proc_macro;
+            use proc_macro::TokenStream;
+
+            #[proc_macro]
+            pub fn make_answer(_item: TokenStream) -> TokenStream {
+                "fn answer() -> u32 { 42 }".parse().unwrap()
+            }
+            "#,
+        )
+        .build();
+
+    p.cargo("build -v")
+        .build_std_arg("core")
+        .run();
+}
+
 #[cargo_test(build_std_real)]
 fn cross_custom() {
     let p = project()