fix(build-std): always link to std when testing proc-macros

src/cargo/core/compiler/compilation.rs

@@ -315,7 +315,10 @@ impl<'gctx> Compilation<'gctx> {
             // libs from the sysroot that ships with rustc. This may not be
             // required (at least I cannot craft a situation where it
             // matters), but is here to be safe.
-            if self.gctx.cli_unstable().build_std.is_none() {
+            if self.gctx.cli_unstable().build_std.is_none() ||
+                // Proc macros dynamically link to std, so set it anyway.
+                pkg.proc_macro()
+            {
                 search_path.push(self.sysroot_target_libdir[&kind].clone());
             }
         }

tests/build-std/main.rs

@@ -360,3 +360,33 @@ fn remap_path_scope() {
         )
         .run();
 }
+
+#[cargo_test(build_std_real)]
+fn test_proc_macro() {
+    // See rust-lang/cargo#14735
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                edition = "2021"
+
+                [lib]
+                proc-macro = true
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("test --lib")
+        .env_remove(cargo_util::paths::dylib_path_envvar())
+        .build_std()
+        .with_stderr_data(str![[r#"
+[COMPILING] foo v0.0.0 ([ROOT]/foo)
+[FINISHED] `test` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+[RUNNING] unittests src/lib.rs (target/debug/deps/foo-[HASH])
+
+"#]])
+        .run();
+}