Add proc_macro to the extern prelude.

src/cargo/core/compiler/build_context/target_info.rs

@@ -39,6 +39,8 @@ pub struct TargetInfo {
     pub rustflags: Vec<String>,
     /// Extra flags to pass to `rustdoc`, see `env_args`.
     pub rustdocflags: Vec<String>,
+    // Remove this when it hits stable (1.41).
+    pub supports_pathless_extern: Option<bool>,
 }
 
 /// Kind of each file generated by a Unit, part of `FileType`.
@@ -101,6 +103,13 @@ impl TargetInfo {
             .args(&rustflags)
             .env_remove("RUSTC_LOG");
 
+        let mut pathless_test = process.clone();
+        pathless_test.args(&["--extern", "proc_macro"]);
+        let supports_pathless_extern = match kind {
+            CompileKind::Host => Some(rustc.cached_output(&pathless_test).is_ok()),
+            _ => None,
+        };
+
         if let CompileKind::Target(target) = kind {
             process.arg("--target").arg(target.rustc_target());
         }
@@ -183,6 +192,7 @@ impl TargetInfo {
                 "RUSTDOCFLAGS",
             )?,
             cfg,
+            supports_pathless_extern,
         })
     }
 

src/cargo/core/compiler/mod.rs

@@ -1010,6 +1010,17 @@ pub fn extern_args<'a>(
             link_to(&dep, dep.extern_crate_name, dep.noprelude)?;
         }
     }
+    if unit.target.proc_macro()
+        && cx
+            .bcx
+            .info(CompileKind::Host)
+            .supports_pathless_extern
+            .unwrap()
+    {
+        // Automatically import `proc_macro`.
+        result.push(OsString::from("--extern"));
+        result.push(OsString::from("proc_macro"));
+    }
 
     Ok(result)
 }

tests/testsuite/proc_macro.rs

@@ -440,3 +440,37 @@ Caused by:
         .with_status(101)
         .run();
 }
+
+#[cargo_test]
+fn proc_macro_extern_prelude() {
+    if !is_nightly() {
+        // remove once pathless `--extern` hits stable (1.41)
+        return;
+    }
+    // Check that proc_macro is in the extern prelude.
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            version = "0.1.0"
+            edition = "2018"
+            [lib]
+            proc-macro = true
+            "#,
+        )
+        .file(
+            "src/lib.rs",
+            r#"
+            use proc_macro::TokenStream;
+            #[proc_macro]
+            pub fn foo(input: TokenStream) -> TokenStream {
+                "".parse().unwrap()
+            }
+            "#,
+        )
+        .build();
+    p.cargo("test").run();
+    p.cargo("doc").run();
+}

tests/testsuite/rustc_info_cache.rs

@@ -6,6 +6,11 @@ use std::env;
 
 #[cargo_test]
 fn rustc_info_cache() {
+    if !cargo_test_support::is_nightly() {
+        // remove once pathless `--extern` hits stable (1.41)
+        return;
+    }
+
     let p = project()
         .file("src/main.rs", r#"fn main() { println!("hello"); }"#)
         .build();