compiletest: Support --extern modifiers with proc-macro directive

So that `pub-priv1.rs` test does not have to (ab)use the `aux-crate`
directive for this purpose.

This is very edge-casey so I don't think we should document this in
rustc-dev-guide. If someone needs to do this they will look at the code
and easily find the functionality.

This is a bit hacky since `--extern priv:pm.rs` is not valid, but we can
make our directives work however we want. And I think this is a fine
pragmatic approach. Doing it "the right way" would be a lot of work for
not much gain. Plus, that work can be done incrementally in small steps
in the future if wanted.

src/tools/compiletest/src/directives/auxiliary.rs

@@ -26,8 +26,12 @@ pub struct AuxCrate {
 }
 
 /// The value of a `proc-macro` directive.
-#[derive(Clone, Debug, Default)]
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
 pub(crate) struct ProcMacro {
+    /// Contains `--extern` modifiers, if any. See the tracking issue for more
+    /// info: <https://github.com/rust-lang/rust/issues/98405>
+    /// With `proc-macro: noprelude:bar.rs` this will be `noprelude`.
+    pub extern_modifiers: Option<String>,
     /// With `proc-macro: bar.rs` this will be `bar.rs`.
     pub path: String,
 }
@@ -108,5 +112,17 @@ fn parse_aux_crate(r: String) -> AuxCrate {
 }
 
 fn parse_proc_macro(r: String) -> ProcMacro {
-    ProcMacro { path: r.trim().to_string() }
+    let r = r.trim();
+
+    // Matches:
+    //   path
+    //   modifiers:path
+    let caps = static_regex!(r"^(?:(?<modifiers>[^=]*?):)?(?<path>.*)$")
+        .captures(r)
+        .expect("can never fail");
+
+    let modifiers = caps.name("modifiers").map(|m| m.as_str().to_string());
+    let path = caps["path"].to_string();
+
+    ProcMacro { extern_modifiers: modifiers, path }
 }

src/tools/compiletest/src/directives/auxiliary/tests.rs

@@ -25,3 +25,19 @@ fn test_aux_crate_value_with_modifiers() {
 fn test_aux_crate_value_invalid() {
     parse_aux_crate("foo.rs".to_string());
 }
+
+#[test]
+fn test_proc_macro_value_no_modifiers() {
+    assert_eq!(
+        ProcMacro { extern_modifiers: None, path: "foo.rs".to_string() },
+        parse_proc_macro("foo.rs".to_string())
+    );
+}
+
+#[test]
+fn test_proc_macro_value_with_modifiers() {
+    assert_eq!(
+        ProcMacro { extern_modifiers: Some("noprelude".to_string()), path: "foo.rs".to_string() },
+        parse_proc_macro("noprelude:foo.rs".to_string())
+    );
+}

src/tools/compiletest/src/runtest.rs

@@ -1302,7 +1302,7 @@ impl<'test> TestCx<'test> {
             let crate_name = path_to_crate_name(&proc_macro.path);
             add_extern(
                 rustc,
-                None, // `extern_modifiers`
+                proc_macro.extern_modifiers.as_deref(),
                 &crate_name,
                 &proc_macro.path,
                 AuxType::ProcMacro,

tests/ui/privacy/pub-priv-dep/auxiliary/pm.rs

@@ -1,8 +1,3 @@
-//@ force-host
-//@ no-prefer-dynamic
-
-#![crate_type = "proc-macro"]
-
 extern crate proc_macro;
 use proc_macro::TokenStream;
 

tests/ui/privacy/pub-priv-dep/pub-priv1.rs

@@ -1,6 +1,6 @@
 //@ aux-crate:priv:priv_dep=priv_dep.rs
 //@ aux-build:pub_dep.rs
-//@ aux-crate:priv:pm=pm.rs
+//@ proc-macro:priv:pm.rs
 //@ compile-flags: -Zunstable-options
 
 // Basic behavior check of exported_private_dependencies from either a public