experiment: add proc_macro to the index

crates/cargo-test-support/src/publish.rs

@@ -229,6 +229,7 @@ pub(crate) fn create_index_line(
     links: Option<String>,
     rust_version: Option<&str>,
     v: Option<u32>,
+    proc_macro: Option<bool>,
 ) -> String {
     // This emulates what crates.io does to retain backwards compatibility.
     let (features, features2) = split_index_features(features.clone());
@@ -240,6 +241,7 @@ pub(crate) fn create_index_line(
         "features": features,
         "yanked": yanked,
         "links": links,
+        "proc_macro": proc_macro,
     });
     if let Some(f2) = &features2 {
         json["features2"] = serde_json::json!(f2);

crates/cargo-test-support/src/registry.rs

@@ -1243,6 +1243,7 @@ fn save_new_crate(
         new_crate.links,
         new_crate.rust_version.as_deref(),
         None,
+        None,
     );
 
     write_to_index(registry_path, &new_crate.name, line, false);
@@ -1527,6 +1528,12 @@ impl Package {
             } else {
                 serde_json::json!(self.name)
             };
+            // simulate alternative registries not having proc_macro in the index
+            let proc_macro = if self.alternative {
+                None
+            } else {
+                Some(self.proc_macro)
+            };
             create_index_line(
                 name,
                 &self.vers,
@@ -1537,6 +1544,7 @@ impl Package {
                 self.links.clone(),
                 self.rust_version.as_deref(),
                 self.v,
+                proc_macro,
             )
         };
 

crates/cargo-util-schemas/index.schema.json

@@ -76,6 +76,12 @@
       ],
       "format": "uint32",
       "minimum": 0
+    },
+    "proc_macro": {
+      "type": [
+        "boolean",
+        "null"
+      ]
     }
   },
   "required": [

crates/cargo-util-schemas/src/index.rs

@@ -69,6 +69,7 @@ pub struct IndexPackage<'a> {
     /// workaround is to downgrade any packages that are incompatible with the
     /// `--precise` flag of `cargo update`.
     pub v: Option<u32>,
+    pub proc_macro: Option<bool>,
 }
 
 /// A dependency as encoded in the [`IndexPackage`] index JSON.

crates/crates-io/lib.rs

@@ -63,6 +63,7 @@ pub struct NewCrate {
     pub badges: BTreeMap<String, BTreeMap<String, String>>,
     pub links: Option<String>,
     pub rust_version: Option<String>,
+    pub proc_macro: Option<bool>,
 }
 
 #[derive(Serialize, Deserialize)]

crates/resolver-tests/src/helpers.rs

@@ -119,7 +119,7 @@ pub fn pkg_dep<T: ToPkgId>(name: T, dep: Vec<Dependency>) -> Summary {
     } else {
         None
     };
-    Summary::new(name.to_pkgid(), dep, &BTreeMap::new(), link, None).unwrap()
+    Summary::new(name.to_pkgid(), dep, &BTreeMap::new(), link, None, None).unwrap()
 }
 
 pub fn pkg_dep_with<T: ToPkgId>(
@@ -137,11 +137,19 @@ pub fn pkg_dep_with<T: ToPkgId>(
         .into_iter()
         .map(|&(name, values)| (name.into(), values.into_iter().map(|&v| v.into()).collect()))
         .collect();
-    Summary::new(name.to_pkgid(), dep, &features, link, None).unwrap()
+    Summary::new(name.to_pkgid(), dep, &features, link, None, None).unwrap()
 }
 
 pub fn pkg_dep_link<T: ToPkgId>(name: T, link: &str, dep: Vec<Dependency>) -> Summary {
-    Summary::new(name.to_pkgid(), dep, &BTreeMap::new(), Some(link), None).unwrap()
+    Summary::new(
+        name.to_pkgid(),
+        dep,
+        &BTreeMap::new(),
+        Some(link),
+        None,
+        None,
+    )
+    .unwrap()
 }
 
 pub fn pkg_id(name: &str) -> PackageId {
@@ -177,6 +185,7 @@ pub fn pkg_loc(name: &str, loc: &str) -> Summary {
         &BTreeMap::new(),
         link,
         None,
+        None,
     )
     .unwrap()
 }
@@ -185,7 +194,15 @@ pub fn remove_dep(sum: &Summary, ind: usize) -> Summary {
     let mut deps = sum.dependencies().to_vec();
     deps.remove(ind);
     // note: more things will need to be copied over in the future, but it works for now.
-    Summary::new(sum.package_id(), deps, &BTreeMap::new(), sum.links(), None).unwrap()
+    Summary::new(
+        sum.package_id(),
+        deps,
+        &BTreeMap::new(),
+        sum.links(),
+        None,
+        None,
+    )
+    .unwrap()
 }
 
 pub fn dep(name: &str) -> Dependency {

crates/resolver-tests/src/lib.rs

@@ -190,8 +190,15 @@ pub fn resolve_with_global_context_raw(
         used: HashSet::new(),
     };
 
-    let root_summary =
-        Summary::new(root_pkg_id, deps, &BTreeMap::new(), None::<&String>, None).unwrap();
+    let root_summary = Summary::new(
+        root_pkg_id,
+        deps,
+        &BTreeMap::new(),
+        None::<&String>,
+        None,
+        None,
+    )
+    .unwrap();
 
     let opts = ResolveOpts::everything();
 

src/cargo/core/resolver/features.rs

@@ -862,6 +862,11 @@ impl<'a, 'gctx> FeatureResolver<'a, 'gctx> {
         self.resolve
             .deps(pkg_id)
             .map(|(dep_id, deps)| {
+                let proc_macro = self
+                    .resolve
+                    .summary(dep_id)
+                    .proc_macro()
+                    .unwrap_or_else(|| self.has_proc_macro_lib(dep_id));
                 let deps = deps
                     .iter()
                     .filter(|dep| {
@@ -907,10 +912,9 @@ impl<'a, 'gctx> FeatureResolver<'a, 'gctx> {
                         // for various targets which are either specified in the manifest
                         // or on the cargo command-line.
                         let lib_fk = if fk == FeaturesFor::default() {
-                            (self.track_for_host
-                                && (dep.is_build() || self.has_proc_macro_lib(dep_id)))
-                            .then(|| FeaturesFor::HostDep)
-                            .unwrap_or_default()
+                            (self.track_for_host && (dep.is_build() || proc_macro))
+                                .then(|| FeaturesFor::HostDep)
+                                .unwrap_or_default()
                         } else {
                             fk
                         };

src/cargo/core/resolver/version_prefs.rs

@@ -143,6 +143,7 @@ mod test {
             &features,
             None::<&String>,
             msrv.map(|m| m.parse().unwrap()),
+            None,
         )
         .unwrap()
     }

src/cargo/core/summary.rs

@@ -29,6 +29,7 @@ struct Inner {
     checksum: Option<String>,
     links: Option<InternedString>,
     rust_version: Option<RustVersion>,
+    proc_macro: Option<bool>,
 }
 
 /// Indicates the dependency inferred from the `dep` syntax that should exist,
@@ -68,6 +69,7 @@ impl Summary {
         features: &BTreeMap<InternedString, Vec<InternedString>>,
         links: Option<impl Into<InternedString>>,
         rust_version: Option<RustVersion>,
+        proc_macro: Option<bool>,
     ) -> CargoResult<Summary> {
         // ****CAUTION**** If you change anything here that may raise a new
         // error, be sure to coordinate that change with either the index
@@ -90,6 +92,7 @@ impl Summary {
                 checksum: None,
                 links: links.map(|l| l.into()),
                 rust_version,
+                proc_macro,
             }),
         })
     }
@@ -112,6 +115,9 @@ impl Summary {
     pub fn features(&self) -> &FeatureMap {
         &self.inner.features
     }
+    pub fn proc_macro(&self) -> &Option<bool> {
+        &self.inner.proc_macro
+    }
 
     pub fn checksum(&self) -> Option<&str> {
         self.inner.checksum.as_deref()

src/cargo/ops/cargo_package/mod.rs

@@ -1212,6 +1212,7 @@ impl<'a> TmpRegistry<'a> {
             yanked: None,
             links: new_crate.links.map(|x| x.into()),
             rust_version: None,
+            proc_macro: Some(package.proc_macro()),
             v: Some(2),
         })?;
 

src/cargo/ops/registry/publish.rs

@@ -634,6 +634,13 @@ pub(crate) fn prepare_transmit(
         None => BTreeMap::new(),
     };
 
+    let proc_macro = manifest
+        .normalized_toml()
+        .lib
+        .as_ref()
+        .and_then(|lib| lib.proc_macro())
+        .unwrap_or_default();
+
     Ok(NewCrate {
         name: publish_pkg.name().to_string(),
         vers: publish_pkg.version().to_string(),
@@ -653,6 +660,7 @@ pub(crate) fn prepare_transmit(
         badges: badges.clone(),
         links: links.clone(),
         rust_version,
+        proc_macro: Some(proc_macro),
     })
 }
 

src/cargo/sources/registry/index/mod.rs

@@ -215,7 +215,14 @@ fn index_package_to_summary(pkg: &IndexPackage<'_>, source_id: SourceId) -> Carg
         .map(|(name, values)| (name.into(), values.into_iter().map(|v| v.into()).collect()))
         .collect::<BTreeMap<_, _>>();
     let links: Option<InternedString> = pkg.links.as_ref().map(|l| l.as_ref().into());
-    let mut summary = Summary::new(pkgid, deps, &features, links, pkg.rust_version.clone())?;
+    let mut summary = Summary::new(
+        pkgid,
+        deps,
+        &features,
+        links,
+        pkg.rust_version.clone(),
+        pkg.proc_macro,
+    )?;
     summary.set_checksum(pkg.cksum.clone());
     Ok(summary)
 }
@@ -674,6 +681,7 @@ impl IndexSummary {
                     cksum: Default::default(),
                     yanked: Default::default(),
                     links: Default::default(),
+                    proc_macro: Default::default(),
                 };
                 let summary = index_package_to_summary(&index, source_id)?;
                 (index, summary, false)

src/cargo/util/toml/mod.rs

@@ -1725,6 +1725,10 @@ pub fn to_real_manifest(
             .unwrap_or_else(|| semver::Version::new(0, 0, 0)),
         source_id,
     );
+    let proc_macro = normalized_toml
+        .lib
+        .as_ref()
+        .and_then(|lib| lib.proc_macro());
     let summary = {
         let summary = Summary::new(
             pkgid,
@@ -1743,6 +1747,7 @@ pub fn to_real_manifest(
                 .collect(),
             normalized_package.links.as_deref(),
             rust_version.clone(),
+            proc_macro,
         );
         // edition2024 stops exposing implicit features, which will strip weak optional dependencies from `dependencies`,
         // need to check whether `dep_name` is stripped as unused dependency