refactor(build-std): extract support core only detection

This is a preparation of reuse for subsequent fix.

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

@@ -619,6 +619,13 @@ impl TargetInfo {
             .iter()
             .any(|sup| sup.as_str() == split.as_str())
     }
+
+    /// Checks if a target support core only.
+    ///
+    /// If no explictly stated in target spec json, we treat it as "maybe support".
+    pub fn support_core_only(&self) -> bool {
+        matches!(self.supports_std, Some(false))
+    }
 }
 
 /// Takes rustc output (using specialized command line args), and calculates the file prefix and

src/cargo/core/compiler/standard_lib.rs

@@ -115,9 +115,9 @@ pub fn generate_std_roots(
 ) -> CargoResult<HashMap<CompileKind, Vec<Unit>>> {
     // Generate a map of Units for each kind requested.
     let mut ret = HashMap::new();
-    let (core_only, maybe_std): (Vec<&CompileKind>, Vec<_>) = kinds.iter().partition(|kind|
-        // Only include targets that explicitly don't support std
-        target_data.info(**kind).supports_std == Some(false));
+    let (core_only, maybe_std): (Vec<&CompileKind>, Vec<_>) = kinds
+        .iter()
+        .partition(|kind| target_data.info(**kind).support_core_only());
     for (default_crate, kinds) in [("core", core_only), ("std", maybe_std)] {
         if kinds.is_empty() {
             continue;