introduce LinkerFeatures

They are a flexible complementary mechanism to linker flavors,
that also avoid the combinatorial explosion of mapping linking features
to actual linker flavors.

compiler/rustc_target/src/spec/mod.rs

@@ -698,6 +698,58 @@ fn to_json(&self) -> Json {
     }
 }
 
+bitflags::bitflags! {
+    /// The `-Z linker-features` components that can individually be enabled or disabled.
+    ///
+    /// They are feature flags intended to be a more flexible mechanism than linker flavors, and
+    /// also to prevent a combinatorial explosion of flavors whenever a new linker feature is
+    /// required. These flags are "generic", in the sense that they can work on multiple targets on
+    /// the CLI. Otherwise, one would have to select different linkers flavors for each target.
+    ///
+    /// Here are some examples of the advantages they offer:
+    /// - default feature sets for principal flavors, or for specific targets.
+    /// - flavor-specific features: for example, clang offers automatic cross-linking with
+    ///   `--target`, which gcc-style compilers don't support. The *flavor* is still a C/C++
+    ///   compiler, and we don't need to multiply the number of flavors for this use-case. Instead,
+    ///   we can have a single `+target` feature.
+    /// - umbrella features: for example if clang accumulates more features in the future than just
+    ///   the `+target` above. That could be modeled as `+clang`.
+    /// - niche features for resolving specific issues: for example, on Apple targets the linker
+    ///   flag implementing the `as-needed` native link modifier (#99424) is only possible on
+    ///   sufficiently recent linker versions.
+    /// - still allows for discovery and automation, for example via feature detection. This can be
+    ///   useful in exotic environments/build systems.
+    #[derive(Clone, Copy, PartialEq, Eq, Default)]
+    pub struct LinkerFeatures: u8 {
+        /// Invoke the linker via a C/C++ compiler (e.g. on most unix targets).
+        const CC  = 1 << 0;
+        /// Use the lld linker, either the system lld or the self-contained linker `rust-lld`.
+        const LLD = 1 << 1;
+    }
+}
+rustc_data_structures::external_bitflags_debug! { LinkerFeatures }
+
+impl LinkerFeatures {
+    /// Parses a single `-Z linker-features` well-known feature, not a set of flags.
+    pub fn from_str(s: &str) -> Option<LinkerFeatures> {
+        Some(match s {
+            "cc" => LinkerFeatures::CC,
+            "lld" => LinkerFeatures::LLD,
+            _ => return None,
+        })
+    }
+
+    /// Returns whether the `lld` linker feature is enabled.
+    pub fn is_lld_enabled(self) -> bool {
+        self.contains(LinkerFeatures::LLD)
+    }
+
+    /// Returns whether the `cc` linker feature is enabled.
+    pub fn is_cc_enabled(self) -> bool {
+        self.contains(LinkerFeatures::CC)
+    }
+}
+
 #[derive(Clone, Copy, Debug, PartialEq, Hash, Encodable, Decodable, HashStable_Generic)]
 pub enum PanicStrategy {
     Unwind,