Fix linker-plugin-lto: use -flto=thin (#1594)

* Fix linker-plugin-lto: use `-flto=thin`

The linker-plugin/lld uses thin LTO by default, and it requires the archive to be compiled
with thin LTO as well, since thin LTO generates function summary
which is needed by thin LTO.

* Clarify purpose of unused 'lto' variable in flags.rs

Added a comment explaining the purpose of the unused 'lto' variable.

* Update comments on thin LTO for linker plugin

Clarify comments regarding thin LTO requirements for linker plugin.

Author: NobodyXu

src/flags.rs

@@ -140,6 +140,8 @@ impl<'this> RustcCodegenFlags<'this> {
             // https://doc.rust-lang.org/rustc/codegen-options/index.html#control-flow-guard
             "-Ccontrol-flow-guard" => self.control_flow_guard = value.or(Some("true")),
             // https://doc.rust-lang.org/rustc/codegen-options/index.html#lto
+            //
+            // This variable is currently unused, we just keep it in case we need it in future
             "-Clto" => self.lto = value.or(Some("true")),
             // https://doc.rust-lang.org/rustc/linker-plugin-lto.html
             "-Clinker-plugin-lto" => self.linker_plugin_lto = Some(true),
@@ -322,11 +324,12 @@ impl<'this> RustcCodegenFlags<'this> {
                 // https://doc.rust-lang.org/rustc/linker-plugin-lto.html
                 if self.linker_plugin_lto.unwrap_or(false) {
                     // https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-flto
-                    let cc_val = match self.lto {
-                        Some("thin") => "thin",
-                        _ => "full",
-                    };
-                    push_if_supported(format!("-flto={cc_val}").into());
+                    // In order to use linker-plugin-lto to achieve cross-lang lto, cc has to use thin LTO
+                    // to compile the c/c++ libraries because llvm linker plugin/lld uses thin LTO by default.
+                    // And for thin LTO in linker plugin to work, the archive also has to be compiled using thin LTO,
+                    // since thin LTO generates extra information that fat LTO does not generate that
+                    // is required for thin LTO process.
+                    push_if_supported("-flto=thin".into());
                 }
                 // https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mguard
                 if let Some(value) = self.control_flow_guard {