When compiling with the flag RUSTFLAGS="-Zautodiff=Enable", lto = "fat" is automatically set.

Author: Hayden602

compiler/rustc_session/src/config.rs

@@ -1507,6 +1507,11 @@ impl Options {
     pub fn get_symbol_mangling_version(&self) -> SymbolManglingVersion {
         self.cg.symbol_mangling_version.unwrap_or(SymbolManglingVersion::Legacy)
     }
+
+    #[inline]
+    pub fn autodiff_enabled(&self) -> bool {
+        self.unstable_opts.autodiff.contains(&AutoDiff::Enable)
+    }
 }
 
 impl UnstableOptions {

compiler/rustc_session/src/session.rs

@@ -658,6 +658,11 @@ impl Session {
 
     /// Calculates the flavor of LTO to use for this compilation.
     pub fn lto(&self) -> config::Lto {
+        // if autodiff is enabled, use fat lto
+        if self.opts.autodiff_enabled() {
+            return config::Lto::Fat;
+        }
+
         // If our target has codegen requirements ignore the command line
         if self.target.requires_lto {
             return config::Lto::Fat;