Fix autodiff incorrectly applying fat-lto to proc-macro crates

Fixes #147487

When -Z autodiff=Enable is used, the compiler automatically enables
fat-lto for all crates. However, proc-macro crates cannot use fat-lto
without the -Zdylib-lto flag, causing compilation errors.

This commit modifies the lto() method in Session to exclude proc-macro
crates from fat-lto when autodiff is enabled, while preserving the
existing behavior for all other crate types.

The fix ensures that:
- Non-proc-macro crates still get fat-lto when autodiff is enabled
- Proc-macro crates are excluded from fat-lto when autodiff is enabled
- Existing autodiff functionality remains unchanged for regular crates

Signed-off-by: Osama Abdelkader <osama.abdelkader@gmail.com>

Author: osamakader

compiler/rustc_session/src/session.rs

@@ -604,7 +604,10 @@ impl Session {
         // fat-lto is the easiest solution to this requirement, but quite expensive.
         // FIXME(autodiff): Make autodiff also work with embed-bc instead of fat-lto.
         if self.opts.autodiff_enabled() {
-            return config::Lto::Fat;
+            // Don't apply fat-lto to proc-macro crates as they cannot use fat-lto without -Zdylib-lto
+            if !self.opts.crate_types.contains(&CrateType::ProcMacro) {
+                return config::Lto::Fat;
+            }
         }
 
         // If our target has codegen requirements ignore the command line