fix: Update proc-macro2 usage for latest nightly

CHANGELOG.md

@@ -15,6 +15,7 @@ The minor version will be incremented upon a breaking change and the patch versi
 
 ### Fixes
 
+- idl: Update `proc-macro2` usage for latest nightly ([#3663](https://github.com/solana-foundation/anchor/pull/3663))
 - cli, docker: Replace `backpackapp/build` Docker image with `solanafoundation/anchor` ([#3619](https://github.com/coral-xyz/anchor/pull/3619)).
 
 ### Breaking

lang/syn/src/idl/defined.rs

@@ -496,7 +496,11 @@ pub fn gen_idl_type(
                 use crate::parser::context::CrateContext;
                 use quote::ToTokens;
 
-                let source_path = proc_macro2::Span::call_site().source_file().path();
+                // If no path was found, just return an empty path and let the find_path function handle it
+                let source_path = proc_macro2::Span::call_site()
+                    .local_file()
+                    .unwrap_or_default();
+
                 if let Ok(Ok(ctx)) = find_path("lib.rs", &source_path).map(CrateContext::parse) {
                     let name = path.path.segments.last().unwrap().ident.to_string();
                     let alias = ctx.type_aliases().find(|ty| ty.ident == name);