slint macro: Use Span::local_file when Rust is 1.88

So we can locate the files relative to the .rs file instead of relative
to the Cargo.toml file.

For compatibility with previous version and older rust, still try to
locate files relative to the Cargo.toml for rust files.

The LSP will always auto-complete relative to the .rs file

The live-preview for Rust need to make the file absolute because it
isn't in the case of a macro

internal/compiler/typeloader.rs

@@ -1520,8 +1520,9 @@ impl TypeLoader {
     ) -> Option<(PathBuf, Option<&'static [u8]>)> {
         // The directory of the current file is the first in the list of include directories.
         referencing_file
-            .map(base_directory)
+            .and_then(|x| x.parent().map(|x| x.to_path_buf()))
             .into_iter()
+            .chain(referencing_file.and_then(maybe_base_directory))
             .chain(self.compiler_config.include_paths.iter().map(PathBuf::as_path).map(
                 |include_path| {
                     let base = referencing_file.map(Path::to_path_buf).unwrap_or_default();
@@ -1671,15 +1672,12 @@ fn get_native_style(all_loaded_files: &mut std::collections::BTreeSet<PathBuf>)
     i_slint_common::get_native_style(false, &std::env::var("TARGET").unwrap_or_default()).into()
 }
 
-/// return the base directory from which imports are loaded
+/// For a .rs file, return the manifest directory
 ///
-/// For a .slint file, this is the parent directory.
-/// For a .rs file, this is relative to the CARGO_MANIFEST_DIR
-///
-/// Note: this function is only called for .rs path as part of the LSP or viewer.
-/// Because from a proc_macro, we don't actually know the path of the current file, and this
-/// is why we must be relative to CARGO_MANIFEST_DIR.
-pub fn base_directory(referencing_file: &Path) -> PathBuf {
+/// This is for compatibility with `slint!` macro as before rust 1.88,
+/// it was not possible for the macro to know the current path and
+/// the Cargo.toml file was used instead
+fn maybe_base_directory(referencing_file: &Path) -> Option<PathBuf> {
     if referencing_file.extension().is_some_and(|e| e == "rs") {
         // For .rs file, this is a rust macro, and rust macro locates the file relative to the CARGO_MANIFEST_DIR which is the directory that has a Cargo.toml file.
         let mut candidate = referencing_file;
@@ -1691,10 +1689,10 @@ pub fn base_directory(referencing_file: &Path) -> PathBuf {
                 break Some(candidate);
             }
         }
+        .map(|x| x.to_path_buf())
     } else {
-        referencing_file.parent()
+        None
     }
-    .map_or_else(Default::default, |p| p.to_path_buf())
 }
 
 #[test]