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

api/rs/macros/Cargo.toml

@@ -23,7 +23,7 @@ default = []
 
 [dependencies]
 i-slint-compiler = { workspace = true, features = ["default", "proc_macro_span", "rust", "display-diagnostics"] }
-
+rustversion = "1.0"
 proc-macro2 = "1.0.17"
 quote = "1.0"
 spin_on = { workspace = true }

api/rs/macros/lib.rs

@@ -13,6 +13,7 @@ use i_slint_compiler::parser::SyntaxKind;
 use i_slint_compiler::*;
 use proc_macro::{Spacing, TokenStream, TokenTree};
 use quote::quote;
+use std::path::PathBuf;
 
 /// Returns true if the two token are touching. For example the two token `foo`and `-` are touching if
 /// it was written like so in the source code: `foo-` but not when written like so `foo -`
@@ -374,8 +375,9 @@ fn extract_compiler_config(
 /// For the documentation about the syntax of the language, see
 #[doc = concat!("[The Slint Language Documentation](https://slint.dev/releases/", env!("CARGO_PKG_VERSION"), "/docs/slint)")]
 ///
-/// When `import`ing `.slint` files or loading images with `@image-url`, the specified paths are relative to the
-/// the directory that contains Cargo.toml.
+/// When Rust 1.88 or later is used, the paths for loading images with `@image-url` and importing `.slint` files
+/// are relative to the `.rs` file that contains the macro.
+/// For compatibility with older rust version, the files are also searched in the manifest directory that contains `Cargo.toml`.
 ///
 /// ### Limitations
 ///
@@ -393,7 +395,18 @@ pub fn slint(stream: TokenStream) -> TokenStream {
     let mut tokens = vec![];
     fill_token_vec(token_iter, &mut tokens);
 
-    let source_file = if let Some(cargo_manifest) = std::env::var_os("CARGO_MANIFEST_DIR") {
+    #[rustversion::since(1.88)]
+    fn local_file(tokens: &[parser::Token]) -> Option<PathBuf> {
+        tokens.first()?.span?.local_file()
+    }
+    #[rustversion::before(1.88)]
+    fn local_file(_: &[parser::Token]) -> Option<PathBuf> {
+        None
+    }
+
+    let source_file = if let Some(path) = local_file(&tokens) {
+        diagnostics::SourceFileInner::from_path_only(path)
+    } else if let Some(cargo_manifest) = std::env::var_os("CARGO_MANIFEST_DIR") {
         let mut path: std::path::PathBuf = cargo_manifest.into();
         path.push("Cargo.toml");
         diagnostics::SourceFileInner::from_path_only(path)