Load templates from relative paths

Templates can now be placed directly next to the source file that they
are defined in as a default. This relies on an unstable rust compiler
feature, which exposes the source file to proc macros. See
<rust-lang/rust#54725> for more info.

This requires the nightly compiler to run, and enabling the
proc_macro_span and procmacro2_semver_exempt cfg flags. To build / test:

```shell
RUSTFLAGS='--cfg proc_macro_span --cfg procmacro2_semver_exempt' \
  cargo +nightly build
```
Fixes: <https://github.com/djc/askama/issues/877>

askama/Cargo.toml

@@ -29,6 +29,12 @@ with-axum = ["askama_derive/with-axum"]
 with-rocket = ["askama_derive/with-rocket"]
 with-warp = ["askama_derive/with-warp"]
 
+## Enables the ability to put templates in a directory relative to the source file that uses them.
+## Requires a nightly compiler and adding:
+## RUSTFLAGS='--cfg proc_macro_span --cfg procmacro2_semver_exempt'
+## to your cargo build command.
+relative-paths = ["askama_derive/relative-paths"]
+
 [dependencies]
 askama_derive = { version = "0.13", path = "../askama_derive" }
 askama_escape = { version = "0.11", path = "../askama_escape" }

askama/tests/relative_paths.rs

@@ -0,0 +1,18 @@
+#[cfg(feature = "relative-paths")]
+mod relative_paths {
+    use askama::Template;
+
+    #[derive(Template)]
+    #[template(path = "relative_paths.txt")]
+    struct RelativePathTemplate {
+        name: String,
+    }
+
+    #[test]
+    fn test_relative_paths() {
+        let t = RelativePathTemplate {
+            name: "world".to_string(),
+        };
+        assert_eq!(t.render().unwrap(), "Hello, world!");
+    }
+}

askama/tests/relative_paths.txt

@@ -0,0 +1 @@
+Hello, {{ name }}!

askama_derive/Cargo.toml

@@ -24,6 +24,12 @@ with-axum = []
 with-rocket = []
 with-warp = []
 
+## Enables the ability to put templates in a directory relative to the source file that uses them.
+## Requires a nightly compiler and adding:
+## RUSTFLAGS='--cfg proc_macro_span --cfg procmacro2_semver_exempt'
+## to your cargo build command.
+relative-paths = []
+
 [dependencies]
 parser = { package = "askama_parser", version = "0.3.1", path = "../askama_parser" }
 mime = "0.3"

askama_derive/src/config.rs

@@ -26,7 +26,22 @@ impl<'a> Config<'a> {
         template_whitespace: Option<&str>,
     ) -> std::result::Result<Config<'a>, CompileError> {
         let root = PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap());
-        let default_dirs = vec![root.join("templates")];
+        let root_path = root.join("templates");
+        let default_dirs;
+        #[cfg(feature = "relative-paths")]
+        {
+            let source = proc_macro2::Span::call_site().source_file();
+            default_dirs = if source.is_real() {
+                let relative_path = source.path().parent().unwrap().to_path_buf();
+                vec![relative_path, root_path]
+            } else {
+                vec![root_path]
+            };
+        }
+        #[cfg(not(feature = "relative-paths"))]
+        {
+            default_dirs = vec![root_path];
+        }
 
         let mut syntaxes = BTreeMap::new();
         syntaxes.insert(DEFAULT_SYNTAX_NAME.to_string(), Syntax::default());