fix: implement dynamic template loader for include functionality

- Add 'loader' feature to minijinja dependency for dynamic template loading
- Implement lazy template loader using env.set_loader() in renderer
- Template loader uses TemplateContext to resolve relative paths
- Security: blocks absolute paths and parent traversal unless --trust mode
- Add comprehensive test suite with 12 integration tests covering:
  * Basic include functionality
  * Nested includes (multiple levels)
  * Include from subdirectories
  * Environment variables in included templates
  * Security restrictions (parent directory and absolute path blocking)
  * Trust mode allowing parent directory access
  * Conditional includes
  * Error handling for missing templates
- Add test fixtures for include templates and expected outputs
- All 267 tests passing (251 unit + 12 integration + 4 doc tests)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: bordeux

Cargo.lock

@@ -10,7 +10,7 @@ keywords = ["template", "minijinja", "cli", "environment", "variables"]
 categories = ["command-line-utilities", "template-engine"]
 
 [dependencies]
-minijinja = { version = "2", features = ["builtins"] }
+minijinja = { version = "2", features = ["builtins", "loader"] }
 clap = { version = "4", features = ["derive"] }
 serde = { version = "1", features = ["derive"] }
 regex = "1"

Cargo.toml

@@ -0,0 +1,6 @@
+Testing relative path resolution:
+
+File content from ./basic.tmpl parsed:
+{% include "./basic.tmpl" %}
+
+End of test.

examples/test-include.tmpl

@@ -85,6 +85,49 @@ fn render(
     // Set strict undefined behavior - fail on undefined variables (like Tera)
     env.set_undefined_behavior(minijinja::UndefinedBehavior::Strict);
 
+    // Clone template_context for use in the loader closure
+    let loader_context = template_context.clone();
+
+    // Set up lazy template loader for includes
+    env.set_loader(move |name: &str| -> Result<Option<String>, minijinja::Error> {
+        // Validate path security unless in trust mode
+        if !loader_context.is_trust_mode() {
+            if name.starts_with('/') {
+                return Err(minijinja::Error::new(
+                    minijinja::ErrorKind::InvalidOperation,
+                    format!(
+                        "Security: Absolute paths are not allowed: {}. Use --trust to bypass this restriction.",
+                        name
+                    ),
+                ));
+            }
+            if name.contains("..") {
+                return Err(minijinja::Error::new(
+                    minijinja::ErrorKind::InvalidOperation,
+                    format!(
+                        "Security: Parent directory (..) traversal is not allowed: {}. Use --trust to bypass this restriction.",
+                        name
+                    ),
+                ));
+            }
+        }
+
+        // Resolve the template path relative to the base directory
+        let resolved_path = loader_context.resolve_path(name);
+
+        // Read the template file
+        match fs::read_to_string(&resolved_path) {
+            Ok(content) => Ok(Some(content)),
+            Err(e) => {
+                // Return a helpful error message
+                Err(minijinja::Error::new(
+                    minijinja::ErrorKind::TemplateNotFound,
+                    format!("Failed to load template '{}': {}", resolved_path.display(), e),
+                ))
+            }
+        }
+    });
+
     // Register all custom functions
     functions::register_all(&mut env, template_context);
 

src/renderer.rs

@@ -0,0 +1,4 @@
+Header: Default Header
+Content from partial template.
+User: guest
+Footer: Done

tests/fixtures/expected/include_base.txt

@@ -0,0 +1,5 @@
+Main template start
+Level 1 start
+Level 2: nested value
+Level 1 end
+Main template end

tests/fixtures/expected/include_nested.txt

@@ -0,0 +1,3 @@
+Header: {{ get_env(name="HEADER", default="Default Header") }}
+{% include "./include_partial.tmpl" %}
+Footer: Done

tests/fixtures/templates/include_base.tmpl

@@ -0,0 +1,3 @@
+Level 1 start
+{% include "./include_nested_level2.tmpl" %}
+Level 1 end

tests/fixtures/templates/include_nested_level1.tmpl

@@ -0,0 +1 @@
+Level 2: nested value

tests/fixtures/templates/include_nested_level2.tmpl

@@ -0,0 +1,3 @@
+Main template start
+{% include "./include_nested_level1.tmpl" %}
+Main template end