Add trybuild test for Captured::state lifetime correctness (#314)

Cargo.toml

@@ -161,6 +161,7 @@ strip-ansi-escapes = "0.2"
 toml = "0.8"
 serde_yaml = "0.9"
 proptest = "1.11.0"
+trybuild = { version = "1", features = ["diff"] }
 
 # Target-specific dev-deps
 [target.'cfg(unix)'.dev-dependencies]

tests/compile_fail_tests.rs

@@ -0,0 +1,13 @@
+//! Compile-time UI tests for `Captured::state()` lifetime correctness.
+//!
+//! `MacroStateGuard` (in `src/manifest/jinja_macros/cache.rs`) relies on the
+//! borrow checker rejecting any programme that lets a `&State` outlive the
+//! owning `Captured`. These trybuild cases pin that guarantee: the dangling
+//! case must fail to compile, and the well-scoped case must compile.
+
+#[test]
+fn captured_state_lifetime_tests() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/ui/captured_state_outlives_captured.rs");
+    t.pass("tests/ui/captured_state_within_scope.rs");
+}

tests/ui/captured_state_outlives_captured.rs

@@ -0,0 +1,15 @@
+//! Attempting to hold a `&State` beyond the owning `Captured`'s lifetime
+//! must be rejected by the borrow checker.
+
+use minijinja::Environment;
+
+fn main() {
+    let mut env = Environment::new();
+    env.add_template("greeting", "{{ 1 }}").expect("template");
+    let template = env.get_template("greeting").expect("template");
+    let state_ref = {
+        let captured = template.render_captured(()).expect("render");
+        captured.state()
+    };
+    let _ = state_ref;
+}

tests/ui/captured_state_outlives_captured.stderr

@@ -0,0 +1,11 @@
+error[E0597]: `captured` does not live long enough
+  --> tests/ui/captured_state_outlives_captured.rs:12:9
+   |
+10 |     let state_ref = {
+   |         --------- borrow later stored here
+11 |         let captured = template.render_captured(()).expect("render");
+   |             -------- binding `captured` declared here
+12 |         captured.state()
+   |         ^^^^^^^^ borrowed value does not live long enough
+13 |     };
+   |     - `captured` dropped here while still borrowed

tests/ui/captured_state_within_scope.rs

@@ -0,0 +1,13 @@
+//! Using `Captured::state()` strictly within the owning `Captured`'s
+//! lifetime compiles cleanly.
+
+use minijinja::Environment;
+
+fn main() {
+    let mut env = Environment::new();
+    env.add_template("greeting", "{{ 1 }}").expect("template");
+    let template = env.get_template("greeting").expect("template");
+    let captured = template.render_captured(()).expect("render");
+    let state = captured.state();
+    let _ = state.lookup("missing");
+}