parser: reject closing delims that look like an op

rinja_derive/src/config.rs

@@ -360,6 +360,26 @@ impl<'a> RawSyntax<'a> {
             }
         }
 
+        for end in [syntax.block_start, syntax.expr_start, syntax.comment_start] {
+            for prefix in ["<<", ">>", "&&", "..", "||"] {
+                if end.starts_with(prefix) {
+                    let msg = if end == prefix {
+                        format!("a closing delimiter must not start with an operator: {end:?}")
+                    } else {
+                        format!(
+                            "a closing delimiter must not start an with operator: \
+                             {end:?} vs {prefix:?}",
+                        )
+                    };
+                    return Err(CompileError::new_with_span(
+                        msg,
+                        file_info.copied(),
+                        config_span,
+                    ));
+                }
+            }
+        }
+
         Ok(syntax)
     }
 }

testing/issue-128-2.toml

@@ -0,0 +1,4 @@
+[[syntax]]
+name = "mwe"
+expr_start = "<<<"
+expr_end = ">>>"

testing/issue-128.toml

@@ -0,0 +1,4 @@
+[[syntax]]
+name = "mwe"
+expr_start = "<<"
+expr_end = ">>"

testing/tests/ui/terminator-operator.rs

@@ -0,0 +1,17 @@
+use rinja::Template;
+
+#[derive(Template)]
+#[template(source = "<<a>> and <<b>>", config = "issue-128.toml", syntax = "mwe", ext="")]
+struct HelloTemplate {
+    a: u32,
+    b: u32,
+}
+
+#[derive(Template)]
+#[template(source = "<<a>> and <<b>>", config = "issue-128-2.toml", syntax = "mwe", ext="")]
+struct HelloTemplate2 {
+    a: u32,
+    b: u32,
+}
+
+fn main() {}

testing/tests/ui/terminator-operator.stderr

@@ -0,0 +1,13 @@
+error: a closing delimiter must not start with an operator: "<<"
+        --> testing/issue-128.toml
+ --> tests/ui/terminator-operator.rs:4:49
+  |
+4 | #[template(source = "<<a>> and <<b>>", config = "issue-128.toml", syntax = "mwe", ext="")]
+  |                                                 ^^^^^^^^^^^^^^^^
+
+error: a closing delimiter must not start an with operator: "<<<" vs "<<"
+        --> testing/issue-128-2.toml
+  --> tests/ui/terminator-operator.rs:11:49
+   |
+11 | #[template(source = "<<a>> and <<b>>", config = "issue-128-2.toml", syntax = "mwe", ext="")]
+   |                                                 ^^^^^^^^^^^^^^^^^^