chore(compiler): add a runnable example for experimental introspectio…

…n implementation (#859)

Wanted to try out some queries so I wrote this 😸

crates/apollo-compiler/examples/introspect.rs

@@ -0,0 +1,48 @@
+use apollo_compiler::execution::SchemaIntrospectionQuery;
+use apollo_compiler::validation::Valid;
+use apollo_compiler::ExecutableDocument;
+use apollo_compiler::Schema;
+
+/// To use, do:
+/// cargo run --example introspect path/to/schema.graphql introspection_query.graphql
+///
+/// You can also provide simple queries inline:
+/// cargo run --example introspect path/to/schema.graphql '{ __schema { types { name } } }'
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let mut args = std::env::args().skip(1);
+    let Some(filename) = args.next() else {
+        return Err("Provide a schema".into());
+    };
+    let schema = Schema::parse_and_validate(std::fs::read_to_string(&filename)?, filename)
+        .map_err(|err| err.errors.to_string())?;
+
+    let Some(filename) = args.next() else {
+        return Err("Provide a query to execute".into());
+    };
+
+    let query = if filename.starts_with('{') {
+        ExecutableDocument::parse_and_validate(&schema, filename, "input.graphql")
+    } else {
+        ExecutableDocument::parse_and_validate(
+            &schema,
+            std::fs::read_to_string(&filename)?,
+            filename,
+        )
+    }
+    .map_err(|err| err.errors.to_string())?;
+
+    let variables = Default::default();
+    let response = SchemaIntrospectionQuery::split_and_execute(
+        &schema,
+        &query,
+        query
+            .get_operation(None)
+            .map_err(|_| "Provided query must be an anonymous introspection query")?,
+        Valid::assume_valid_ref(&variables),
+        |_| panic!("Provided query must not have non-introspection elements"),
+    );
+
+    serde_json::to_writer_pretty(std::io::stdout().lock(), &response)?;
+
+    Ok(())
+}