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# Example: Type checking through `rustc_interface`
`rustc_interface` allows you to interact with Rust code at various stages of compilation.
## Getting the type of an expression
To get the type of an expression, use the `global_ctxt` to get a `TyCtxt`.
The following was tested with <!-- date-check: Jan 2023 --> `nightly-2022-12-19`
(see [here][example] for the complete example):
[example]: https://github.com/rust-lang/rustc-dev-guide/blob/master/examples/rustc-driver-interacting-with-the-ast.rs
```rust
let config = rustc_interface::Config {
input: config::Input::Str {
name: source_map::FileName::Custom("main.rs".to_string()),
input: "fn main() { let message = \"Hello, world!\"; println!(\"{}\", message); }"
.to_string(),
},
/* other config */
};
rustc_interface::run_compiler(config, |compiler| {
compiler.enter(|queries| {
// Analyze the crate and inspect the types under the cursor.
queries.global_ctxt().unwrap().take().enter(|tcx| {
// Every compilation contains a single crate.
let hir_krate = tcx.hir();
// Iterate over the top-level items in the crate, looking for the main function.
for id in hir_krate.items() {
let item = hir_krate.item(id);
// Use pattern-matching to find a specific node inside the main function.
if let rustc_hir::ItemKind::Fn(_, _, body_id) = item.kind {
let expr = &tcx.hir().body(body_id).value;
if let rustc_hir::ExprKind::Block(block, _) = expr.kind {
if let rustc_hir::StmtKind::Local(local) = block.stmts[0].kind {
if let Some(expr) = local.init {
let hir_id = expr.hir_id; // hir_id identifies the string "Hello, world!"
let def_id = tcx.hir().local_def_id(item.hir_id()); // def_id identifies the main function
let ty = tcx.typeck(def_id).node_type(hir_id);
println!("{:?}: {:?}", expr, ty);
}
}
}
}
}
})
});
});
```
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