src/doc/rustc-dev-guide/src/rustc-driver-interacting-with-the-ast.md

   1 # Example: Type checking through `rustc_interface`
   2
   3 `rustc_interface` allows you to interact with Rust code at various stages of compilation.
   4
   5 ## Getting the type of an expression
   6
   7 To get the type of an expression, use the `global_ctxt` to get a `TyCtxt`.
   8 The following was tested with <!-- date: 2022-06 --> `nightly-2022-06-05`
   9 (see [here][example] for the complete example):
  10
  11 [example]: https://github.com/rust-lang/rustc-dev-guide/blob/master/examples/rustc-driver-interacting-with-the-ast.rs
  12
  13 ```rust
  14 let config = rustc_interface::Config {
  15     input: config::Input::Str {
  16         name: source_map::FileName::Custom("main.rs".to_string()),
  17         input: "fn main() { let message = \"Hello, world!\"; println!(\"{}\", message); }"
  18             .to_string(),
  19     },
  20     /* other config */
  21 };
  22 rustc_interface::run_compiler(config, |compiler| {
  23     compiler.enter(|queries| {
  24         // Analyze the crate and inspect the types under the cursor.
  25         queries.global_ctxt().unwrap().take().enter(|tcx| {
  26             // Every compilation contains a single crate.
  27             let hir_krate = tcx.hir();
  28             // Iterate over the top-level items in the crate, looking for the main function.
  29             for id in hir_krate.items() {
  30                 let item = hir_krate.item(id);
  31                 // Use pattern-matching to find a specific node inside the main function.
  32                 if let rustc_hir::ItemKind::Fn(_, _, body_id) = item.kind {
  33                     let expr = &tcx.hir().body(body_id).value;
  34                     if let rustc_hir::ExprKind::Block(block, _) = expr.kind {
  35                         if let rustc_hir::StmtKind::Local(local) = block.stmts[0].kind {
  36                             if let Some(expr) = local.init {
  37                                 let hir_id = expr.hir_id; // hir_id identifies the string "Hello, world!"
  38                                 let def_id = tcx.hir().local_def_id(item.hir_id()); // def_id identifies the main function
  39                                 let ty = tcx.typeck(def_id).node_type(hir_id);
  40                                 println!("{:?}: {:?}", expr, ty);
  41                             }
  42                         }
  43                     }
  44                 }
  45             }
  46         })
  47     });
  48 });
  49 ```