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 NOTE: For the example to compile, you will need to first run the following:
   8
   9     rustup component add rustc-dev
  10
  11 To get the type of an expression, use the `global_ctxt` to get a `TyCtxt`:
  12
  13 ```rust
  14 // In this example, config specifies the rust program:
  15 //   fn main() { let message = \"Hello, world!\"; println!(\"{}\", message); }
  16 // Our goal is to get the type of the string literal "Hello, world!".
  17 //
  18 // See https://github.com/rust-lang/rustc-dev-guide/blob/master/examples/rustc-driver-example.rs for a complete example of configuring rustc_interface
  19 rustc_interface::run_compiler(config, |compiler| {
  20     compiler.enter(|queries| {
  21         // Analyze the crate and inspect the types under the cursor.
  22         queries.global_ctxt().unwrap().take().enter(|tcx| {
  23             // Every compilation contains a single crate.
  24             let krate = tcx.hir().krate();
  25             // Iterate over the top-level items in the crate, looking for the main function.
  26             for (_, item) in &krate.items {
  27                 // Use pattern-matching to find a specific node inside the main function.
  28                 if let rustc_hir::ItemKind::Fn(_, _, body_id) = item.kind {
  29                     let expr = &tcx.hir().body(body_id).value;
  30                     if let rustc_hir::ExprKind::Block(block, _) = expr.kind {
  31                         if let rustc_hir::StmtKind::Local(local) = block.stmts[0].kind {
  32                             if let Some(expr) = local.init {
  33                                 let hir_id = expr.hir_id; // hir_id identifies the string "Hello, world!"
  34                                 let def_id = tcx.hir().local_def_id(item.hir_id); // def_id identifies the main function
  35                                 let ty = tcx.typeck_tables_of(def_id).node_type(hir_id);
  36                                 println!("{:?}: {:?}", expr, ty); // prints expr(HirId { owner: DefIndex(3), local_id: 4 }: "Hello, world!"): &'static str
  37                             }
  38                         }
  39                     }
  40                 }
  41             }
  42         })
  43     });
  44 });
  45 ```