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The HIR

The HIR – "High-level IR" – is the primary IR used in most of rustc. It is a desugared version of the "abstract syntax tree" (AST) that is generated after parsing, macro expansion, and name resolution have completed. Many parts of HIR resemble Rust surface syntax quite closely, with the exception that some of Rust's expression forms have been desugared away (as an example, for loops are converted into a loop and do not appear in the HIR).

This chapter covers the main concepts of the HIR.

You can view the HIR representation of your code by passing the -Zunpretty=hir-tree flag to rustc:

cargo rustc -- -Zunpretty=hir-tree

Out-of-band storage and the Crate type

The top-level data-structure in the HIR is the Crate, which stores the contents of the crate currently being compiled (we only ever construct HIR for the current crate). Whereas in the AST the crate data structure basically just contains the root module, the HIR Crate structure contains a number of maps and other things that serve to organize the content of the crate for easier access.

For example, the contents of individual items (e.g. modules, functions, traits, impls, etc) in the HIR are not immediately accessible in the parents. So, for example, if there is a module item foo containing a function bar():

mod foo {
    fn bar() { }
}

then in the HIR the representation of module foo (the Mod stuct) would only have the ItemId I of bar(). To get the details of the function bar(), we would lookup I in the items map.

One nice result from this representation is that one can iterate over all items in the crate by iterating over the key-value pairs in these maps (without the need to trawl through the whole HIR). There are similar maps for things like trait items and impl items, as well as "bodies" (explained below).

The other reason to set up the representation this way is for better integration with incremental compilation. This way, if you gain access to an &hir::Item (e.g. for the mod foo), you do not immediately gain access to the contents of the function bar(). Instead, you only gain access to the id for bar(), and you must invoke some function to lookup the contents of bar() given its id; this gives the compiler a chance to observe that you accessed the data for bar(), and then record the dependency.

Identifiers in the HIR

Most of the code that has to deal with things in HIR tends not to carry around references into the HIR, but rather to carry around identifier numbers (or just "ids"). Right now, you will find four sorts of identifiers in active use:

  • DefId – primarily names "definitions" or top-level items.
    • You can think of a DefId as shorthand for a very explicit and complete path, like std::collections::HashMap. However, these paths are able to name things that are not nameable in normal Rust (e.g. impls), and they also include extra information about the crate (such as its version number, since two versions of the same crate can co-exist).
    • A DefId really consists of two parts, a CrateNum (which identifies the crate) and a DefIndex (which indexes into a list of items that is maintained per crate).
  • HirId – combines the index of a particular item with an offset within that item.
    • The key point of an HirId is that it is relative to some item (which is named via a DefId).
  • BodyId – an absolute identifier that refers to a specific body (definition of a function or constant) in the crate. It is currently effectively a "newtype'd" NodeId.
  • NodeId – an absolute ID that identifies a single node in the HIR tree.
    • While these are still in common use, they are being slowly phased out.
    • Since they are absolute within the crate, adding a new node anywhere in the tree causes the NodeIds of all subsequent code in the crate to change. This is terrible for incremental compilation, as you can perhaps imagine.

The HIR Map

Most of the time when you are working with the HIR, you will do so via the HIR Map, accessible in the tcx via tcx.hir (and defined in the hir::map module). The HIR map contains a number of methods to convert between IDs of various kinds and to lookup data associated with an HIR node.

For example, if you have a DefId, and you would like to convert it to a NodeId, you can use tcx.hir.as_local_node_id(def_id). This returns an Option<NodeId> – this will be None if the def-id refers to something outside of the current crate (since then it has no HIR node), but otherwise returns Some(n) where n is the node-id of the definition.

Similarly, you can use tcx.hir.find(n) to lookup the node for a NodeId. This returns a Option<Node<'tcx>>, where Node is an enum defined in the map; by matching on this you can find out what sort of node the node-id referred to and also get a pointer to the data itself. Often, you know what sort of node n is – e.g. if you know that n must be some HIR expression, you can do tcx.hir.expect_expr(n), which will extract and return the &hir::Expr, panicking if n is not in fact an expression.

Finally, you can use the HIR map to find the parents of nodes, via calls like tcx.hir.get_parent_node(n).

HIR Bodies

A body represents some kind of executable code, such as the body of a function/closure or the definition of a constant. Bodies are associated with an owner, which is typically some kind of item (e.g. an fn() or const), but could also be a closure expression (e.g. |x, y| x + y). You can use the HIR map to find the body associated with a given DefId (maybe_body_owned_by()) or to find the owner of a body (body_owner_def_id()).