Module rustc_middle::mir::visit
source · Expand description
The MIR Visitor
Overview
There are two visitors, one for immutable and one for mutable references, but both are generated by the following macro. The code is written according to the following conventions:
- introduce a
visit_foo
and asuper_foo
method for every MIR type visit_foo
, by default, callssuper_foo
super_foo
, by default, destructures thefoo
and callsvisit_foo
This allows you as a user to override visit_foo
for types are
interested in, and invoke (within that method) call
self.super_foo
to get the default behavior. Just as in an OO
language, you should never call super
methods ordinarily except
in that circumstance.
For the most part, we do not destructure things external to the
MIR, e.g., types, spans, etc, but simply visit them and stop. This
avoids duplication with other visitors like TypeFoldable
.
Updating
The code is written in a very deliberate style intended to minimize the chance of things being overlooked. You’ll notice that we always use pattern matching to reference fields and we ensure that all matches are exhaustive.
For example, the super_basic_block_data
method begins like this:
fn super_basic_block_data(
&mut self,
block: BasicBlock,
data: & $($mutability)? BasicBlockData<'tcx>
) {
let BasicBlockData {
statements,
terminator,
is_cleanup: _
} = *data;
for statement in statements {
self.visit_statement(block, statement);
}
...
}
Here we used let BasicBlockData { <fields> } = *data
deliberately,
rather than writing data.statements
in the body. This is because if one
adds a new field to BasicBlockData
, one will be forced to revise this code,
and hence one will (hopefully) invoke the correct visit methods (if any).
For this to work, ALL MATCHES MUST BE EXHAUSTIVE IN FIELDS AND VARIANTS.
That means you never write ..
to skip over fields, nor do you write _
to skip over variants in a match
.
The only place that _
is acceptable is to match a field (or
variant argument) that does not require visiting, as in
is_cleanup
above.
Macros
Enums
visit_ty
and friends to give context
about where the type etc appears.