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|
% Macro Hygiene rules and notes
Use-cases
=========
A variable defined within the macro won't collide with one in a capture
```rust
macro_rules! foo {
($v:expr) => {{ let a = 123; a + $v }};
}
fn bar(a: usize) -> usize {
// `a` here must resolve to the argument, not the macro parameter
foo!(a)
}
```
Macros within functions can access values defined before the invocation
```rust
fn bar(a: usize) -> usize {
let b = 1;
macro_rules! foo {
($v:expr) => {{ a + b + $v }};
}
let a = 123;
// `a` here must resolve to the local above, while the one in the macro resolves to the argument
foo!(a)
}
```
Macros in outer scope can access any outer-scope name
Identifiers introduced within macros are inaccessible outside
```rust
fn bar() {
macro_rules! foo {
($v:expr) => {let a = $v;};
}
foo!(123);
//let b = a; // ERROR
}
```
Implementation Notes
===================
Idea 1: Index list
-------------------
Lexer maintains a stack of counts - incremented after every TT end (closing bracket) and for every IDENT lexed. The stack grows into each TT.
This stack is the hygiene information for an identifier. During lookup a symbol is only usable if its hygiene information is a superset or before yours.
```
if( item_hygine[..-1].is_subset_of(my_hygine) && item_hygiene[-1] < my_hygiene[item_hygiene.len] ) {
/* allowed */
}
```
Hygiene storage points:
- Relative paths (of the leading node)
- Pattern bindings (variable definitions)
- _TODO: Where else?_
- ?Generic definitions
- Items? (when would that be checked? They're "globally" addressable)
Problem: Destructuring patterns end up with an incorrect hygiene using this system.
Idea 2: Hygiene is directly related to the parser state
--------------------------
- Each lexer has a hygiene tag
- Token trees carry with them the hygiene tag for the contained tokens
- :ident captures a TT with associated hygiene context
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