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|
/// This module takes a (parsed) defenition of `macro_rules` invocation, a
/// `tt::TokenTree` representing an argument of macro invocation, and produces a
/// `tt::TokenTree` for the result of the expansion.
use rustc_hash::FxHashMap;
use ra_syntax::SmolStr;
use crate::tt_cursor::TtCursor;
pub(crate) fn exapnd(rules: &crate::MacroRules, input: &tt::Subtree) -> Option<tt::Subtree> {
rules.rules.iter().find_map(|it| expand_rule(it, input))
}
fn expand_rule(rule: &crate::Rule, input: &tt::Subtree) -> Option<tt::Subtree> {
let mut input = TtCursor::new(input);
let bindings = match_lhs(&rule.lhs, &mut input)?;
if !input.is_eof() {
return None;
}
expand_subtree(&rule.rhs, &bindings, &mut Vec::new())
}
/// The actual algorithm for expansion is not too hard, but is pretty tricky.
/// `Bindings` structure is the key to understanding what we are doing here.
///
/// On the high level, it stores mapping from meta variables to the bits of
/// syntax it should be substituted with. For example, if `$e:expr` is matched
/// with `1 + 1` by macro_rules, the `Binding` will store `$e -> 1 + 1`.
///
/// The tricky bit is dealing with repetitions (`$()*`). Consider this example:
///
/// ```not_rust
/// macro_rules! foo {
/// ($($ i:ident $($ e:expr),*);*) => {
/// $(fn $ i() { $($ e);*; })*
/// }
/// }
/// foo! { foo 1,2,3; bar 4,5,6 }
/// ```
///
/// Here, the `$i` meta variable is matched first with `foo` and then with
/// `bar`, and `$e` is matched in turn with `1`, `2`, `3`, `4`, `5`, `6`.
///
/// To represent such "multi-mappings", we use a recursive structures: we map
/// variables not to values, but to *lists* of values or other lists (that is,
/// to the trees).
///
/// For the above example, the bindings would store
///
/// ```not_rust
/// i -> [foo, bar]
/// e -> [[1, 2, 3], [4, 5, 6]]
/// ```
///
/// We construct `Bindings` in the `match_lhs`. The interesting case is
/// `TokenTree::Repeat`, where we use `push_nested` to create the desired
/// nesting structure.
///
/// The other side of the puzzle is `expand_subtree`, where we use the bindings
/// to substitute meta variables in the output template. When expanding, we
/// maintain a `nesteing` stack of indicies whihc tells us which occurence from
/// the `Bindings` we should take. We push to the stack when we enter a
/// repetition.
///
/// In other words, `Bindings` is a *multi* mapping from `SmolStr` to
/// `tt::TokenTree`, where the index to select a particular `TokenTree` among
/// many is not a plain `usize`, but an `&[usize]`.
#[derive(Debug, Default)]
struct Bindings {
inner: FxHashMap<SmolStr, Binding>,
}
#[derive(Debug)]
enum Binding {
Simple(tt::TokenTree),
Nested(Vec<Binding>),
}
impl Bindings {
fn get(&self, name: &SmolStr, nesting: &[usize]) -> Option<&tt::TokenTree> {
let mut b = self.inner.get(name)?;
for &idx in nesting.iter() {
b = match b {
Binding::Simple(_) => break,
Binding::Nested(bs) => bs.get(idx)?,
};
}
match b {
Binding::Simple(it) => Some(it),
Binding::Nested(_) => None,
}
}
fn push_nested(&mut self, nested: Bindings) -> Option<()> {
for (key, value) in nested.inner {
if !self.inner.contains_key(&key) {
self.inner.insert(key.clone(), Binding::Nested(Vec::new()));
}
match self.inner.get_mut(&key) {
Some(Binding::Nested(it)) => it.push(value),
_ => return None,
}
}
Some(())
}
}
fn match_lhs(pattern: &crate::Subtree, input: &mut TtCursor) -> Option<Bindings> {
let mut res = Bindings::default();
for pat in pattern.token_trees.iter() {
match pat {
crate::TokenTree::Leaf(leaf) => match leaf {
crate::Leaf::Var(crate::Var { text, kind }) => {
let kind = kind.clone()?;
match kind.as_str() {
"ident" => {
let ident = input.eat_ident()?.clone();
res.inner.insert(
text.clone(),
Binding::Simple(tt::Leaf::from(ident).into()),
);
}
_ => return None,
}
}
crate::Leaf::Punct(punct) => {
if input.eat_punct()? != punct {
return None;
}
}
crate::Leaf::Ident(ident) => {
if input.eat_ident()?.text != ident.text {
return None;
}
}
_ => return None,
},
crate::TokenTree::Repeat(crate::Repeat {
subtree,
kind: _,
separator,
}) => {
while let Some(nested) = match_lhs(subtree, input) {
res.push_nested(nested)?;
if let Some(separator) = *separator {
if !input.is_eof() {
if input.eat_punct()?.char != separator {
return None;
}
}
}
}
}
_ => {}
}
}
Some(res)
}
fn expand_subtree(
template: &crate::Subtree,
bindings: &Bindings,
nesting: &mut Vec<usize>,
) -> Option<tt::Subtree> {
let token_trees = template
.token_trees
.iter()
.map(|it| expand_tt(it, bindings, nesting))
.collect::<Option<Vec<_>>>()?;
Some(tt::Subtree {
token_trees,
delimiter: template.delimiter,
})
}
fn expand_tt(
template: &crate::TokenTree,
bindings: &Bindings,
nesting: &mut Vec<usize>,
) -> Option<tt::TokenTree> {
let res: tt::TokenTree = match template {
crate::TokenTree::Subtree(subtree) => expand_subtree(subtree, bindings, nesting)?.into(),
crate::TokenTree::Repeat(repeat) => {
let mut token_trees = Vec::new();
nesting.push(0);
while let Some(t) = expand_subtree(&repeat.subtree, bindings, nesting) {
let idx = nesting.pop().unwrap();
nesting.push(idx + 1);
token_trees.push(t.into())
}
nesting.pop().unwrap();
tt::Subtree {
token_trees,
delimiter: tt::Delimiter::None,
}
.into()
}
crate::TokenTree::Leaf(leaf) => match leaf {
crate::Leaf::Ident(ident) => tt::Leaf::from(tt::Ident {
text: ident.text.clone(),
})
.into(),
crate::Leaf::Punct(punct) => tt::Leaf::from(punct.clone()).into(),
crate::Leaf::Var(v) => bindings.get(&v.text, nesting)?.clone(),
crate::Leaf::Literal(l) => tt::Leaf::from(tt::Literal {
text: l.text.clone(),
})
.into(),
},
};
Some(res)
}
|
