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libsyntax: Do not derive Hash for Ident

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This commit is contained in:
Vadim Petrochenkov 2015-10-03 12:44:47 +03:00
parent f492ec4bf4
commit f37d6fc6f3
2 changed files with 29 additions and 15 deletions
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40
src/libsyntax/ast.rs
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@ -65,6 +65,7 @@ use ptr::P;
use std::fmt;
use std::rc::Rc;
use std::borrow::Cow;
use std::hash::{Hash, Hasher};
use serialize::{Encodable, Decodable, Encoder, Decoder};
/// A name is a part of an identifier, representing a string or gensym. It's
@ -84,7 +85,7 @@ pub struct SyntaxContext(pub u32);
/// An identifier contains a Name (index into the interner
/// table) and a SyntaxContext to track renaming and
/// macro expansion per Flatt et al., "Macros That Work Together"
#[derive(Clone, Copy, Eq, Hash)]
#[derive(Clone, Copy, Eq)]
pub struct Ident {
pub name: Name,
pub ctxt: SyntaxContext
@ -133,22 +134,35 @@ impl Ident {
impl PartialEq for Ident {
fn eq(&self, other: &Ident) -> bool {
if self.ctxt == other.ctxt {
self.name == other.name
} else {
// IF YOU SEE ONE OF THESE FAILS: it means that you're comparing
// idents that have different contexts. You can't fix this without
// knowing whether the comparison should be hygienic or non-hygienic.
// if it should be non-hygienic (most things are), just compare the
// 'name' fields of the idents.
if self.ctxt != other.ctxt {
// There's no one true way to compare Idents. They can be compared
// non-hygienically `id1.name == id2.name`, hygienically
// `mtwt::resolve(id1) == mtwt::resolve(id2)`, or even member-wise
// `(id1.name, id1.ctxt) == (id2.name, id2.ctxt)` depending on the situation.
// Ideally, PartialEq should not be implemented for Ident at all, but that
// would be too impractical, because many larger structures (Token, in particular)
// including Idents as their parts derive PartialEq and use it for non-hygienic
// comparisons. That's why PartialEq is implemented and defaults to non-hygienic
// comparison. Hash is implemented too and is consistent with PartialEq, i.e. only
// the name of Ident is hashed. Still try to avoid comparing idents in your code
// (especially as keys in hash maps), use one of the three methods listed above
// explicitly.
//
// On the other hand, if the comparison does need to be hygienic,
// one example and its non-hygienic counterpart would be:
// syntax::parse::token::Token::mtwt_eq
// syntax::ext::tt::macro_parser::token_name_eq
// If you see this panic, then some idents from different contexts were compared
// non-hygienically. It's likely a bug. Use one of the three comparison methods
// listed above explicitly.
panic!("idents with different contexts are compared with operator `==`: \
{:?}, {:?}.", self, other);
}
self.name == other.name
}
}
impl Hash for Ident {
fn hash<H: Hasher>(&self, state: &mut H) {
self.name.hash(state)
}
}