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style: Make Rust static atoms able to be used in const contexts.

I see atom dropping code generated in release builds for stuff like dropping the
"class" atom here:

https://searchfox.org/mozilla-central/rev/4df8821c1b824db5f40f381f48432f219d99ae36/servo/components/style/gecko/wrapper.rs#592

That is silly, and I hope making Atom be able to be used in const context will
help the compiler see that yeah, we're not doing anything interesting and the
atom shouldn't get dropped.

It also allows us to get rid of a few lazy_static!s, so we should do it anyway.

In order to accomplish this, compute the offset into gGkAtoms manually instead
of going through the static_atoms() array and then back to the byte offset.

Differential Revision: https://phabricator.services.mozilla.com/D55039
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emilio committed Nov 27, 2019
1 parent 001c511 commit 31837a1efa276a735ef9d0371af762dcc4d0a540
@@ -132,7 +132,7 @@ def __exit__(self, type, value, traceback):
RULE_TEMPLATE = '''
("{atom}") => {{{{
#[allow(unsafe_code)] #[allow(unused_unsafe)]
unsafe {{ $crate::string_cache::Atom::from_index({index}) }}
unsafe {{ $crate::string_cache::Atom::from_index_unchecked({index}) }}
}}}};
'''[1:]

@@ -49,10 +49,15 @@ macro_rules! local_name {
};
}

/// A handle to a Gecko atom.
/// A handle to a Gecko atom. This is a type that can represent either:
///
/// * A strong reference to a dynamic atom (an `nsAtom` pointer), in which case
/// the `usize` just holds the pointer value.
///
/// * A byte offset from `gGkAtoms` to the `nsStaticAtom` object (shifted to
/// the left one bit, and with the lower bit set to `1` to differentiate it
/// from the above), so `(offset << 1 | 1)`.
///
/// This is either a strong reference to a dynamic atom (an nsAtom pointer),
/// or an offset from gGkAtoms to the nsStaticAtom object.
#[derive(Eq, PartialEq)]
#[repr(C)]
pub struct Atom(NonZeroUsize);
@@ -87,7 +92,7 @@ fn static_atoms() -> &'static [nsStaticAtom; STATIC_ATOM_COUNT] {
fn valid_static_atom_addr(addr: usize) -> bool {
unsafe {
let atoms = static_atoms();
let start = atoms.get_unchecked(0) as *const _;
let start = atoms.as_ptr();
let end = atoms.get_unchecked(STATIC_ATOM_COUNT) as *const _;
let in_range = addr >= start as usize && addr < end as usize;
let aligned = addr % mem::align_of::<nsStaticAtom>() == 0;
@@ -379,17 +384,16 @@ impl Atom {
}

/// Creates a static atom from its index in the static atom table, without
/// checking in release builds.
/// checking.
#[inline]
pub unsafe fn from_index(index: u16) -> Self {
let ptr = static_atoms().get_unchecked(index as usize) as *const _;
let handle = make_static_handle(ptr);
let atom = Atom(handle);
debug_assert!(valid_static_atom_addr(ptr as usize));
debug_assert!(atom.is_static());
debug_assert!((*atom).is_static());
debug_assert!(handle == make_handle(atom.as_ptr()));
atom
pub const unsafe fn from_index_unchecked(index: u16) -> Self {
// FIXME(emilio): No support for debug_assert! in const fn for now. Note
// that violating this invariant will debug-assert in the `Deref` impl
// though.
//
// debug_assert!((index as usize) < STATIC_ATOM_COUNT);
let offset = (index as usize) * std::mem::size_of::<nsStaticAtom>() + kGkAtomsArrayOffset as usize;
Atom(NonZeroUsize::new_unchecked((offset << 1) | 1))
}

/// Creates an atom from an atom pointer.
@@ -63,43 +63,38 @@ where
{
use crate::Atom;

// FIXME(emilio): This should be an actual static.
lazy_static! {
static ref SPECIAL_HTML_ELEMENTS: [Atom; 16] = [
atom!("br"),
atom!("wbr"),
atom!("meter"),
atom!("progress"),
atom!("canvas"),
atom!("embed"),
atom!("object"),
atom!("audio"),
atom!("iframe"),
atom!("img"),
atom!("video"),
atom!("frame"),
atom!("frameset"),
atom!("input"),
atom!("textarea"),
atom!("select"),
];
}
const SPECIAL_HTML_ELEMENTS: [Atom; 16] = [
atom!("br"),
atom!("wbr"),
atom!("meter"),
atom!("progress"),
atom!("canvas"),
atom!("embed"),
atom!("object"),
atom!("audio"),
atom!("iframe"),
atom!("img"),
atom!("video"),
atom!("frame"),
atom!("frameset"),
atom!("input"),
atom!("textarea"),
atom!("select"),
];

// https://drafts.csswg.org/css-display/#unbox-svg
//
// There's a note about "Unknown elements", but there's not a good way to
// know what that means, or to get that information from here, and no other
// UA implements this either.
lazy_static! {
static ref SPECIAL_SVG_ELEMENTS: [Atom; 6] = [
atom!("svg"),
atom!("a"),
atom!("g"),
atom!("use"),
atom!("tspan"),
atom!("textPath"),
];
}
const SPECIAL_SVG_ELEMENTS: [Atom; 6] = [
atom!("svg"),
atom!("a"),
atom!("g"),
atom!("use"),
atom!("tspan"),
atom!("textPath"),
];

// https://drafts.csswg.org/css-display/#unbox-html
if element.is_html_element() {

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