Jorge Aparicio 4c7e277340 add an #[used] attribute ...

similar to GCC's __attribute((used))__. This attribute prevents LLVM from
optimizing away a non-exported symbol, within a compilation unit (object file),
when there are no references to it.

This is better explained with an example:

```
#[used]
static LIVE: i32 = 0;

static REFERENCED: i32 = 0;

static DEAD: i32 = 0;

fn internal() {}

pub fn exported() -> &'static i32 {
    &REFERENCED
}
```

Without optimizations, LLVM pretty much preserves all the static variables and
functions within the compilation unit.

```
$ rustc --crate-type=lib --emit=obj symbols.rs && nm -C symbols.o
0000000000000000 t drop::h1be0f8f27a2ba94a
0000000000000000 r symbols::REFERENCED::hb3bdfd46050bc84c
0000000000000000 r symbols::DEAD::hc2ea8f9bd06f380b
0000000000000000 r symbols::LIVE::h0970cf9889edb56e
0000000000000000 T symbols::exported::h6f096c2b1fc292b2
0000000000000000 t symbols::internal::h0ac1aadbc1e3a494
```

With optimizations, LLVM will drop dead code. Here `internal` is dropped because
it's not a exported function/symbol (i.e. not `pub`lic). `DEAD` is dropped for
the same reason. `REFERENCED` is preserved, even though it's not exported,
because it's referenced by the `exported` function. Finally, `LIVE` survives
because of the `#[used]` attribute even though it's not exported or referenced.

```
$ rustc --crate-type=lib -C opt-level=3 --emit=obj symbols.rs && nm -C symbols.o
0000000000000000 r symbols::REFERENCED::hb3bdfd46050bc84c
0000000000000000 r symbols::LIVE::h0970cf9889edb56e
0000000000000000 T symbols::exported::h6f096c2b1fc292b2
```

Note that the linker knows nothing about `#[used]` and will drop `LIVE`
because no other object references to it.

```
$ echo 'fn main() {}' >> symbols.rs
$ rustc symbols.rs && nm -C symbols | grep LIVE
```

At this time, `#[used]` only works on `static` variables.

2017-04-05 13:40:11 -05:00

..

diagnostics

Use Symbol instead of InternedString in the AST, HIR, and various other places.

2016-11-21 09:00:55 +00:00

ext

Improve Path spans.

2017-03-30 05:44:56 +00:00

parse

Auto merge of #40620 - laumann:slash-in-diagnostics-path, r=BurntSushi

2017-03-31 03:22:39 +00:00

print

Auto merge of #40346 - jseyfried:path_and_tokenstream_attr, r=nrc

2017-03-19 10:56:08 +00:00

util

Refactor out ast::ItemKind::MacroDef.

2017-03-10 08:08:32 -08:00

abi.rs

Add support for x86-interrupt calling convention

2017-03-02 19:01:15 +01:00

ast.rs

Refactor how spans are combined in the parser.

2017-03-29 11:17:59 +00:00

attr.rs

Improve Path spans.

2017-03-30 05:44:56 +00:00

Cargo.toml

Remove internal liblog

2017-03-23 11:28:00 -07:00

codemap.rs

Refactor how spans are combined in the parser.

2017-03-29 11:17:59 +00:00

config.rs

Liberalize attributes.

2017-03-14 04:39:21 +00:00

diagnostic_list.rs

Add long error explanations

2017-02-21 15:52:14 +01:00

entry.rs

Cleanup InternedString.

2016-11-21 09:00:56 +00:00

feature_gate.rs

add an #[used] attribute

2017-04-05 13:40:11 -05:00

fold.rs

Auto merge of #40346 - jseyfried:path_and_tokenstream_attr, r=nrc

2017-03-19 10:56:08 +00:00

json.rs

Merge ExpnId and SyntaxContext.

2017-03-29 00:41:10 +00:00

lib.rs

Merge ExpnId and SyntaxContext.

2017-03-29 00:41:10 +00:00

ptr.rs

Documentation for default types modified

2016-09-11 22:58:01 +05:30

show_span.rs

annotate stricter lifetimes on LateLintPass methods to allow them to forward to a Visitor

2016-12-06 11:28:51 +01:00

std_inject.rs

Merge ExpnId and SyntaxContext.

2017-03-29 00:41:10 +00:00

str.rs

syntax: Copy unstable str::char_at into libsyntax

2015-04-21 10:23:53 -07:00

test_snippet.rs

Merge ExpnId and SyntaxContext.

2017-03-29 00:41:10 +00:00

test.rs

Merge ExpnId and SyntaxContext.

2017-03-29 00:41:10 +00:00

tokenstream.rs

Merge ExpnId and SyntaxContext.

2017-03-29 00:41:10 +00:00

visit.rs

Add catch expr to AST and disallow catch as a struct name

2017-03-11 22:26:52 -08:00