Propose add function level inline assembly for rust.

[lygstate]

For example:

extern "C" {
pub fn get_registers() inline_assembly = 'ByteArray Literal for the Object file to be linked directly';
}

and the ByteArray Literal could be generated a procedure macro.

extern "C" {
pub fn get_registers() inline_assembly = get_registers_macro_assembly!();
}

[lygstate]

By this, we can implement section and other platform specific things directly without burden the rustc
For example, the following is a MSVC construct for section implement.

void some_fucntion_implement_init_by_rust(void);
#pragma section(".CRT$XCU",read)
void (*some_fucntion_implement_init)(void) = some_fucntion_implement_init_by_rust;
#pragma comment(linker,"/include:some_fucntion_implement_init")

And the following is the HEX string generated by MSVC compiler:
64 86 03 20 19 05 2a 58 5b 01 20 20 0a 20 20 20 20 20 20 20 2e 64 72 65 63 74 76 65 20 20 20 20 20 20 20 20 55 20 20 20 8c 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 0a 10 20 2e 64 65 62 75 67 24 53 20 20 20 20 20 20 20 20 68 20 20 20 e1 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 40 20 10 42 2e 64 61 74 61 20 20 20 20 20 20 20 20 20 20 20 08 20 20 20 49 01 20 20 51 01 20 20 20 20 20 20 01 20 20 20 40 20 40 c0 20 20 20 2f 69 6e 63 6c 75 64 65 3a 73 6f 6d 65 5f 66 75 63 6e 74 69 6f 6e 5f 69 6d 70 6c 65 6d 65 6e 74 5f 69 6e 69 74 20 2f 44 45 46 41 55 4c 54 4c 49 42 3a 22 4c 49 42 43 4d 54 22 20 2f 44 45 46 41 55 4c 54 4c 49 42 3a 22 4f 4c 44 4e 41 4d 45 53 22 20 04 20 20 20 f1 20 20 20 5a 20 20 20 1c 20 01 11 20 20 20 20 45 3a 5c 6a 73 2d 65 67 6e 69 6e 65 5c 74 65 73 74 2e 6f 62 6a 20 3a 20 3c 11 20 62 20 20 d0 20 13 20 20 20 95 5e 01 20 13 20 20 20 95 5e 01 20 4d 69 63 72 6f 73 6f 66 74 20 28 52 29 20 4f 70 74 69 6d 69 7a 69 6e 67 20 43 6f 6d 70 69 6c 65 72 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 06 20 20 20 01 20 40 63 6f 6d 70 2e 69 64 95 5e 04 01 ff ff 20 20 03 20 40 66 65 61 74 2e 30 30 90 01 20 80 ff ff 20 20 03 20 2e 64 72 65 63 74 76 65 20 20 20 20 01 20 20 20 03 01 55 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 2e 64 65 62 75 67 24 53 20 20 20 20 02 20 20 20 03 01 68 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 04 20 20 20 20 20 20 20 20 20 20 20 02 20 2e 64 61 74 61 20 20 20 20 20 20 20 03 20 20 20 03 01 08 20 20 20 01 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 29 20 20 20 20 20 20 20 03 20 20 20 02 20 46 20 20 20 73 6f 6d 65 5f 66 75 63 6e 74 69 6f 6e 5f 69 6d 70 6c 65 6d 65 6e 74 5f 69 6e 69 74 5f 62 79 5f 72 75 73 74 20 73 6f 6d 65 5f 66 75 63 6e 74 69 6f 6e 5f 69 6d 70 6c 65 6d 65 6e 74 5f 69 6e 69 74 20

And following is the dump from dumpbin of MSVC

Dump of file test.obj

File Type: COFF OBJECT

FILE HEADER VALUES
8664 machine (x64)
3 number of sections
582A0519 time date stamp Tue Nov 15 02:40:25 2016
15B file pointer to symbol table
A number of symbols
0 size of optional header
0 characteristics

SECTION HEADER #1
.drectve name
0 physical address
0 virtual address
55 size of raw data
8C file pointer to raw data (0000008C to 000000E0)
0 file pointer to relocation table
0 file pointer to line numbers
0 number of relocations
0 number of line numbers
100A00 flags
Info
Remove
1 byte align

RAW DATA #1
00000000: 20 20 20 2F 69 6E 63 6C 75 64 65 3A 73 6F 6D 65 /include:some
00000010: 5F 66 75 63 6E 74 69 6F 6E 5F 69 6D 70 6C 65 6D _fucntion_implem
00000020: 65 6E 74 5F 69 6E 69 74 20 2F 44 45 46 41 55 4C ent_init /DEFAUL
00000030: 54 4C 49 42 3A 22 4C 49 42 43 4D 54 22 20 2F 44 TLIB:"LIBCMT" /D
00000040: 45 46 41 55 4C 54 4C 49 42 3A 22 4F 4C 44 4E 41 EFAULTLIB:"OLDNA
00000050: 4D 45 53 22 20 MES"

Linker Directives

---

/include:some_fucntion_implement_init
/DEFAULTLIB:LIBCMT
/DEFAULTLIB:OLDNAMES

SECTION HEADER #2
.debug$S name
0 physical address
0 virtual address
68 size of raw data
E1 file pointer to raw data (000000E1 to 00000148)
0 file pointer to relocation table
0 file pointer to line numbers
0 number of relocations
0 number of line numbers
42100040 flags
Initialized Data
Discardable
1 byte align
Read Only

RAW DATA #2
00000000: 04 00 00 00 F1 00 00 00 5A 00 00 00 1C 00 01 11 ....ñ...Z.......
00000010: 00 00 00 00 45 3A 5C 6A 73 2D 65 67 6E 69 6E 65 ....E:\js-egnine
00000020: 5C 74 65 73 74 2E 6F 62 6A 00 3A 00 3C 11 00 62 \test.obj.:.<..b
00000030: 00 00 D0 00 13 00 00 00 95 5E 01 00 13 00 00 00 ..Ð......^......
00000040: 95 5E 01 00 4D 69 63 72 6F 73 6F 66 74 20 28 52 .^..Microsoft (R
00000050: 29 20 4F 70 74 69 6D 69 7A 69 6E 67 20 43 6F 6D ) Optimizing Com
00000060: 70 69 6C 65 72 00 00 00 piler...

SECTION HEADER #3
.data name
0 physical address
0 virtual address
8 size of raw data
149 file pointer to raw data (00000149 to 00000150)
151 file pointer to relocation table
0 file pointer to line numbers
1 number of relocations
0 number of line numbers
C0400040 flags
Initialized Data
8 byte align
Read Write

RAW DATA #3
00000000: 00 00 00 00 00 00 00 00 ........

RELOCATIONS #3
Symbol Symbol
Offset Type Applied To Index Name

---

00000000 ADDR64 00000000 00000000 6 some_fucntion_implement_init_by_rust

COFF SYMBOL TABLE
000 01045E95 ABS notype Static | @comp.id
001 80000190 ABS notype Static | @feat.00
002 00000000 SECT1 notype Static | .drectve
Section length 55, #relocs 0, #linenums 0, checksum 0
004 00000000 SECT2 notype Static | .debug$S
Section length 68, #relocs 0, #linenums 0, checksum 0
006 00000000 UNDEF notype () External | some_fucntion_implement_init_by_rust
007 00000000 SECT3 notype Static | .data
Section length 8, #relocs 1, #linenums 0, checksum 0
009 00000000 SECT3 notype External | some_fucntion_implement_init

String Table Size = 0x46 bytes

Summary

       8 .data
      68 .debug$S
      55 .drectve

So we just need a macro to construct such a object file directly.
The compiler doesn't care about how the Macro Procedure generate the file, it's just output the
object file of this function, and link it latter.
The only things we need to care is `some_fucntion_implement_init` & `some_fucntion_implement_init_by_rust`
and some offsets. and some literal about the MSVC compiler.

[durka]

I think we have this already (at least in Accepted RFC Rust, not sure about Implemented RFC Rust). See #1548 and #1201.

[Amanieu]

Isn't this the same thing as just linking an external .o file using build.rs?

[lygstate]

@Amanieu it's different, cause we can creating the .o at the compile time without modifing the build.rs and
natrually platoform indepent.