Implement `asm!` macro

[CohenArthur]

There are a few things needed for proper inline assembly handling

• Create an AST node for inline assembly #1567
• Create an HIR node for inline assembly #1568
• Implement proper parsing of the asm string
• Implement conversion to gcc-like inline assembly

The HIR node will definitely help for the unsafe checks regarding asm!()

[liushuyu]

References:

• https://rustc-dev-guide.rust-lang.org/asm.html
• https://github.com/rust-lang/rust/blob/master/compiler/rustc_codegen_gcc/src/asm.rs
• https://github.com/rust-lang/rust/blob/master/compiler/rustc_builtin_macros/src/asm.rs (How Rustc parses the inline assembly)
• https://github.com/rust-lang/rust/blob/master/compiler/rustc_parse_format/src/lib.rs (Parsing format strings, which contains logic for parsing inline assembly syntax)

[mvvsmk]

@CohenArthur , I am trying to understand how macro parsing work in order to impliment
Implement proper parsing of the asm string could you give me some pointers as to which files I should take a look at ? and at which stage macro parsing begins, because as I understand it it happnes lazy-ly but at which stage?

[CohenArthur]

@mvvsmk take a look at gcc/rust/expand/rust-macro-builtins.cc. The tables at the top help understand the structure of builtin macro handlers, so for example for include_str we have the following entry:

    {"include_str", MacroBuiltin::include_str_handler},

meaning that the include_str_handler will be called upon seeing a call to include_str!(). If you have a look at the handler, it is responsible for parsing its input and transforming it:

tl::optional<AST::Fragment>
MacroBuiltin::include_str_handler (location_t invoc_locus,
				   AST::MacroInvocData &invoc)
{
  /* Get target filename from the macro invocation, which is treated as a path
     relative to the include!-ing file (currently being compiled).  */
  auto lit_expr
    = parse_single_string_literal (BuiltinMacro::IncludeStr,
				   invoc.get_delim_tok_tree (), invoc_locus,
				   invoc.get_expander ());
  if (lit_expr == nullptr)
    return AST::Fragment::create_error ();

so the goal would be to do the same for asm!() and implement proper parsing of the inline assembly format. at the moment this is the entry for asm!():

    {"asm", MacroBuiltin::sorry},

because it's not implemented yet and there is no handler for it