Inline assembly: Clarify references to quoted regs

For `lateout` and similar operand direction specifiers, a quoted argument must be an explicit register name, per the EBNF grammar we give. As such, we should not use quotes when describing syntax used with register classes.

Further, correct the EBNF: in EBNF, quoted strings are always terminals, but `"<explicit register>"` is intended to be a nonterminal surrounded by literal (terminal) quotation marks. Moving the quotes to their own terminal symbols does require escaping, but makes the terminal/nonterminal distinction clearer. The only other place double-quotes are present in the grammar is inside `STRING_LITERAL`/`RAW_STRING_LITERAL`, so calling attention to this case is worthwhile for clarity.

Author: fw-immunant

src/inline-assembly.md

@@ -41,7 +41,7 @@ The following ABNF specifies the general syntax:
 ```text
 format_string := STRING_LITERAL / RAW_STRING_LITERAL
 dir_spec := "in" / "out" / "lateout" / "inout" / "inlateout"
-reg_spec := <register class> / "<explicit register>"
+reg_spec := <register class> / "\"" <explicit register> "\""
 operand_expr := expr / "_" / expr "=>" expr / expr "=>" "_"
 reg_operand := dir_spec "(" reg_spec ")" operand_expr
 operand := reg_operand
@@ -184,7 +184,7 @@ Here is the list of currently supported register classes:
 >
 > - On x86-64 the high byte registers (e.g. `ah`) are not available in the `reg_byte` register class.
 >
-> - Some register classes are marked as "Only clobbers" which means that they cannot be used for inputs or outputs, only clobbers of the form `out("reg") _` or `lateout("reg") _`.
+> - Some register classes are marked as "Only clobbers" which means that they cannot be used for inputs or outputs, only clobbers of the form `out(<register class>) _` or `lateout(<register class>) _`.
 
 Each register class has constraints on which value types they can be used with.
 This is necessary because the way a value is loaded into a register depends on its type.
@@ -356,7 +356,7 @@ If all references to an operand already have modifiers then the warning is suppr
 ## ABI clobbers
 
 The `clobber_abi` keyword can be used to apply a default set of clobbers to an `asm!` block.
-This will automatically insert the necessary clobber constraints as needed for calling a function with a particular calling convention: if the calling convention does not fully preserve the value of a register across a call then a `lateout("reg") _` is implicitly added to the operands list.
+This will automatically insert the necessary clobber constraints as needed for calling a function with a particular calling convention: if the calling convention does not fully preserve the value of a register across a call then `lateout("...") _` is implicitly added to the operands list (where the `...` is replaced by the register's name).
 
 `clobber_abi` may be specified any number of times. It will insert a clobber for all unique registers in the union of all specified calling conventions.