Match CharEscape discriminants to ESCAPE table entries

[dtolnay]

In agreement with #1273 (comment), this shows no discernible performance change on my machines, despite seeming like it might make a difference in each of the following conversions:

json/src/ser.rs

Lines 1770 to 1782 in 6843c36

	use self::CharEscape::*;
	let escape_char = match char_escape {
	Quote => b'"',
	ReverseSolidus => b'\\',
	Solidus => b'/',
	Backspace => b'b',
	FormFeed => b'f',
	LineFeed => b'n',
	CarriageReturn => b'r',
	Tab => b't',
	AsciiControl(_) => b'u',
	};

json/src/ser.rs

Lines 2111 to 2122 in 6843c36

	let char_escape = match escape {
	self::BB => CharEscape::Backspace,
	self::TT => CharEscape::Tab,
	self::NN => CharEscape::LineFeed,
	self::FF => CharEscape::FormFeed,
	self::RR => CharEscape::CarriageReturn,
	self::QU => CharEscape::Quote,
	self::BS => CharEscape::ReverseSolidus,
	self::UU => CharEscape::AsciiControl(byte),
	// Safety: the escape table does not contain any other type of character.
	_ => unsafe { hint::unreachable_unchecked() },
	};

Compiler Explorer

use core::hint;

pub enum CharEscape1 {
    Quote,
    ReverseSolidus,
    Solidus,
    Backspace,
    FormFeed,
    LineFeed,
    CarriageReturn,
    Tab,
    AsciiControl(u8),
}

#[repr(u8)]
pub enum CharEscape2 {
    Quote = b'"',
    ReverseSolidus = b'\\',
    Solidus = b'/',
    Backspace = b'b',
    FormFeed = b'f',
    LineFeed = b'n',
    CarriageReturn = b'r',
    Tab = b't',
    AsciiControl(u8) = b'u',
}

#[unsafe(no_mangle)]
pub fn charescape1_to_u8(char_escape: CharEscape1) -> u8 {
    match char_escape { 
        CharEscape1::Quote => b'"', 
        CharEscape1::ReverseSolidus => b'\\', 
        CharEscape1::Solidus => b'/', 
        CharEscape1::Backspace => b'b', 
        CharEscape1::FormFeed => b'f', 
        CharEscape1::LineFeed => b'n', 
        CharEscape1::CarriageReturn => b'r', 
        CharEscape1::Tab => b't', 
        CharEscape1::AsciiControl(_) => b'u', 
    }
}

#[unsafe(no_mangle)]
pub fn charescape2_to_u8(char_escape: CharEscape2) -> u8 {
    match char_escape { 
        CharEscape2::Quote => b'"', 
        CharEscape2::ReverseSolidus => b'\\', 
        CharEscape2::Solidus => b'/', 
        CharEscape2::Backspace => b'b', 
        CharEscape2::FormFeed => b'f', 
        CharEscape2::LineFeed => b'n', 
        CharEscape2::CarriageReturn => b'r', 
        CharEscape2::Tab => b't', 
        CharEscape2::AsciiControl(_) => b'u', 
    }
}

#[unsafe(no_mangle)]
pub unsafe fn u8_to_charescape1(escape: u8, byte: u8) -> CharEscape1 {
    match escape { 
        self::BB => CharEscape1::Backspace, 
        self::TT => CharEscape1::Tab, 
        self::NN => CharEscape1::LineFeed, 
        self::FF => CharEscape1::FormFeed, 
        self::RR => CharEscape1::CarriageReturn, 
        self::QU => CharEscape1::Quote, 
        self::BS => CharEscape1::ReverseSolidus, 
        self::UU => CharEscape1::AsciiControl(byte), 
        _ => unsafe { hint::unreachable_unchecked() }, 
    }
}

#[unsafe(no_mangle)]
pub unsafe fn u8_to_charescape2(escape: u8, byte: u8) -> CharEscape2 {
    match escape { 
        self::BB => CharEscape2::Backspace, 
        self::TT => CharEscape2::Tab, 
        self::NN => CharEscape2::LineFeed, 
        self::FF => CharEscape2::FormFeed, 
        self::RR => CharEscape2::CarriageReturn, 
        self::QU => CharEscape2::Quote, 
        self::BS => CharEscape2::ReverseSolidus, 
        self::UU => CharEscape2::AsciiControl(byte), 
        _ => unsafe { hint::unreachable_unchecked() }, 
    }
}

const BB: u8 = b'b';
const TT: u8 = b't';
const NN: u8 = b'n';
const FF: u8 = b'f';
const RR: u8 = b'r';
const QU: u8 = b'"';
const BS: u8 = b'\\';
const UU: u8 = b'u';

charescape1_to_u8:
        movzx   eax, dil
        lea     rcx, [rip + .Lswitch.table.charescape1_to_u8]
        movzx   eax, byte ptr [rax + rcx]
        ret

charescape2_to_u8:
        mov     eax, edi
        ret

u8_to_charescape1:
        mov     edx, esi
        movzx   eax, dil
        add     eax, -92
        cmp     eax, 25
        ja      .LBB2_8
        lea     rcx, [rip + .LJTI2_0]
        movsxd  rax, dword ptr [rcx + 4*rax]
        add     rax, rcx
        jmp     rax
.LBB2_9:
        mov     al, 1
        ret
.LBB2_5:
        mov     al, 5
        ret
.LBB2_8:
        xor     eax, eax
        ret
.LBB2_6:
        mov     al, 4
        ret
.LBB2_2:
        mov     al, 3
        ret
.LBB2_4:
        mov     al, 7
        ret
.LBB2_10:
        mov     al, 8
        ret
.LBB2_7:
        mov     al, 6
        ret
.LBB2_3:
        ud2
.LJTI2_0:
        .long   .LBB2_9-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_2-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_6-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_5-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_7-.LJTI2_0
        .long   .LBB2_3-.LJTI2_0
        .long   .LBB2_4-.LJTI2_0
        .long   .LBB2_10-.LJTI2_0

u8_to_charescape2:
        mov     edx, esi
        mov     eax, edi
        ret

.Lswitch.table.charescape1_to_u8:
        .ascii  "\"\\/bfnrtu"