servo: Merge #15992 - Rewrite PropertyDeclaration::id to help the optimizer (from servo:id-table); r=bholley

If I’m reading the release-mode assembly correctly, before this change `PropertyDeclaration::id` is implemented with a computed jump:

```assembly
	lea	rcx, [rip + .LJTI117_0]
	movsxd	rax, dword ptr [rcx + 4*rax]
	add	rax, rcx
	jmp	rax
.LBB117_3:
	mov	dword ptr [rdi], 65536
	mov	rax, rdi
	ret
.LBB117_2:
	mov	dword ptr [rdi], 0
	mov	rax, rdi
	ret
.LBB117_4:
	mov	dword ptr [rdi], 131072
	mov	rax, rdi
	ret
.LBB117_6:
	mov	dword ptr [rdi], 262144
	mov	rax, rdi
	ret
.LBB117_7:
	mov	dword ptr [rdi], 327680
	mov	rax, rdi
	ret

; Four similar lines repeated for each of the few hundred variants...
```

With Rust 1.15 (currently used for geckolib) this doesn’t change significantly. In Nightly 1.17 however, the compiled code uses a lookup table, possibly thanks to rust-lang/rust#39456.

```assembly
	movq	(%rsi), %rax
	cmpq	$171, %rax
	jne	.LBB23_1
	addq	$8, %rsi
	movq	%rsi, 8(%rdi)
	movb	$1, %al
	jmp	.LBB23_3
.LBB23_1:
	xorq	$128, %rax
	leaq	.Lswitch.table.6(%rip), %rcx
	movb	(%rax,%rcx), %al
	movb	%al, 1(%rdi)
	xorl	%eax, %eax
.LBB23_3:
	movb	%al, (%rdi)
	movq	%rdi, %rax
	retq
```

Source-Repo: https://github.com/servo/servo
Source-Revision: 9e8e1a47241c6906b4f5777da0d04e3655982fae

UltraBlame original commit: af90f70b05df8c81f440b222728ae48c2e2bf5f8

Author: marco-c

servo/components/style/properties/properties.mako.rs

@@ -8208,6 +8208,69 @@ match
 *
 self
 {
+PropertyDeclaration
+:
+:
+Custom
+(
+ref
+name
+_
+)
+=
+>
+{
+return
+PropertyDeclarationId
+:
+:
+Custom
+(
+name
+)
+}
+PropertyDeclaration
+:
+:
+CSSWideKeyword
+(
+id
+_
+)
+|
+PropertyDeclaration
+:
+:
+WithVariables
+(
+id
+_
+)
+=
+>
+{
+return
+PropertyDeclarationId
+:
+:
+Longhand
+(
+id
+)
+}
+_
+=
+>
+{
+}
+}
+let
+longhand_id
+=
+match
+*
+self
+{
 %
 for
 property
@@ -8231,11 +8294,6 @@ camel_case
 =
 >
 {
-PropertyDeclarationId
-:
-:
-Longhand
-(
 LonghandId
 :
 :
@@ -8244,7 +8302,6 @@ property
 .
 camel_case
 }
-)
 }
 %
 endfor
@@ -8253,57 +8310,91 @@ PropertyDeclaration
 :
 CSSWideKeyword
 (
-id
-_
-)
-=
->
-PropertyDeclarationId
-:
-:
-Longhand
-(
-id
+.
+.
 )
+|
 PropertyDeclaration
 :
 :
 WithVariables
 (
-id
-_
-)
-=
->
-PropertyDeclarationId
-:
-:
-Longhand
-(
-id
+.
+.
 )
+|
 PropertyDeclaration
 :
 :
 Custom
 (
-ref
-name
-_
+.
+.
 )
 =
 >
 {
+debug_assert
+!
+(
+false
+"
+unreachable
+"
+)
+;
+/
+/
+This
+value
+is
+never
+used
+but
+having
+an
+expression
+of
+the
+same
+"
+shape
+"
+/
+/
+as
+for
+other
+variants
+helps
+the
+optimizer
+compile
+this
+match
+expression
+/
+/
+to
+a
+lookup
+table
+.
+LonghandId
+:
+:
+BackgroundColor
+}
+}
+;
 PropertyDeclarationId
 :
 :
-Custom
+Longhand
 (
-name
+longhand_id
 )
 }
-}
-}
 fn
 with_variables_from_shorthand
 (