Replace usage of std::intrinsics::discriminant_value with `match`

[SimonSapin]

Replace usage of std::intrinsics::discriminant_value with per-enum generated code that uses match expressions. The LLVM IR shows that this optimizes well in release mode.

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix part of #11815.

• There are tests for these changes OR
• These changes do not require new tests because they shouldn’t change any behavior.

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This change is

[highfive]

Heads up! This PR modifies the following files:

• @bholley: components/style/properties/properties.mako.rs

[highfive]

Warning

• These commits modify style code, but no tests are modified. Please consider adding a test!

[SimonSapin]

@bors-servo try

[bors-servo]

⌛ Trying commit 5fe3e85 with merge fdbcd64...

[SimonSapin]

Test crate:

#![feature(core_intrinsics)]

extern crate style;

use std::intrinsics::discriminant_value;

pub fn with_match(decl: &style::properties::PropertyDeclaration) -> usize {
    decl.discriminant_value()
}

pub fn with_intrinsic(decl: &style::properties::PropertyDeclaration) -> usize {
    unsafe { discriminant_value(decl) as usize }
}

cargo rustc -- --emit=llvm-ir produces a .ll file that has 6843 lines.

cargo rustc --release -- --emit=llvm-ir produces this :)

; ModuleID = 'style_stable.0.rs'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

%"11.style::properties::PropertyDeclaration" = type { i64, [0 x i64], [60 x i64] }

; Function Attrs: norecurse nounwind readonly uwtable
define i64 @_ZN12style_stable10with_match17h5ee51730fa1d4ed4E(%"11.style::properties::PropertyDeclaration"* noalias nocapture readonly dereferenceable(488)) unnamed_addr #0 {
entry-block:
  %.idx = getelementptr %"11.style::properties::PropertyDeclaration", %"11.style::properties::PropertyDeclaration"* %0, i64 0, i32 0
  %.idx.val = load i64, i64* %.idx, align 8
  ret i64 %.idx.val
}

; Function Attrs: norecurse nounwind readonly uwtable
define i64 @_ZN12style_stable14with_intrinsic17h38553922905ee296E(%"11.style::properties::PropertyDeclaration"* noalias nocapture readonly dereferenceable(488)) unnamed_addr #0 {
entry-block:
  %1 = getelementptr inbounds %"11.style::properties::PropertyDeclaration", %"11.style::properties::PropertyDeclaration"* %0, i64 0, i32 0
  %2 = load i64, i64* %1, align 8, !range !0
  ret i64 %2
}

attributes #0 = { norecurse nounwind readonly uwtable }

!0 = !{i64 0, i64 129}

[bors-servo]

☀️ Test successful - android, arm32, arm64, linux-dev, linux-rel, mac-dev-unit, mac-rel-css, mac-rel-wpt, windows

[SimonSapin]

r? @bholley

[bors-servo]

☔ The latest upstream changes (presumably #11834) made this pull request unmergeable. Please resolve the merge conflicts.

[nox]

@bors-servo r+

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Reviewed 2 of 2 files at r1, 1 of 1 files at r2, 2 of 2 files at r3, 2 of 2 files at r4, 2 of 2 files at r5.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[bors-servo]

📌 Commit 3932a82 has been approved by nox

[bors-servo]

⌛ Testing commit 3932a82 with merge 810735a...

[bors-servo]

☀️ Test successful - android, arm32, arm64, linux-dev, linux-rel, mac-dev-unit, mac-rel-css, mac-rel-wpt, windows