1855819 - Eliminate the use of `let` and `const` bindings in self hosted code, and disallow them entirely

Status: RESOLVED FIXED

(Core :: JavaScript Engine, task, P2)

Description

[Matthew Gaudet (he/him) [:mgaudet]]

Since we don't want to pay TDZ costs in self hosted code where we simply should just do a better job, we should disallow let and var bindings in self hosted code.

Comment 1

[Matthew Gaudet (he/him) [:mgaudet]]

Attachment: Bug 1855819 - Emit syntax error on use of lexical variable types in self-hosted code r?arai

Comment 2

[Matthew Gaudet (he/him) [:mgaudet]]

Attachment: Bug 1855819 - Replace use of `let` and `const` in self-hosted code with `var` r?arai

Patch mechanically produced with regexp; changes to defines are coming from
prettier or eslint, whichever is auto-formatting JS.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/5152bad4c966
Replace use of `let` and `const` in self-hosted code with `var` r=arai
https://hg.mozilla.org/integration/autoland/rev/2351d7a1a27d
Emit syntax error on use of lexical variable types in self-hosted code r=arai

Comment 4

[Sandor Molnar[:smolnar]]

https://hg.mozilla.org/mozilla-central/rev/5152bad4c966
https://hg.mozilla.org/mozilla-central/rev/2351d7a1a27d

Comment 5

[Acasandrei Beatrice (needinfo me)]

(In reply to Pulsebot from comment #3)

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/5152bad4c966
Replace use of let and const in self-hosted code with var r=arai
https://hg.mozilla.org/integration/autoland/rev/2351d7a1a27d
Emit syntax error on use of lexical variable types in self-hosted code r=arai

== Change summary for alert #39865 (as of Mon, 09 Oct 2023 09:40:49 GMT) ==

Improvements:

Ratio	Test	Platform	Options	Absolute values (old vs new)
0.31%	Base Content JS	windows11-64-2009-shippable-qr	fission	1,579,648.00 -> 1,574,808.00
0.30%	Base Content JS	linux1804-64-shippable-qr	fission	1,577,462.67 -> 1,572,728.00
0.30%	Base Content JS	macosx1015-64-shippable-qr	fission	1,578,272.00 -> 1,573,544.00
0.28%	Base Content JS	linux1804-64-shippable-qr	fission	1,577,134.67 -> 1,572,728.00

For up to date results, see: https://treeherder.mozilla.org/perfherder/alerts?id=39865