1452818 - Eliminate the need for BytecodeCompiler::startPosition to be default-constructable

Status: RESOLVED FIXED

(Core :: JavaScript Engine, enhancement)

Description

[Jeff Walden [:Waldo]]

Some of the Unicode character-type work wants to be able to create character iterators that have Rust-like borrow-checking semantics.  Part of this requires that iterators always be initialized, which means they should never be default-constructed.

Unfortunately, TokenStreamCharsBase::Position *is* default-constructed now.  This can be fixed: move the single Position where this is necessary to a location where it can be constructed fully initialized, and then initialize those positions by directly constructing them from a |tokenStream|.

Comment 1

[Jeff Walden [:Waldo]]

Attachment: Move BytecodeCompiler::startPosition into more locally-scoped stack-frame variables

This doesn't really change anything, but it does get rid of a very small amount of useless code-execution when not compiling things that don't have handleParseFailure failure modes.

Comment 2

[Jeff Walden [:Waldo]]

Attachment: Make tokenstream positions be created by constructor-call so that their members don't have to be default-constructible

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/mozilla-inbound/rev/d21bb4d700d8
Move BytecodeCompiler::startPosition into more locally-scoped stack-frame variables.  r=arai
https://hg.mozilla.org/integration/mozilla-inbound/rev/4f2a7bac2733
Make tokenstream positions be created by constructor-call so that their members don't have to be default-constructible.  r=arai

Comment 4

[Andreea Pavel [:apavel]]

https://hg.mozilla.org/mozilla-central/rev/d21bb4d700d8
https://hg.mozilla.org/mozilla-central/rev/4f2a7bac2733

Comment 5

[Jeff Walden [:Waldo]]

Oh, for anyone wondering -- we have to have a |Position foo(...)| local declared that way, not a function that returns a Position, because returning a Position that way pre-C++17 is not guaranteed to not need to invoke a copy constructor or similar, and I may need to have that requirement for the work that motivated this.