352921 - Label misplaced after hoisting with "L: for(var i = x in w)"

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Jesse Ruderman]

js> function() { L: for(let i = x in w) break L }
function () {
L:
    x;
    for (let i in w) {
        break L;
    }
}

(Is this the only construct that causes a statement to be hoisted, or are there others?)

Comment 1

[Jesse Ruderman]

The problem is more clear with "continue" than "break", because you can only continue to a loop.

Comment 2

[Brendan Eich [:brendan]]

Yes, only in the bizarre case of an initializer for a for-in variable do we hoist the initalizer (and the var; let must remain in the loop of course), to simplify the source annotations needed for decompilation.

This is a minor bug.

/be

Comment 3

[Jesse Ruderman]

Something similar happens with |var|:

js> function() { L: for(var i = x in w) break L } 
function () {
L:
    var i = x;
    for (i in w) {
        break L;
    }
}

Comment 4

[Ryan VanderMeulen [:RyanVM]]

Comment #0 now gives:
js> (function() { L: for(let i = x in w) break L })
typein:23: SyntaxError: for-in loop let declaration may not have an initializer:
typein:23: (function() { L: for(let i = x in w) break L })
typein:23: .........................^

Comment #3 still shows the bug.

Comment 5

[Luke Wagner [:luke]]

BytecodeEmitter.cpp: "'for (let x = i in o)' is mercifully banned."

Comment 6

[Jan de Mooij [:jandem]]

Fixed by saving the source (bug 761723).