expand: fix sequential '\' in double quote literals

[cfogrady]

Fix for #1106

[mvdan]

Thanks! I actually avoid unit tests where possible, and instead test the exposed API. In this case I would add integration tests in the interpreter, for example adding to these around line 400 in interp_test.go:

	// escaped chars
	{"echo a\\b", "ab\n"},
	{"echo a\\ b", "a b\n"},
	{"echo \\$a", "$a\n"},
	{"echo \"a\\b\"", "a\\b\n"},
	{"echo 'a\\b'", "a\\b\n"},
	{"echo \"a\\\nb\"", "ab\n"},
	{"echo 'a\\\nb'", "a\\\nb\n"},
	{`echo "\""`, "\"\n"},
	{`echo \\`, "\\\n"},
	{`echo \\\\`, "\\\\\n"},
	{`echo \`, "\n"},

I know that means there isn't test coverage in the same package, but I think proper coverage across the module is still fine. And, ultimately, most of these bugs in the expand package only matter for users of the interp package.

[mvdan]

And avoiding unit tests also means that you don't need to add one more function to test.

[cfogrady]

And avoiding unit tests also means that you don't need to add one more function to test.

For clarity, do you prefer to keep the logic inlined? My default is to split out what I would consider to be tangential logic because I think it helps readability, but you're the maintainer.

[mvdan]

Yes, please keep the code where it was - the func isn't particularly large or very indented, so I don't think it requires splitting up just yet.

[cfogrady]

Function is inlined once again, and the (in my opinion) essential test cases have been added to the interp_test.go test cases.

[mvdan]

Thank you! The change is subtle but I do see how my previous logic was wrong with repeated backslashes, as rather than treating them like escape pairs, I would keep on skipping over all of them.

[mvdan]

There was a minor gofmt issue which I fixed. I will squash-merge, as I prefer to keep the git history tidy. For future PRs, feel free to squash into a single commit yourself with a good commit message, and I'll merge as-is.

[cfogrady]

Sounds good!

[mvdan]

(as usual got sidetracked waiting for CI to pass, merging now)