testing: Testable examples collapse `Output` newlines, making them impossible to pass

[keilin-anz]

What version of Go are you using (go version)?

$ go version
go version go1.20.2 darwin/amd64

⚠️ This also appears to affect go1.19, though I believe it worked at some point in the past

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

❌ Not relevant, this happens in multiple environments including Go Playground with Go 1.19 and Go 1.20

What did you do?

1. Provide a testable example of a function which returns output containing successive newlines
2. Example cannot pass tests, since the Output annotation is modified by the testing parser

Minimal example

Viewable here: https://go.dev/play/p/LFktKVr75lz

✅ The ExampleFoo function returns data containing successive newlines:

Single newline:

Double newline:


Final text

✅ The ExampleFoo contains an embedded Output: annotation with the same expected output:

	//Output:
	//Single newline:
	//
	//Double newline:
	//
	//
	//Final text

✅ The Output and stdout are compared, with leading and trailing whitespace removed
❌ Multiple newlines are collapsed into single newlines, making the test fail - only solution is to modify the function under test (which is not an option)

What did you expect to see?

Output: should be parsed as-is, not modified (beyond trimming leading and trailing whitespace)

What did you see instead?

The multiple empty lines are collapsed into a single empty line

Additional context

Did a little hunting to see if I could find a root cause, no success yet

Currently suspect it's either hidden somewhere within the testing package, or is an unintended side effect of how chains of single line comments are parsed by ast

[keilin-anz]

Potential solutions

1. Make the testing library collapse newlines in both the stdout result (AKA got:) as well as the Output: comments (AKA want:)
2. Find root cause of the newline collapse and revert it
   1. Uncertain when this started happening, so unsure of the potential impact of the change

I'm guessing (1) has the least likelihood of breaking things for people, but it is still non-optimal - since it's not technically testing that output is correct (in some scenarios it's vitally important to be sure the output contains multiple newlines, so the test is effectively not a test.. whilst that clearly should be covered by unit tests - it's still odd to have a formalised test scenario which ignores the validity of output)

[seankhliao]

2 should be the way forward.
Maybe it's using the new structured doc parser that collapses newlines?

[heschi]

cc @bcmills

[bcmills]

The documented behavior is that “The comparison ignores leading and trailing space”, so collapsing interior newlines does seem incorrect.

However, the Output: parsing does at least need to account for different line endings on different platforms (\n vs. \r\n).

To minimize the number of existing examples that break, I think this should probably be gated on a GODEBUG setting (see #56986).

[gopherbot]

Change https://go.dev/cl/479119 mentions this issue: go/ast: allow Example output to not reduce empty lines