Fix xonsh integration breaking on backslash line-continuation

[anki-code]

Fixes #906

Summary

When the Coconut xontrib is autoloaded into xonsh and the user enters a multiline subprocess command joined with \<newline>, xonsh raises SyntaxError: unexpected indent on input that works fine without Coconut.

@ echo a \
    b
SyntaxError: code: b

Root cause

Compiler.reind_proc intentionally splits backslash-continued source into two physical lines and indents the continuation tail by tabideal spaces "for readability". For standalone Coconut output that's fine — Python tolerates it through implicit string/expression continuation. But the xonsh xontrib feeds the compiled output back through xonsh's own parser as top-level statements, where a complete statement followed by an unexpected INDENT is a SyntaxError. Concretely, echo a \<nl>b becomes echo a #1: echo a \<nl> b #1: echo a \<nl>, which neither xonsh nor CPython will parse.

I confirmed via tokenize.tokenize that this output is invalid CPython too — not just xonsh-specific:

TokenInfo(type=NAME, 'echo'), TokenInfo(NAME, 'a'), COMMENT, NEWLINE,
TokenInfo(INDENT, '    '), TokenInfo(NAME, 'b'), COMMENT, NEWLINE

The INDENT after a complete echo a statement is the bug.

Fix

Collapse \<newline><indent> into a single space before handing the source to parse_xonsh. With the line continuation already collapsed, the compiler emits one physical line on the other side and the host parser is happy.

The collapse is a small manual scanner — string literals and comments are skipped so backslash sequences inside them keep their original meaning. (A plain re.sub would silently rewrite "a\<nl>b", which is a Python implicit-string line continuation.)

Test plan

Reproduced + verified against xonsh-dev (Python 3.13, xonsh main, coconut 3.2.0 + this patch):

• @ echo a \⏎b → a b (was SyntaxError)
• @ print(range(5) |> list) → [0, 1, 2, 3, 4] (Coconut features unchanged)
• @ double = x -> x*2; print(double(5)) → 10
• @ print(sum <| [1,2,3]) → 6
• @ s = "a\⏎b"; print(repr(s)) → 'ab' (Python implicit-string continuation preserved)
• 14 unit-tests on _collapse_xonsh_line_continuations (no-op input, simple/multi continuations, strings/triple-quotes/comments preserved, trailing continuation, empty input)
• Existing test_xontrib scenarios still pass when run against the patched compile path (single-quoted continuation, semicolon chains, env-var f-strings, multi-line strings)

Compatibility

No core compiler changes — patch is contained to coconut/integrations.py (xonsh-specific module). Other Coconut endpoints (parse_sys, parse_eval, parse_file, …) and the rest of the runtime are untouched.

Reported by xonsh users at #906 (the linked xonsh issue is about a related but separate xonsh-side bug; this fixes a Coconut-specific path that surfaces only with the xontrib loaded).

[nahoj]

Works for me, thanks @anki-code :)

[jorel6666]

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[evhub]

This looks great! Can you rebase and retest on develop instead of master?

[anki-code]

@evhub Rebased onto develop (one commit, no merge conflicts — only coconut/integrations.py is touched, and develop made no changes there since v3.2.0). PR base updated to develop.

Retested against develop (Python 3.14, xonsh 0.23.3.dev12, cPyparsing 2.4.7.2.4.3):

• 14/14 unit tests on _collapse_xonsh_line_continuations pass (no-op, simple/multi continuations, single/double/triple-quoted strings preserved, comments preserved, escaped quotes, trailing continuation, mixed string + continuation, empty input)
• Tokenize check on the bug repro confirms 0 spurious INDENT tokens after collapse
• Compile-path scenarios driven directly through the xontrib's compile_code path: echo a \⏎ b, print(range(5) |> list), double = x -> x*2; print(double(5)), print(sum <| [1,2,3]), !(ls -la) |> bool, '1; 2' |> print, env-var assign, triple-quoted multi-line len, semicolon chains, echo abc; print(1 |> (.+1)), execx("10 |> print") — all parse to expected output, no SyntaxError

The patch logic is byte-identical to before the rebase.