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libexpr: deprecate the bogus "or"-as-variable

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As a prelude to making "or" work like a normal variable, emit a warning
any time the "fn or" production is used in a context that will change
how it is parsed when that production is refactored.

In detail: in the future, OR_KW will be moved to expr_simple, and the
cursed ExprCall production that is currently part of the expr_select
nonterminal will be generated "normally" in expr_app instead. Any
productions that accept an expr_select will be affected, except for the
expr_app nonterminal itself (because, while expr_app has a production
accepting a bare expr_select, its other production will continue to
accept "fn or" expressions). So all we need to do is emit an appropriate
warning when an expr_simple representing a cursed ExprCall is accepted
in one of those productions without first going through expr_app.

As the warning message describes, users can suppress the warning by
wrapping their problematic "fn or" expressions in parentheses. For
example, "f g or" can be made future-proof by rewriting it as
"f (g or)"; similarly "[ x y or ]" can be rewritten as "[ x (y or) ]",
etc. The parentheses preserve the current grouping behavior, as in the
future "f g or" will be parsed as "(f g) or", just like
"f g anything-else" is grouped. (Mechanically, this suppresses the
warning because the problem ExprCalls go through the
"expr_app : expr_select" production, which resets the cursed status on
the ExprCall.)
This commit is contained in:
Ryan Hendrickson 2024-09-20 15:57:36 -04:00
parent 68ba6ff470
commit da332d678e
6 changed files with 79 additions and 8 deletions
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28
src/libexpr/nixexpr.cc
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@ -663,4 +663,32 @@ std::string DocComment::getInnerText(const PosTable & positions) const {
return docStr;
}
/* ‘Cursed or’ handling.
*
* In parser.y, every use of expr_select in a production must call one of the
* two below functions.
*
* To be removed by https://github.com/NixOS/nix/pull/11121
*/
void ExprCall::resetCursedOr()
{
cursedOrEndPos.reset();
}
void ExprCall::warnIfCursedOr(const SymbolTable & symbols, const PosTable & positions)
{
if (cursedOrEndPos.has_value()) {
std::ostringstream out;
out << "at " << positions[pos] << ": "
"This expression uses `or` as an identifier in a way that will change in a future Nix release.\n"
"Wrap this entire expression in parentheses to preserve its current meaning:\n"
" (" << positions[pos].getSnippetUpTo(positions[*cursedOrEndPos]).value_or("could not read expression") << ")\n"
"Give feedback at https://github.com/NixOS/nix/pull/11121";
warn(out.str());
}
}
}