diagnose "unexpected EOF" at EOF

this needs a string comparison because there seems to be no other way to
get that information out of bison. usually the location info is going to
be correct (pointing at a bad token), but since EOF isn't a token as
such it'll be wrong in that this case.

this hasn't shown up much so far because a single line ending *is* a
token, so any file formatted in the usual manner (ie, ending in a line
ending) would have its EOF position reported correctly.

src/libexpr/parser.y

@@ -64,6 +64,10 @@ using namespace nix;
 
 void yyerror(YYLTYPE * loc, yyscan_t scanner, ParserState * state, const char * error)
 {
+    if (std::string_view(error).starts_with("syntax error, unexpected end of file")) {
+        loc->first_column = loc->last_column;
+        loc->first_line = loc->last_line;
+    }
     throw ParseError({
         .msg = HintFmt(error),
         .pos = state->positions[state->at(*loc)]

tests/functional/lang/parse-fail-eof-in-string.err.exp

@@ -1,5 +1,5 @@
 error: syntax error, unexpected end of file, expecting '"'
-       at «stdin»:3:5:
+       at «stdin»:3:6:
             2| # Note that this file must not end with a newline.
             3| a 1"$
              |      ^

tests/functional/lang/parse-fail-eof-pos.err.exp

@@ -0,0 +1,5 @@
+error: syntax error, unexpected end of file
+       at «stdin»:3:1:
+            2| # no content
+            3|
+             | ^

tests/functional/lang/parse-fail-eof-pos.nix

@@ -0,0 +1,2 @@
+(
+# no content