Fix error messages for unterminated ( and {

If we hit an EOF token, and the character before the EOF is a newline,
we should make EOF token start at the previous newline.  That way any
errors reported will occur on that line.

For example "foo(\n" should report an error on line 1 even though the
EOF technically occurs on line 2.

[Bug #20918]
https://bugs.ruby-lang.org/issues/20918

Author: tenderlove

src/prism.c

@@ -10700,6 +10700,14 @@ parser_lex(pm_parser_t *parser) {
             // We'll check if we're at the end of the file. If we are, then we
             // need to return the EOF token.
             if (parser->current.end >= parser->end) {
+                // If we hit EOF, but the EOF came immediately after a newline,
+                // set the start of the token to the newline.  This way any EOF
+                // errors will be reported as happening on that line rather than
+                // a line after.  For example "foo(\n" should report an error
+                // on line 1 even though EOF technically occurs on line 2.
+                if (parser->current.start > parser->start && (*(parser->current.start - 1) == '\n')) {
+                    parser->current.start -= 1;
+                }
                 LEX(PM_TOKEN_EOF);
             }
 

test/prism/errors/unterminated_block.txt

@@ -0,0 +1,4 @@
+foo {
+     ^ unexpected end-of-input, assuming it is closing the parent top level context
+     ^ expected a block beginning with `{` to end with `}`
+

test/prism/errors/unterminated_method_parameters.txt

@@ -0,0 +1,3 @@
+foo(
+    ^ unexpected end-of-input; expected a `)` to close the arguments
+

test/prism/errors_test.rb

@@ -32,6 +32,17 @@ class ErrorsTest < TestCase
       end
     end
 
+    def test_newline_preceding_eof
+      err = Prism.parse("foo(").errors.first
+      assert_equal 1, err.location.start_line
+
+      err = Prism.parse("foo(\n").errors.first
+      assert_equal 1, err.location.start_line
+
+      err = Prism.parse("foo(\n\n\n\n\n").errors.first
+      assert_equal 5, err.location.start_line
+    end
+
     def test_embdoc_ending
       source = <<~RUBY
         =begin\n=end