[ruby/prism] Convert start line to signed integers

Ruby allows for 0 or negative line start, this is often used
with `eval` calls to get a correct offset when prefixing a snippet.

e.g.

```ruby
caller = caller_locations(1, 1).first
class_eval <<~RUBY, caller.path, caller.line - 2
  # frozen_string_literal: true
  def some_method
    #{caller_provided_code_snippet}
  end
RUBY
```

https://github.com/ruby/prism/commit/0d14ed1452

2 files changed, 17 insertions, 0 deletions

diff --git a/prism/util/pm_buffer.c b/prism/util/pm_buffer.c
index dcdf1770bb..307b55d030 100644
--- a/prism/util/pm_buffer.c
+++ b/prism/util/pm_buffer.c
@@ -152,6 +152,15 @@ pm_buffer_append_varuint(pm_buffer_t *buffer, uint32_t value) {
 }
 
 /**
+ * Append a 32-bit signed integer to the buffer as a variable-length integer.
+ */
+void
+pm_buffer_append_varsint(pm_buffer_t *buffer, int32_t value) {
+    uint32_t unsigned_int = ((uint32_t)(value) << 1) ^ ((uint32_t)(value >> 31));
+    pm_buffer_append_varuint(buffer, unsigned_int);
+}
+
+/**
  * Concatenate one buffer onto another.
  */
 void
diff --git a/prism/util/pm_buffer.h b/prism/util/pm_buffer.h
index a8596be476..ec11d05e9b 100644
--- a/prism/util/pm_buffer.h
+++ b/prism/util/pm_buffer.h
@@ -121,6 +121,14 @@ void pm_buffer_append_byte(pm_buffer_t *buffer, uint8_t value);
 void pm_buffer_append_varuint(pm_buffer_t *buffer, uint32_t value);
 
 /**
+ * Append a 32-bit signed integer to the buffer as a variable-length integer.
+ *
+ * @param buffer The buffer to append to.
+ * @param value The integer to append.
+ */
+void pm_buffer_append_varsint(pm_buffer_t *buffer, int32_t value);
+
+/**
  * Concatenate one buffer onto another.
  *
  * @param destination The buffer to concatenate onto.