Merge #1092

1092: Add function to get nul-terminated strings from memory r=MarkMcCaskey a=srenatus

## Description

This is meant to fix #1086.

❓ I'm a bit new to this -- is this how you'd do it? Happy to take directions! 😃 

I've added a note regarding the special case in the comment.

Are there existing tests to expand?

## Review

- [x] Add a short description of the the change to the CHANGELOG.md file


Co-authored-by: Stephan Renatus <srenatus@chef.io>

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## **[Unreleased]**
 
+- [#1092](https://github.com/wasmerio/wasmer/pull/1092) Add `get_utf8_string_with_nul` to `WasmPtr` to read nul-terminated strings from memory.
 
 ## 0.12.0 - 2019-12-18
 

lib/runtime-core/src/memory/ptr.rs

@@ -137,6 +137,17 @@ impl<T: Copy + ValueType> WasmPtr<T, Array> {
         let slice: &[u8] = unsafe { std::slice::from_raw_parts(ptr, str_len as usize) };
         std::str::from_utf8(slice).ok()
     }
+
+    /// Get a UTF-8 string representation of this `WasmPtr`, where the string is nul-terminated.
+    /// Note that this does not account for UTF-8 strings that _contain_ nul themselves,
+    /// [`get_utf8_string`] has to be used for those.
+    pub fn get_utf8_string_with_nul<'a>(self, memory: &'a Memory) -> Option<&'a str> {
+        memory.view::<u8>()[(self.offset as usize)..]
+            .iter()
+            .map(|cell| cell.get())
+            .position(|byte| byte == 0)
+            .and_then(|length| self.get_utf8_string(memory, length as u32))
+    }
 }
 
 unsafe impl<T: Copy, Ty> WasmExternType for WasmPtr<T, Ty> {