Rollup merge of rust-lang#58629 - euclio:debug-empty-str, r=alexcrichton

rust-lldb: fix crash when printing empty string

Fixes rust-lang#52185.

~Re-enables the pretty-std debuginfo test and tweaks the test as necessary to get it to pass again. This reveals that lldb's formatting of enums is broken (rust-lang#58492). I also removed the emoji from the test because I couldn't get the docker image's gdb to print the emoji, just octal escapes (https://github.com/rust-lang/rust/pull/53154/files#r208263904).~

src/etc/lldb_rust_formatters.py

@@ -290,6 +290,8 @@ def render_element(i):
 
 
 def read_utf8_string(ptr_val, byte_count):
+    if byte_count == 0:
+        return '""'
     error = lldb.SBError()
     process = ptr_val.get_wrapped_value().GetProcess()
     data = process.ReadMemory(ptr_val.as_integer(), byte_count, error)

src/test/debuginfo/empty-string.rs

@@ -0,0 +1,33 @@
+// compile-flags:-g
+// min-gdb-version: 7.7
+// min-lldb-version: 310
+
+// === GDB TESTS ===================================================================================
+
+// gdb-command: run
+
+// gdb-command: print empty_string
+// gdb-check:$1 = ""
+
+// gdb-command: print empty_str
+// gdb-check:$2 = ""
+
+// === LLDB TESTS ==================================================================================
+
+// lldb-command: run
+
+// lldb-command: fr v empty_string
+// lldb-check:[...]empty_string = ""
+
+// lldb-command: fr v empty_str
+// lldb-check:[...]empty_str = ""
+
+fn main() {
+    let empty_string = String::new();
+
+    let empty_str = "";
+
+    zzz(); // #break
+}
+
+fn zzz() {}