Auto merge of #34550 - cynicaldevil:master, r=Manishearth

Added a pretty printer for &mut slices

Fixes #30232
I have added a test which checks for correctness in gdb, but I need some help to do the same for lldb.

r? @Manishearth

src/etc/debugger_pretty_printers_common.py

@@ -139,7 +139,7 @@ class Type(object):
             return TYPE_KIND_STR_SLICE
 
         # REGULAR SLICE
-        if (unqualified_type_name.startswith("&[") and
+        if (unqualified_type_name.startswith(("&[", "&mut [")) and
             unqualified_type_name.endswith("]") and
             self.__conforms_to_field_layout(SLICE_FIELD_NAMES)):
             return TYPE_KIND_SLICE

src/test/debuginfo/vec-slices.rs

@@ -53,6 +53,11 @@
 // gdb-command:print *((int64_t[2]*)('vec_slices::MUT_VECT_SLICE'.data_ptr))
 // gdb-check:$15 = {64, 65}
 
+//gdb-command:print mut_slice.length
+//gdb-check:$16 = 5
+//gdb-command:print *((int64_t[5]*)(mut_slice.data_ptr))
+//gdb-check:$17 = {1, 2, 3, 4, 5}
+
 
 // === LLDB TESTS ==================================================================================
 
@@ -106,6 +111,8 @@ fn main() {
         MUT_VECT_SLICE = VECT_SLICE;
     }
 
+    let mut_slice: &mut [i64] = &mut [1, 2, 3, 4, 5];
+
     zzz(); // #break
 }