Improve miri's error reporting in check_in_alloc

Author: LooMaclin

src/librustc/mir/interpret/allocation.rs

@@ -32,8 +32,10 @@ pub enum CheckInAllocMsg {
 }
 
 impl Display for CheckInAllocMsg {
+    /// When this printed as an error the context looks like this
+    /// "{test name} test failed: pointer must be in-bounds at offset..."
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{}", match *self {
+        write!(f, "{} test", match *self {
             CheckInAllocMsg::MemoryAccessTest => "Memory access",
             CheckInAllocMsg::NullPointerTest => "Null pointer",
             CheckInAllocMsg::PointerArithmeticTest => "Pointer arithmetic",

src/librustc/mir/interpret/error.rs

@@ -461,7 +461,7 @@ impl<'tcx, O: fmt::Debug> fmt::Debug for InterpError<'tcx, O> {
         use self::InterpError::*;
         match *self {
             PointerOutOfBounds { ptr, msg, allocation_size } => {
-                write!(f, "{} test failed: pointer must be in-bounds at offset {}, \
+                write!(f, "{} failed: pointer must be in-bounds at offset {}, \
                           but is outside bounds of allocation {} which has size {}",
                     msg, ptr.offset.bytes(), ptr.alloc_id, allocation_size.bytes())
             },