reduce default uninit_const_chunk_threshold to 16 (from 256)

Author: erikdesjardins

compiler/rustc_session/src/options.rs

@@ -1485,9 +1485,9 @@ options! {
         "in diagnostics, use heuristics to shorten paths referring to items"),
     ui_testing: bool = (false, parse_bool, [UNTRACKED],
         "emit compiler diagnostics in a form suitable for UI testing (default: no)"),
-    uninit_const_chunk_threshold: usize = (256, parse_number, [TRACKED],
+    uninit_const_chunk_threshold: usize = (16, parse_number, [TRACKED],
         "allow generating const initializers with mixed init/uninit chunks, \
-        and set the maximum number of chunks for which this is allowed (default: 256)"),
+        and set the maximum number of chunks for which this is allowed (default: 16)"),
     unleash_the_miri_inside_of_you: bool = (false, parse_bool, [TRACKED],
         "take the brakes off const evaluation. NOTE: this is unsound (default: no)"),
     unpretty: Option<String> = (None, parse_unpretty, [UNTRACKED],

src/test/codegen/uninit-consts.rs

@@ -15,8 +15,8 @@ pub struct PartiallyUninit {
 
 // CHECK: [[PARTIALLY_UNINIT:@[0-9]+]] = private unnamed_addr constant <{ [4 x i8], [12 x i8] }> <{ [4 x i8] c"\EF\BE\AD\DE", [12 x i8] undef }>, align 4
 
-// This shouldn't contain undef, since it contains more than 256 chunks
-// (the default value of uninit_const_chunk_threshold).
+// This shouldn't contain undef, since it contains more chunks
+// than the default value of uninit_const_chunk_threshold.
 // CHECK: [[UNINIT_PADDING_HUGE:@[0-9]+]] = private unnamed_addr constant <{ [32768 x i8] }> <{ [32768 x i8] c"{{.+}}" }>, align 4
 
 // CHECK: [[FULLY_UNINIT_HUGE:@[0-9]+]] = private unnamed_addr constant <{ [16384 x i8] }> undef