Sema: avoid error return traces when possible

stage2 was adding bogus error return trace frames when an error was not
being returned. This commit makes several improvements:

 * Make a runtime check if necessary to only emit a frame into the error
   return trace when an actual error is returned.
 * Use the `analyzeIsNonErrComptimeOnly` machinery to avoid runtime
   checks when it is compile-time-known that the value is an error, or a
   non-error.
 * Make std.builtin.returnError take a non-optional stack trace pointer.

closes #12174

1 files changed, 1 insertions, 2 deletions

diff --git a/lib/std/builtin.zig b/lib/std/builtin.zig
index 047c65439c..ef716c6972 100644
--- a/lib/std/builtin.zig
+++ b/lib/std/builtin.zig
@@ -867,10 +867,9 @@ pub fn panicOutOfBounds(index: usize, len: usize) noreturn {
     std.debug.panic("attempt to index out of bound: index {d}, len {d}", .{ index, len });
 }
 
-pub noinline fn returnError(maybe_st: ?*StackTrace) void {
+pub noinline fn returnError(st: *StackTrace) void {
     @setCold(true);
     @setRuntimeSafety(false);
-    const st = maybe_st orelse return;
     addErrRetTraceAddr(st, @returnAddress());
 }