toolchain/58728: lint objects to __SHIFTIN/__BITS of values in known-limited range without cast

[campbell+netbsd%mumble.net@localhost]

>Number:         58728
>Category:       toolchain
>Synopsis:       lint objects to __SHIFTIN/__BITS of values in known-limited range without cast
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    toolchain-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Oct 08 13:05:01 +0000 2024
>Originator:     Taylor R Campbell
>Release:        current
>Organization:
The LintBSD Shiftination
>Environment:
>Description:
    178 		const char16_t w1 = S->surrogate;
    179 		const char16_t w2 = c16;
    180 		c32 = (char32_t)(
    181 		    __SHIFTIN(__SHIFTOUT(w1, __BITS(9,0)), __BITS(19,10)) |
    182 		    __SHIFTIN(__SHIFTOUT(w2, __BITS(9,0)), __BITS(9,0)));

https://nxr.netbsd.org/xref/src/lib/libc/locale/c16rtomb.c?r=1.7#178

Without this (char32_t) cast, lint objects:

/home/riastradh/netbsd/current/src/lib/libc/locale/c16rtomb.c(182): warning: conversion from 'unsigned long' to 'unsigned int' may lose accuracy [132]

But it should be statically obvious from a local range analysis that truncation is impossible here, so the cast is unnecessary.
>How-To-Repeat:
omit needless cast in lib/libc/locale/c16rtomb.c, make -C lib/libc lint
>Fix:
Yes, please!