Do not clean (in buildworld/buildkernel) by default

As discussed on the freebsd-arch mailing list[1].  For historical
reasons FreeBSD's buildworld and buildkernel targets started by cleaning
the object tree, for traditional (non-metamode) builds.

Cleaning is not necessary when dependencies are properly tracked, and we
have a somewhat kludgey script[2] to handle some known cases where deps
were mishandled by traditional builds.  Be consistent with the vast
majority of open source build systems by default, and do not clean at
the beginning of buildworld or buildkernel.

Users may set WITH_CLEAN in src.conf(5) to restore the previous
behaviour, or run `make cleanworld` and/or `make cleankernel` before
starting a build.

[1] https://lists.freebsd.org/archives/freebsd-arch/2024-July/000727.html
[2] tools/build/depend-cleanup.sh

Reviewed by:	jhb, imp
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46172

UPDATING

@@ -27,6 +27,16 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 15.x IS SLOW:
 	world, or to merely disable the most expensive debugging functionality
 	at runtime, run "ln -s 'abort:false,junk:false' /etc/malloc.conf".)
 
+20240729:
+	The build now defaults to WITHOUT_CLEAN - i.e., no automatic clean
+	is performed at the beginning of buildworld or buildkernel.  The
+	WITH_CLEAN src.conf(5) knob can be used to restore the previous
+	behaviour.
+
+	If you encounter incremental build issues, please report them to the
+	freebsd-current mailing list so that a special-case dependency can be
+	added, if necessary.
+
 20240712:
 	Support for armv6 has been disconnected and is being removed.
 

share/man/man5/src.conf.5

@@ -1,5 +1,5 @@
 .\" DO NOT EDIT-- this file is @generated by tools/build/options/makeman.
-.Dd May 22, 2024
+.Dd July 29, 2024
 .Dt SRC.CONF 5
 .Os
 .Sh NAME
@@ -383,8 +383,8 @@ Build clang-format.
 .It Va WITHOUT_CLANG_FULL
 Avoid building the ARCMigrate, Rewriter and StaticAnalyzer components of
 the Clang C/C++ compiler.
-.It Va WITHOUT_CLEAN
-Do not clean before building world and/or kernel.
+.It Va WITH_CLEAN
+Clean before building world and/or kernel.
 .It Va WITHOUT_CPP
 Do not build
 .Xr cpp 1 .

share/mk/src.opts.mk

@@ -80,7 +80,6 @@ __DEFAULT_YES_OPTIONS = \
     CDDL \
     CLANG \
     CLANG_BOOTSTRAP \
-    CLEAN \
     CPP \
     CROSS_COMPILER \
     CRYPT \
@@ -195,6 +194,7 @@ __DEFAULT_NO_OPTIONS = \
     BHYVE_SNAPSHOT \
     CLANG_EXTRAS \
     CLANG_FORMAT \
+    CLEAN \
     DIALOG \
     DETECT_TZ_CHANGES \
     DISK_IMAGE_TOOLS_BOOTSTRAP \

tools/build/options/WITH_CLEAN

@@ -0,0 +1 @@
+Clean before building world and/or kernel.