build: Use C23 if possible

For broader compiler support, we try both -std=c23 and -std=c2x.  If
neither is supported, we fall back to -std=c17, but we will start
requiring some C23 features as extensions in this mode.

5 files changed, 23 insertions, 10 deletions

diff --git a/.github/workflows/codecov.yml b/.github/workflows/codecov.yml
index e4e8f71..7231aa0 100644
--- a/.github/workflows/codecov.yml
+++ b/.github/workflows/codecov.yml
@@ -24,7 +24,7 @@ jobs:
 
       - name: Generate coverage
         run: |
-          ./configure --enable-gcov
+          ./configure --enable-gcov EXTRA_CFLAGS="-std=gnu2x"
           make -j$(nproc) check TEST_FLAGS="--sudo"
           gcov -abcfpu obj/*/*.o
 
diff --git a/build/flags.mk b/build/flags.mk
index d6f9499..00876d1 100644
--- a/build/flags.mk
+++ b/build/flags.mk
@@ -8,7 +8,7 @@ include gen/vars.mk
 
 # Internal flags
 _CPPFLAGS := -Isrc -Igen -include src/prelude.h
-_CFLAGS := -std=c17
+_CFLAGS :=
 _LDFLAGS :=
 _LDLIBS :=
 
@@ -58,8 +58,7 @@ _CFLAGS += ${SAN_CFLAGS,${SAN}}
 YESLIBS := ${NOT,${_MSAN}${_TSAN}${_TYSAN}}
 NOLIBS ?= ${NOT,${YESLIBS}}
 
-# gcov only intercepts fork()/exec() with -std=gnu*
-GCOV_CFLAGS,y := -std=gnu17 --coverage
+GCOV_CFLAGS,y := --coverage
 _CFLAGS += ${GCOV_CFLAGS,${_GCOV}}
 
 LINT_CPPFLAGS,y := -D_FORTIFY_SOURCE=3 -DBFS_LINT
@@ -91,6 +90,10 @@ include build/exports.mk
 
 # Conditionally-supported flags
 AUTO_FLAGS := \
+    gen/flags/std.mk \
+    gen/flags/bind-now.mk \
+    gen/flags/deps.mk \
+    gen/flags/pthread.mk \
     gen/flags/Wformat.mk \
     gen/flags/Wimplicit-fallthrough.mk \
     gen/flags/Wimplicit.mk \
@@ -99,10 +102,7 @@ AUTO_FLAGS := \
     gen/flags/Wshadow.mk \
     gen/flags/Wsign-compare.mk \
     gen/flags/Wstrict-prototypes.mk \
-    gen/flags/Wundef-prefix.mk \
-    gen/flags/bind-now.mk \
-    gen/flags/deps.mk \
-    gen/flags/pthread.mk
+    gen/flags/Wundef-prefix.mk
 
 gen/flags.mk: ${AUTO_FLAGS}
 	${MSG} "[ GEN] $@"
diff --git a/build/flags/std.c b/build/flags/std.c
new file mode 100644
index 0000000..6030d1f
--- /dev/null
+++ b/build/flags/std.c
@@ -0,0 +1,12 @@
+// Copyright © Tavian Barnes <tavianator@tavianator.com>
+// SPDX-License-Identifier: 0BSD
+
+/// _CFLAGS += -std=c23
+/// ---
+/// _CFLAGS += -std=c2x
+/// ---
+/// _CFLAGS += -std=c17
+
+int main(void) {
+	return 0;
+}
diff --git a/docs/CONTRIBUTING.md b/docs/CONTRIBUTING.md
index 099157d..e411fde 100644
--- a/docs/CONTRIBUTING.md
+++ b/docs/CONTRIBUTING.md
@@ -17,7 +17,8 @@ This enables machine processing of license information based on the SPDX License
 Implementation
 --------------
 
-`bfs` is written in [C](https://en.wikipedia.org/wiki/C_(programming_language)), specifically [C17](https://en.wikipedia.org/wiki/C17_(C_standard_revision)).
+`bfs` is written in [C](https://en.wikipedia.org/wiki/C_(programming_language)).
+The build defaults to [C23](https://en.wikipedia.org/wiki/C23_(C_standard_revision)), but it also supports [C17](https://en.wikipedia.org/wiki/C17_(C_standard_revision)) as long as some popular C23 features are available as extensions.
 You can get a feel for the coding style by skimming the source code.
 [`main.c`](/src/main.c) contains an overview of the rest of source files.
 A quick summary:
diff --git a/src/diag.h b/src/diag.h
index 0a4d3b9..9f3e8e1 100644
--- a/src/diag.h
+++ b/src/diag.h
@@ -55,9 +55,9 @@ void bfs_diagf(const char *format, ...);
 /**
  * Print a message to standard error and abort.
  */
+_noreturn
 _cold
 _printf(1, 2)
-_noreturn
 void bfs_abortf(const char *format, ...);
 
 /**