core: add unstable no_fp_fmt_parse to disable float fmt/parse code

In some projects (e.g. kernel), floating point is forbidden. They can disable
hardware floating point support and use `+soft-float` to avoid fp instructions
from being generated, but as libcore contains the formatting code for `f32`
and `f64`, some fp intrinsics are depended. One could define stubs for these
intrinsics that just panic [1], but it means that if any formatting functions
are accidentally used, mistake can only be caught during the runtime rather
than during compile-time or link-time, and they consume a lot of space without
LTO.

This patch provides an unstable cfg `no_fp_fmt_parse` to disable these.
A panicking stub is still provided for the `Debug` implementation (unfortunately)
because there are some SIMD types that use `#[derive(Debug)]`.

[1]: https://lkml.org/lkml/2021/4/14/1028

library/core/src/fmt/mod.rs

@@ -13,7 +13,10 @@ use crate::result;
 use crate::str;
 
 mod builders;
+#[cfg(not(no_fp_fmt_parse))]
 mod float;
+#[cfg(no_fp_fmt_parse)]
+mod nofloat;
 mod num;
 
 #[stable(feature = "fmt_flags_align", since = "1.28.0")]

library/core/src/fmt/nofloat.rs

@@ -0,0 +1,15 @@
+use crate::fmt::{Debug, Formatter, Result};
+
+macro_rules! floating {
+    ($ty:ident) => {
+        #[stable(feature = "rust1", since = "1.0.0")]
+        impl Debug for $ty {
+            fn fmt(&self, _fmt: &mut Formatter<'_>) -> Result {
+                panic!("floating point support is turned off");
+            }
+        }
+    };
+}
+
+floating! { f32 }
+floating! { f64 }

library/core/src/num/mod.rs

@@ -25,9 +25,13 @@ macro_rules! unlikely {
 }
 
 // All these modules are technically private and only exposed for coretests:
+#[cfg(not(no_fp_fmt_parse))]
 pub mod bignum;
+#[cfg(not(no_fp_fmt_parse))]
 pub mod dec2flt;
+#[cfg(not(no_fp_fmt_parse))]
 pub mod diy_float;
+#[cfg(not(no_fp_fmt_parse))]
 pub mod flt2dec;
 pub mod fmt;
 
@@ -44,6 +48,7 @@ mod wrapping;
 pub use wrapping::Wrapping;
 
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg(not(no_fp_fmt_parse))]
 pub use dec2flt::ParseFloatError;
 
 #[stable(feature = "rust1", since = "1.0.0")]

src/test/run-make-fulldeps/core-no-fp-fmt-parse/Makefile

@@ -0,0 +1,4 @@
+-include ../tools.mk
+
+all:
+	$(RUSTC) --edition=2018 --crate-type=rlib ../../../../library/core/src/lib.rs --cfg no_fp_fmt_parse