Add more detail regarding i686 and FreeBSD version requirements

Author: asomers

src/doc/rustc/src/platform-support.md

@@ -100,7 +100,7 @@ target | notes
 [`riscv64gc-unknown-linux-gnu`](platform-support/riscv64gc-unknown-linux-gnu.md) | RISC-V Linux (kernel 4.20, glibc 2.29)
 [`riscv64gc-unknown-linux-musl`](platform-support/riscv64gc-unknown-linux-musl.md) | RISC-V Linux (kernel 4.20, musl 1.2.3)
 `s390x-unknown-linux-gnu` | S390x Linux (kernel 3.2, glibc 2.17)
-[`x86_64-unknown-freebsd`](platform-support/freebsd.md) | 64-bit FreeBSD
+[`x86_64-unknown-freebsd`](platform-support/freebsd.md) | 64-bit amd64 FreeBSD
 `x86_64-unknown-illumos` | illumos
 `x86_64-unknown-linux-musl` | 64-bit Linux with musl 1.2.3
 [`x86_64-unknown-netbsd`](platform-support/netbsd.md) | NetBSD/amd64
@@ -166,7 +166,7 @@ target | std | notes
 `i586-unknown-linux-musl` | ✓ | 32-bit Linux w/o SSE, musl 1.2.3 [^x86_32-floats-x87]
 [`i686-linux-android`](platform-support/android.md) | ✓ | 32-bit x86 Android [^x86_32-floats-return-ABI]
 [`i686-pc-windows-gnullvm`](platform-support/pc-windows-gnullvm.md) | ✓ | 32-bit x86 MinGW (Windows 10+), LLVM ABI [^x86_32-floats-return-ABI]
-[`i686-unknown-freebsd`](platform-support/freebsd.md) | ✓ | 32-bit FreeBSD [^x86_32-floats-return-ABI]
+[`i686-unknown-freebsd`](platform-support/freebsd.md) | ✓ | 32-bit x86 FreeBSD [^x86_32-floats-return-ABI]
 `i686-unknown-linux-musl` | ✓ | 32-bit Linux with musl 1.2.3 [^x86_32-floats-return-ABI]
 [`i686-unknown-uefi`](platform-support/unknown-uefi.md) | ? | 32-bit UEFI
 [`loongarch64-unknown-none`](platform-support/loongarch-none.md) | * | LoongArch64 Bare-metal (LP64D ABI)

src/doc/rustc/src/platform-support/freebsd.md

@@ -4,8 +4,6 @@
 
 [FreeBSD] multi-platform 4.4BSD-based UNIX-like operating system.
 
-[FreeBSD]: https://www.FreeBSD.org/
-
 ## Target maintainers
 
 - Alan Somers `asomers@FreeBSD.org`, https://github.com/asomers
@@ -17,23 +15,42 @@ The `x86_64-unknown-freebsd` target is Tier 2 with host tools.
 `i686-unknown-freebsd` is Tier 2 without host tools.  Other targets are Tier 3.
 See [platform-support.md](../platform-support.md) for the full list.
 
-On all architectures, rustc requires FreeBSD version 12 or later to run and
-produces binaries that require FreeBSD version 12, too.  Prior to Rust 1.78.0,
-rustc would run on FreeBSD 10 or later, but build binaries that required
-FreeBSD 11.  libc requires FreeBSD 11 or later.
+We commit that rustc will run on all currently supported releases of
+FreeBSD[supported-releases] .  EoL releases may be supported for a time, too.
+The same guarantees apply for the standard library and the libc crate.
+
+Specific release support matrix, as of Rust 1.82.0:
+
+| FreeBSD Release | rustc    | std      | libc    |
+| --------------- | -------- | -------- | ------- |
+| 10              | < 1.78.0 | ?        | ?       |
+| 11              | < 1.78.0 | < 1.78.0 | current |
+| 12+             | current  | current  | current |
 
-`extern "C"` uses the official calling convention of the respective architectures.
+`extern "C"` uses the official calling convention of the respective
+architectures.
 
 FreeBSD OS binaries use the ELF file format.
 
 ## Building Rust programs
 
-The `x86_64-unknown-freebsd` artifacts are distributed by the rust project and
-may be installed with rustup.  Other targets are built by the ports system and
-may be installed with [pkg(7)](https://man.freebsd.org/cgi/man.cgi?query=pkg)
-or [ports(7)](https://man.freebsd.org/cgi/man.cgi?query=ports).
+The `x86_64-unknown-freebsd` and `i686-unknown-freebsd` artifacts are
+distributed by the rust project and may be installed with rustup.  Other
+targets are built by the ports system and may be installed with
+[pkg(7)][pkg] or [ports(7)][ports].
+
+By default the `i686-unknown-freebsd` target uses SSE2 instructions.  To build
+code that does not require SSE2, build lang/rust from ports[ports] and disable the
+`SSE2` option at build time.  That will produce non-compliant behavior.  See
+[issue #114479][x86-32-float-issue].
 
 ## Testing
 
 The Rust test suite can be run natively. It can also be run from the ports tree
 with the `make test` command from within the lang/rust directory.
+
+[FreeBSD]: https://www.FreeBSD.org/
+[supported-releases]: https://www.freebsd.org/security/#sup
+[ports]: https://man.freebsd.org/cgi/man.cgi?query=ports
+[pkg]: https://man.freebsd.org/cgi/man.cgi?query=pkg
+[x86-32-float-issue]: https://github.com/rust-lang/rust/issues/114479