deprecate -Csoft-float because it is unsound (and not fixable)

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -185,7 +185,13 @@ pub(crate) fn target_machine_factory(
     let reloc_model = to_llvm_relocation_model(sess.relocation_model());
 
     let (opt_level, _) = to_llvm_opt_settings(optlvl);
-    let use_softfp = sess.opts.cg.soft_float;
+    let use_softfp = if sess.target.arch == "arm" && sess.target.abi == "eabihf" {
+        sess.opts.cg.soft_float
+    } else {
+        // `validate_commandline_args_with_session_available` has already warned about this being ignored.
+        // Let's make sure LLVM doesn't suddenly start using this flag on more targets.
+        false
+    };
 
     let ffunction_sections =
         sess.opts.unstable_opts.function_sections.unwrap_or(sess.target.function_sections);

compiler/rustc_session/messages.ftl

@@ -107,6 +107,16 @@ session_sanitizer_not_supported = {$us} sanitizer is not supported for this targ
 session_sanitizers_not_supported = {$us} sanitizers are not supported for this target
 
 session_skipping_const_checks = skipping const checks
+
+session_soft_float_deprecated =
+    `-Csoft-float` is unsound and deprecated; use a corresponding *eabi target instead
+    .note = it will be removed or ignored in a future version of Rust
+session_soft_float_deprecated_issue = see issue #129893 <https://github.com/rust-lang/rust/issues/129893> for more information
+
+session_soft_float_ignored =
+    `-Csoft-float` is ignored on this target; it only has an effect on *eabihf targets
+    .note = this may become a hard error in a future version of Rust
+
 session_split_debuginfo_unstable_platform = `-Csplit-debuginfo={$debuginfo}` is unstable on this platform
 
 session_split_lto_unit_requires_lto = `-Zsplit-lto-unit` requires `-Clto`, `-Clto=thin`, or `-Clinker-plugin-lto`

compiler/rustc_session/src/errors.rs

@@ -484,3 +484,14 @@ pub(crate) struct FunctionReturnThunkExternRequiresNonLargeCodeModel;
 pub(crate) struct FailedToCreateProfiler {
     pub(crate) err: String,
 }
+
+#[derive(Diagnostic)]
+#[diag(session_soft_float_ignored)]
+#[note]
+pub(crate) struct SoftFloatIgnored;
+
+#[derive(Diagnostic)]
+#[diag(session_soft_float_deprecated)]
+#[note]
+#[note(session_soft_float_deprecated_issue)]
+pub(crate) struct SoftFloatDeprecated;

compiler/rustc_session/src/options.rs

@@ -1615,7 +1615,7 @@ options! {
     save_temps: bool = (false, parse_bool, [UNTRACKED],
         "save all temporary output files during compilation (default: no)"),
     soft_float: bool = (false, parse_bool, [TRACKED],
-        "use soft float ABI (*eabihf targets only) (default: no)"),
+        "deprecated option: use soft float ABI (*eabihf targets only) (default: no)"),
     #[rustc_lint_opt_deny_field_access("use `Session::split_debuginfo` instead of this field")]
     split_debuginfo: Option<SplitDebuginfo> = (None, parse_split_debuginfo, [TRACKED],
         "how to handle split-debuginfo, a platform-specific option"),

compiler/rustc_session/src/session.rs

@@ -1340,6 +1340,16 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
             }
         }
     }
+
+    if sess.opts.cg.soft_float {
+        if sess.target.arch == "arm" && sess.target.abi == "eabihf" {
+            sess.dcx().emit_warn(errors::SoftFloatDeprecated);
+        } else {
+            // All `use_softfp` does is the equivalent of `-mfloat-abi` in GCC/clang, which only exists on ARM targets.
+            // We document this flag to only affect `*eabihf` targets, so let's show a warning for all other targets.
+            sess.dcx().emit_warn(errors::SoftFloatIgnored);
+        }
+    }
 }
 
 /// Holds data on the current incremental compilation session, if there is one.