show a warning when -Csoft-float is used on a non-eabihf target

compiler/rustc_codegen_llvm/messages.ftl

@@ -60,6 +60,9 @@ codegen_llvm_sanitizer_memtag_requires_mte =
 codegen_llvm_serialize_module = failed to serialize module {$name}
 codegen_llvm_serialize_module_with_llvm_err = failed to serialize module {$name}: {$llvm_err}
 
+codegen_llvm_soft_float_ignored =
+    `-Csoft-float` is ignored on this target; it only has an effect on `*eabihf` targets
+
 codegen_llvm_symbol_already_defined =
     symbol `{$symbol_name}` is already defined
 

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -35,7 +35,7 @@ use crate::back::profiling::{
     selfprofile_after_pass_callback, selfprofile_before_pass_callback, LlvmSelfProfiler,
 };
 use crate::errors::{
-    CopyBitcode, FromLlvmDiag, FromLlvmOptimizationDiag, LlvmError, UnknownCompression,
+    self, CopyBitcode, FromLlvmDiag, FromLlvmOptimizationDiag, LlvmError, UnknownCompression,
     WithLlvmError, WriteBytecode,
 };
 use crate::llvm::diagnostic::OptimizationDiagnosticKind;
@@ -185,7 +185,17 @@ 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 {
+        // 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.
+        if sess.opts.cg.soft_float {
+            sess.dcx().emit_warn(errors::SoftFloatIgnored);
+        }
+        // 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_codegen_llvm/src/errors.rs

@@ -231,3 +231,7 @@ pub(crate) struct InvalidTargetFeaturePrefix<'a> {
 pub(crate) struct FixedX18InvalidArch<'a> {
     pub arch: &'a str,
 }
+
+#[derive(Diagnostic)]
+#[diag(codegen_llvm_soft_float_ignored)]
+pub(crate) struct SoftFloatIgnored;