rustc_target: Add target feature constraints for LoongArch

Part of #116344

Author: heiher

compiler/rustc_target/src/target_features.rs

@@ -923,6 +923,28 @@ impl Target {
                     _ => unreachable!(),
                 }
             }
+            "loongarch64" => {
+                // LoongArch handles ABI in a very sane way, being fully explicit via `llvm_abiname`
+                // about what the intended ABI is.
+                match &*self.llvm_abiname {
+                    "ilp32d" | "lp64d" => {
+                        // Requires d (which implies f), incompatible with nothing.
+                        FeatureConstraints { required: &["d"], incompatible: &[] }
+                    }
+                    "ilp32f" | "lp64f" => {
+                        // Requires f, incompatible with nothing.
+                        FeatureConstraints { required: &["f"], incompatible: &[] }
+                    }
+                    "ilp32s" | "lp64s" => {
+                        // The soft-float ABI does not require any features and is also not
+                        // incompatible with any features. Rust targets explicitly specify the
+                        // LLVM ABI names, which allows for enabling hard-float support even on
+                        // soft-float targets, and ensures that the ABI behavior is as expected.
+                        NOTHING
+                    }
+                    _ => unreachable!(),
+                }
+            }
             _ => NOTHING,
         }
     }

tests/ui/target-feature/forbidden-hardfloat-target-feature-flag-disable-d.rs

@@ -0,0 +1,12 @@
+//@ compile-flags: --target=loongarch64-unknown-linux-gnu --crate-type=lib
+//@ needs-llvm-components: loongarch
+//@ compile-flags: -Ctarget-feature=-d
+//@ min-llvm-version: 20
+// For now this is just a warning.
+//@ build-pass
+//@error-pattern: must be enabled to ensure that the ABI
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang = "sized"]
+pub trait Sized {}

tests/ui/target-feature/forbidden-hardfloat-target-feature-flag-disable-d.stderr

@@ -0,0 +1,13 @@
+warning: target feature `d` must be enabled to ensure that the ABI of the current target can be implemented correctly
+   |
+   = note: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>
+
+warning: unstable feature specified for `-Ctarget-feature`: `d`
+   |
+   = note: this feature is not stably supported; its behavior can change in the future
+
+warning: both target-abi and the triple-implied ABI are invalid, ignoring and using feature-implied ABI
+warning: 'lp64f' has not been standardized
+warning: 2 warnings emitted
+