inline at the callsite & warn when target features mismatch

Co-authored-by: Jamie Cunliffe <Jamie.Cunliffe@arm.com>

1 files changed, 54 insertions, 0 deletions

diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index 97aa1065967..ac646aba156 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/compiler/rustc_lint_defs/src/builtin.rs
@@ -5138,3 +5138,57 @@ declare_lint! {
     "detects tail calls of functions marked with `#[track_caller]`",
     @feature_gate = explicit_tail_calls;
 }
+declare_lint! {
+    /// The `inline_always_mismatching_target_features` lint will trigger when a
+    /// function with the `#[inline(always)]` and `#[target_feature(enable = "...")]`
+    /// attributes is called and cannot be inlined due to missing target features in the caller.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,ignore (fails on x86_64)
+    /// #[inline(always)]
+    /// #[target_feature(enable = "fp16")]
+    /// unsafe fn callee() {
+    ///     // operations using fp16 types
+    /// }
+    ///
+    /// // Caller does not enable the required target feature
+    /// fn caller() {
+    ///     unsafe { callee(); }
+    /// }
+    ///
+    /// fn main() {
+    ///     caller();
+    /// }
+    /// ```
+    ///
+    /// This will produce:
+    ///
+    /// ```text
+    /// warning: call to `#[inline(always)]`-annotated `callee` requires the same target features. Function will not have `alwaysinline` attribute applied
+    ///   --> $DIR/builtin.rs:5192:14
+    ///    |
+    /// 10 |     unsafe { callee(); }
+    ///    |              ^^^^^^^^
+    ///    |
+    /// note: `fp16` target feature enabled in `callee` here but missing from `caller`
+    ///   --> $DIR/builtin.rs:5185:1
+    ///    |
+    /// 3  | #[target_feature(enable = "fp16")]
+    ///    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    /// 4  | unsafe fn callee() {
+    ///    | ------------------
+    ///    = note: `#[warn(inline_always_mismatching_target_features)]` on by default
+    /// warning: 1 warning emitted
+    /// ```
+    ///
+    /// ### Explanation
+    ///
+    /// Inlining a function with a target feature attribute into a caller that
+    /// lacks the corresponding target feature can lead to unsound behavior.
+    /// LLVM may select the wrong instructions or registers, or reorder
+    /// operations, potentially resulting in runtime errors.
+    pub INLINE_ALWAYS_MISMATCHING_TARGET_FEATURES,
+    Warn,
+    r#"detects when a function annotated with `#[inline(always)]` and `#[target_feature(enable = "..")]` is inlined into a caller without the required target feature"#,
+}