mir_transform: implement forced inlining

Adds `#[rustc_force_inline]` which is similar to always inlining but
reports an error if the inlining was not possible, and which always
attempts to inline annotated items, regardless of optimisation levels.
It can only be applied to free functions to guarantee that the MIR
inliner will be able to resolve calls.

2 files changed, 14 insertions, 4 deletions

diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
index d279590d45a..53300c95fa7 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
@@ -2294,7 +2294,7 @@ impl<'tcx> ObligationCause<'tcx> {
                 {
                     FailureCode::Error0644
                 }
-                TypeError::IntrinsicCast => FailureCode::Error0308,
+                TypeError::IntrinsicCast | TypeError::ForceInlineCast => FailureCode::Error0308,
                 _ => FailureCode::Error0308,
             },
         }
@@ -2360,8 +2360,11 @@ impl<'tcx> ObligationCause<'tcx> {
                 {
                     ObligationCauseFailureCode::ClosureSelfref { span }
                 }
+                TypeError::ForceInlineCast => {
+                    ObligationCauseFailureCode::CantCoerceForceInline { span, subdiags }
+                }
                 TypeError::IntrinsicCast => {
-                    ObligationCauseFailureCode::CantCoerce { span, subdiags }
+                    ObligationCauseFailureCode::CantCoerceIntrinsic { span, subdiags }
                 }
                 _ => ObligationCauseFailureCode::Generic { span, subdiags },
             },
diff --git a/compiler/rustc_trait_selection/src/errors.rs b/compiler/rustc_trait_selection/src/errors.rs
index a8fddff4e4a..53a4e5031c6 100644
--- a/compiler/rustc_trait_selection/src/errors.rs
+++ b/compiler/rustc_trait_selection/src/errors.rs
@@ -1729,8 +1729,15 @@ pub enum ObligationCauseFailureCode {
         #[primary_span]
         span: Span,
     },
-    #[diag(trait_selection_oc_cant_coerce, code = E0308)]
-    CantCoerce {
+    #[diag(trait_selection_oc_cant_coerce_force_inline, code = E0308)]
+    CantCoerceForceInline {
+        #[primary_span]
+        span: Span,
+        #[subdiagnostic]
+        subdiags: Vec<TypeErrorAdditionalDiags>,
+    },
+    #[diag(trait_selection_oc_cant_coerce_intrinsic, code = E0308)]
+    CantCoerceIntrinsic {
         #[primary_span]
         span: Span,
         #[subdiagnostic]