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.

1 files changed, 23 insertions, 1 deletions

diff --git a/compiler/rustc_mir_transform/src/validate.rs b/compiler/rustc_mir_transform/src/validate.rs
index a670da94fcc..9444c5b61dd 100644
--- a/compiler/rustc_mir_transform/src/validate.rs
+++ b/compiler/rustc_mir_transform/src/validate.rs
@@ -1,6 +1,7 @@
 //! Validates the MIR to ensure that invariants are upheld.
 
 use rustc_abi::{ExternAbi, FIRST_VARIANT, Size};
+use rustc_attr_parsing::InlineAttr;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir::LangItem;
 use rustc_index::IndexVec;
@@ -79,7 +80,7 @@ impl<'tcx> crate::MirPass<'tcx> for Validator {
             cfg_checker.fail(location, msg);
         }
 
-        if let MirPhase::Runtime(_) = body.phase {
+        if let MirPhase::Runtime(phase) = body.phase {
             if let ty::InstanceKind::Item(_) = body.source.instance {
                 if body.has_free_regions() {
                     cfg_checker.fail(
@@ -88,6 +89,27 @@ impl<'tcx> crate::MirPass<'tcx> for Validator {
                     );
                 }
             }
+
+            if phase >= RuntimePhase::Optimized
+                && body
+                    .basic_blocks
+                    .iter()
+                    .filter_map(|bb| match &bb.terminator().kind {
+                        TerminatorKind::Call { func, .. }
+                        | TerminatorKind::TailCall { func, .. } => Some(func),
+                        _ => None,
+                    })
+                    .filter_map(|func| match func.ty(&body.local_decls, tcx).kind() {
+                        ty::FnDef(did, ..) => Some(did),
+                        _ => None,
+                    })
+                    .any(|did| matches!(tcx.codegen_fn_attrs(did).inline, InlineAttr::Force { .. }))
+            {
+                cfg_checker.fail(
+                    Location::START,
+                    "`#[rustc_force_inline]`-annotated function not inlined",
+                );
+            }
         }
     }
 }