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.

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -11,6 +11,22 @@ pub enum InlineAttr {
     Hint,
     Always,
     Never,
+    /// `#[rustc_force_inline]` forces inlining to happen in the MIR inliner - it reports an error
+    /// if the inlining cannot happen. It is limited to only free functions so that the calls
+    /// can always be resolved.
+    Force {
+        attr_span: Span,
+        reason: Option<Symbol>,
+    },
+}
+
+impl InlineAttr {
+    pub fn always(&self) -> bool {
+        match self {
+            InlineAttr::Always | InlineAttr::Force { .. } => true,
+            InlineAttr::None | InlineAttr::Hint | InlineAttr::Never => false,
+        }
+    }
 }
 
 #[derive(Clone, Encodable, Decodable, Debug, PartialEq, Eq, HashStable_Generic)]