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_monomorphize/src/collector.rs

@@ -207,6 +207,7 @@
 
 use std::path::PathBuf;
 
+use rustc_attr_parsing::InlineAttr;
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::sync::{LRef, MTLock, par_for_each_in};
 use rustc_data_structures::unord::{UnordMap, UnordSet};
@@ -959,6 +960,14 @@ fn should_codegen_locally<'tcx>(tcx: TyCtxtAt<'tcx>, instance: Instance<'tcx>) -
         return false;
     }
 
+    if tcx.def_kind(def_id).has_codegen_attrs()
+        && matches!(tcx.codegen_fn_attrs(def_id).inline, InlineAttr::Force { .. })
+    {
+        // `#[rustc_force_inline]` items should never be codegened. This should be caught by
+        // the MIR validator.
+        return false;
+    }
+
     if def_id.is_local() {
         // Local items cannot be referred to locally without monomorphizing them locally.
         return true;