diff --git a/compiler/rustc_const_eval/src/interpret/util.rs b/compiler/rustc_const_eval/src/interpret/util.rs
index eb0fdebb665fa..8685aba393fd1 100644
--- a/compiler/rustc_const_eval/src/interpret/util.rs
+++ b/compiler/rustc_const_eval/src/interpret/util.rs
@@ -43,22 +43,9 @@ where
                         let is_used = unused_params.contains(index).map_or(true, |unused| !unused);
                         // Only recurse when generic parameters in fns, closures and generators
                         // are used and require substitution.
-                        match (is_used, subst.definitely_needs_subst(self.tcx)) {
-                            // Just in case there are closures or generators within this subst,
-                            // recurse.
-                            (true, true) => return subst.super_visit_with(self),
-                            // Confirm that polymorphization replaced the parameter with
-                            // `ty::Param`/`ty::ConstKind::Param`.
-                            (false, true) if cfg!(debug_assertions) => match subst.unpack() {
-                                ty::subst::GenericArgKind::Type(ty) => {
-                                    assert!(matches!(ty.kind(), ty::Param(_)))
-                                }
-                                ty::subst::GenericArgKind::Const(ct) => {
-                                    assert!(matches!(ct.val, ty::ConstKind::Param(_)))
-                                }
-                                ty::subst::GenericArgKind::Lifetime(..) => (),
-                            },
-                            _ => {}
+                        // Just in case there are closures or generators within this subst, recurse.
+                        if is_used && subst.definitely_needs_subst(self.tcx) {
+                            return subst.super_visit_with(self);
                         }
                     }
                     ControlFlow::CONTINUE
diff --git a/compiler/rustc_mir_transform/src/inline.rs b/compiler/rustc_mir_transform/src/inline.rs
index d43528a1cf098..d88eac3cf15c6 100644
--- a/compiler/rustc_mir_transform/src/inline.rs
+++ b/compiler/rustc_mir_transform/src/inline.rs
@@ -43,7 +43,14 @@ crate fn is_enabled(tcx: TyCtxt<'_>) -> bool {
         return enabled;
     }
 
-    tcx.sess.mir_opt_level() >= 3
+    // If you change this optimization level, also change the level in
+    // `mir_drops_elaborated_and_const_checked` for the call to `mir_inliner_callees`.
+    // Otherwise you will get an ICE about stolen MIR.
+    match tcx.sess.mir_opt_level() {
+        0 | 1 => false,
+        2 => tcx.sess.opts.incremental == None,
+        _ => true,
+    }
 }
 
 impl<'tcx> MirPass<'tcx> for Inline {
@@ -325,6 +332,17 @@ impl Inliner<'tcx> {
             }
         }
 
+        // At mir-opt-level=1, only inline `#[inline(always)]` functions
+        if self.tcx.sess.mir_opt_level() == 1 && callee_attrs.inline != InlineAttr::Always {
+            return Err("at mir-opt-level=1, only inline(always) is inlined");
+        }
+
+        if self.tcx.sess.mir_opt_level() == 1
+            && self.tcx.sess.opts.optimize != rustc_session::config::OptLevel::Aggressive
+        {
+            return Err("at mir-opt-level=1, only inline if -O is specified");
+        }
+
         Ok(())
     }
 
@@ -470,8 +488,24 @@ impl Inliner<'tcx> {
         }
 
         if let InlineAttr::Always = callee_attrs.inline {
-            debug!("INLINING {:?} because inline(always) [cost={}]", callsite, cost);
-            Ok(())
+            if self.tcx.sess.mir_opt_level() == 1 {
+                if cost <= 25 {
+                    debug!(
+                        "INLINING {:?} because inline(always) and [cost={} <= threshold=25]",
+                        callsite, cost
+                    );
+                    Ok(())
+                } else {
+                    debug!(
+                        "NOT inlining {:?} because inline(always) but [cost={} > threshold=25]",
+                        callsite, cost
+                    );
+                    Err("cost above threshold")
+                }
+            } else {
+                debug!("INLINING {:?} because inline(always) [cost={}]", callsite, cost);
+                Ok(())
+            }
         } else {
             if cost <= threshold {
                 debug!("INLINING {:?} [cost={} <= threshold={}]", callsite, cost, threshold);