Emit a lint for small functions without #[inline]

compiler/rustc_builtin_macros/src/deriving/debug.rs

@@ -33,7 +33,7 @@ pub fn expand_deriving_debug(
             explicit_self: true,
             nonself_args: vec![(fmtr, sym::f)],
             ret_ty: Path(path_std!(fmt::Result)),
-            attributes: ast::AttrVec::new(),
+            attributes: thin_vec![cx.attr_word(sym::inline, span)],
             fieldless_variants_strategy:
                 FieldlessVariantsStrategy::SpecializeIfAllVariantsFieldless,
             combine_substructure: combine_substructure(Box::new(|a, b, c| {

compiler/rustc_mir_transform/messages.ftl

@@ -42,6 +42,9 @@ mir_transform_requires_unsafe = {$details} is unsafe and requires unsafe {$op_in
     }
     .not_inherited = items do not inherit unsafety from separate enclosing items
 
+mir_transform_small_fn_without_inline = this function looks small ({$statements}) but doesn't have #[inline], consider adding it
+    .suggestion = add the inline attribute
+
 mir_transform_target_feature_call_label = call to function with `#[target_feature]`
 mir_transform_target_feature_call_note = can only be called if the required target features are available
 

compiler/rustc_mir_transform/src/cross_crate_inline.rs

@@ -55,14 +55,37 @@ fn cross_crate_inlinable(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
     }
 
     let mir = tcx.optimized_mir(def_id);
-    let mut checker =
-        CostChecker { tcx, callee_body: mir, calls: 0, statements: 0, landing_pads: 0, resumes: 0 };
+    let mut checker = CostChecker {
+        tcx,
+        callee_body: mir,
+        calls: 0,
+        statements: 0,
+        landing_pads: 0,
+        resumes: 0,
+        branches: 0,
+        asserts: 0,
+    };
     checker.visit_body(mir);
-    checker.calls == 0
+    let is_leaf = checker.calls == 0
         && checker.resumes == 0
         && checker.landing_pads == 0
         && checker.statements
-            <= tcx.sess.opts.unstable_opts.cross_crate_inline_threshold.unwrap_or(100)
+            <= tcx.sess.opts.unstable_opts.cross_crate_inline_threshold.unwrap_or(100);
+
+    let is_trivial_wrapper = checker.calls == 1
+        && checker.resumes == 0
+        && checker.landing_pads == 0
+        && mir.basic_blocks.len() == 2;
+
+    if is_trivial_wrapper {
+        let span = tcx.def_span(def_id);
+        tcx.sess.emit_warning(crate::errors::SuggestAddingInline {
+            place: span,
+            suggest_inline: span.with_hi(span.lo()),
+            statements: checker.statements,
+        });
+    }
+    is_leaf
 }
 
 struct CostChecker<'b, 'tcx> {
@@ -72,6 +95,8 @@ struct CostChecker<'b, 'tcx> {
     statements: usize,
     landing_pads: usize,
     resumes: usize,
+    branches: usize,
+    asserts: usize,
 }
 
 impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
@@ -105,7 +130,7 @@ impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
                 }
             }
             TerminatorKind::Assert { unwind, .. } => {
-                self.calls += 1;
+                self.asserts += 1;
                 if let UnwindAction::Cleanup(_) = unwind {
                     self.landing_pads += 1;
                 }
@@ -117,6 +142,10 @@ impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
                     self.landing_pads += 1;
                 }
             }
+            TerminatorKind::SwitchInt { .. } => {
+                self.statements += 1;
+                self.branches += 1;
+            }
             TerminatorKind::Return => {}
             _ => self.statements += 1,
         }

compiler/rustc_mir_transform/src/errors.rs

@@ -278,3 +278,13 @@ pub(crate) struct MustNotSuspendReason {
     pub span: Span,
     pub reason: String,
 }
+
+#[derive(Diagnostic)]
+#[diag(mir_transform_small_fn_without_inline)]
+pub struct SuggestAddingInline {
+    #[primary_span]
+    pub place: Span,
+    #[suggestion(code = "#[inline]\n", applicability = "machine-applicable")]
+    pub suggest_inline: Span,
+    pub statements: usize,
+}