MIR inliner maintains unused var_debug_info

compiler/rustc_mir_transform/src/inline.rs

@@ -15,7 +15,7 @@ use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrs;
 use rustc_middle::mir::visit::*;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, Instance, InstanceKind, Ty, TyCtxt, TypeFlags, TypeVisitableExt};
-use rustc_session::config::{DebugInfo, OptLevel};
+use rustc_session::config::OptLevel;
 use rustc_span::source_map::Spanned;
 use tracing::{debug, instrument, trace, trace_span};
 
@@ -982,16 +982,7 @@ fn inline_call<'tcx, I: Inliner<'tcx>>(
     // Insert all of the (mapped) parts of the callee body into the caller.
     caller_body.local_decls.extend(callee_body.drain_vars_and_temps());
     caller_body.source_scopes.append(&mut callee_body.source_scopes);
-    if tcx
-        .sess
-        .opts
-        .unstable_opts
-        .inline_mir_preserve_debug
-        .unwrap_or(tcx.sess.opts.debuginfo != DebugInfo::None)
-    {
-        // Note that we need to preserve these in the standard library so that
-        // people working on rust can build with or without debuginfo while
-        // still getting consistent results from the mir-opt tests.
+    if tcx.sess.keep_var_debug_info() {
         caller_body.var_debug_info.append(&mut callee_body.var_debug_info);
     }
     caller_body.basic_blocks_mut().append(callee_body.basic_blocks_mut());

compiler/rustc_session/src/session.rs

@@ -765,6 +765,18 @@ impl Session {
             .unwrap_or(self.target.default_dwarf_version)
     }
 
+    /// Whether to maintain `Body.var_debug_info` that keeps track where variables were declared
+    pub fn keep_var_debug_info(&self) -> bool {
+        // -Zinline-mir-preserve-debug is enabled when building the standard library, so that
+        // people working on rust can build with or without debuginfo while
+        // still getting consistent results from the mir-opt tests.
+        self.opts
+            .unstable_opts
+            .inline_mir_preserve_debug
+            // only "full" promises any variable-level information
+            .unwrap_or(self.opts.debuginfo == DebugInfo::Full)
+    }
+
     pub fn stack_protector(&self) -> StackProtector {
         if self.target.options.supports_stack_protector {
             self.opts.unstable_opts.stack_protector

tests/mir-opt/inline_var_debug_info_kept.rs

@@ -0,0 +1,50 @@
+//@ test-mir-pass: Inline
+//@ revisions: PRESERVE FULL NONE LIMITED
+//@ [PRESERVE]compile-flags: -O -C debuginfo=0 -Zinline-mir-preserve-debug
+//@ [FULL]compile-flags: -O -C debuginfo=2
+//@ [NONE]compile-flags: -O -C debuginfo=0
+//@ [LIMITED]compile-flags: -O -C debuginfo=1
+
+#[inline(always)]
+fn inline_fn1(arg1: i32) -> i32 {
+    let local1 = arg1 + 1;
+    let _local2 = 10;
+    arg1 + local1
+}
+
+#[inline(always)]
+fn inline_fn2(binding: i32) -> i32 {
+    {
+        let binding = inline_fn1(binding);
+        binding
+    }
+}
+
+#[inline(never)]
+fn test() -> i32 {
+    // CHECK-LABEL: fn test
+    inline_fn2(1)
+    // CHECK-LABEL: (inlined inline_fn2)
+
+    // PRESERVE: debug binding =>
+    // FULL: debug binding =>
+    // NONE-NOT: debug binding =>
+    // LIMITED-NOT: debug binding =>
+
+    // CHECK-LABEL: (inlined inline_fn1)
+
+    // PRESERVE: debug arg1 =>
+    // FULL: debug arg1 =>
+    // NONE-NOT: debug arg1 =>
+    // LIMITED-NOT: debug arg1 =>
+
+    // PRESERVE: debug local1 =>
+    // FULL: debug local1 =>
+    // NONE-NOT: debug local1 =>
+    // LIMITED-NOT: debug local1 =>
+
+    // PRESERVE: debug _local2 =>
+    // FULL: debug _local2 =>
+    // NONE-NOT: debug _local2 =>
+    // LIMITED-NOT: debug _local2 =>
+}