Auto merge of rust-lang#116810 - cjgillot:mir-opt-check, r=oli-obk

Add FileCheck annotations to mir-opt tests. This PR makes compiletest run LLVM `FileCheck` tool on mir-opt tests. The check is *run by default*, except if disabled using `// skip-filecheck` comment. This ensures that we do not have a silently broken test. For now, the check is only run on the output of `--emit=mir`, ie. on PreCodegen MIR. I give an example on `reference_prop.rs`. r? `@oli-obk` cc `@RalfJung` Fixes rust-lang#85180
rust-lang-ci · Oct 19, 2023 · e76cb8c · e76cb8c
2 parents c104861 + 328192b
commit e76cb8c
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diff --git a/src/tools/compiletest/src/runtest.rs b/src/tools/compiletest/src/runtest.rs
@@ -15,6 +15,7 @@ use crate::json;
 use crate::read2::{read2_abbreviated, Truncated};
 use crate::util::{add_dylib_path, dylib_env_var, logv, PathBufExt};
 use crate::ColorConfig;
+use miropt_test_tools::{files_for_miropt_test, MiroptTest, MiroptTestFile};
 use regex::{Captures, Regex};
 use rustfix::{apply_suggestions, get_suggestions_from_json, Filter};
 
@@ -229,6 +230,7 @@ enum Emit {
     None,
     Metadata,
     LlvmIr,
+    Mir,
     Asm,
     LinkArgsAsm,
 }
@@ -2506,6 +2508,9 @@ impl<'test> TestCx<'test> {
             Emit::LlvmIr => {
                 rustc.args(&["--emit", "llvm-ir"]);
             }
+            Emit::Mir => {
+                rustc.args(&["--emit", "mir"]);
+            }
             Emit::Asm => {
                 rustc.args(&["--emit", "asm"]);
             }
@@ -3984,11 +3989,17 @@ impl<'test> TestCx<'test> {
     fn run_mir_opt_test(&self) {
         let pm = self.pass_mode();
         let should_run = self.should_run(pm);
-        let emit_metadata = self.should_emit_metadata(pm);
-        let passes = self.get_passes();
 
-        let proc_res = self.compile_test_with_passes(should_run, emit_metadata, passes);
-        self.check_mir_dump();
+        let mut test_info = files_for_miropt_test(
+            &self.testpaths.file,
+            self.config.get_pointer_width(),
+            self.config.target_cfg().panic.for_miropt_test_tools(),
+        );
+
+        let passes = std::mem::take(&mut test_info.passes);
+
+        let proc_res = self.compile_test_with_passes(should_run, Emit::Mir, passes);
+        self.check_mir_dump(test_info);
         if !proc_res.status.success() {
             self.fatal_proc_rec("compilation failed!", &proc_res);
         }
@@ -4002,37 +4013,12 @@ impl<'test> TestCx<'test> {
         }
     }
 
-    fn get_passes(&self) -> Vec<String> {
-        let files = miropt_test_tools::files_for_miropt_test(
-            &self.testpaths.file,
-            self.config.get_pointer_width(),
-            self.config.target_cfg().panic.for_miropt_test_tools(),
-        );
-
-        let mut out = Vec::new();
-
-        for miropt_test_tools::MiroptTestFiles {
-            from_file: _,
-            to_file: _,
-            expected_file: _,
-            passes,
-        } in files
-        {
-            out.extend(passes);
-        }
-        out
-    }
-
-    fn check_mir_dump(&self) {
+    fn check_mir_dump(&self, test_info: MiroptTest) {
         let test_dir = self.testpaths.file.parent().unwrap();
         let test_crate =
             self.testpaths.file.file_stem().unwrap().to_str().unwrap().replace("-", "_");
 
-        let suffix = miropt_test_tools::output_file_suffix(
-            &self.testpaths.file,
-            self.config.get_pointer_width(),
-            self.config.target_cfg().panic.for_miropt_test_tools(),
-        );
+        let MiroptTest { run_filecheck, suffix, files, passes: _ } = test_info;
 
         if self.config.bless {
             for e in
@@ -4047,14 +4033,7 @@ impl<'test> TestCx<'test> {
             }
         }
 
-        let files = miropt_test_tools::files_for_miropt_test(
-            &self.testpaths.file,
-            self.config.get_pointer_width(),
-            self.config.target_cfg().panic.for_miropt_test_tools(),
-        );
-        for miropt_test_tools::MiroptTestFiles { from_file, to_file, expected_file, passes: _ } in
-            files
-        {
+        for MiroptTestFile { from_file, to_file, expected_file } in files {
             let dumped_string = if let Some(after) = to_file {
                 self.diff_mir_files(from_file.into(), after.into())
             } else {
@@ -4095,6 +4074,14 @@ impl<'test> TestCx<'test> {
                 }
             }
         }
+
+        if run_filecheck {
+            let output_path = self.output_base_name().with_extension("mir");
+            let proc_res = self.verify_with_filecheck(&output_path);
+            if !proc_res.status.success() {
+                self.fatal_proc_rec("verification with 'FileCheck' failed", &proc_res);
+            }
+        }
     }
 
     fn diff_mir_files(&self, before: PathBuf, after: PathBuf) -> String {

diff --git a/src/tools/miropt-test-tools/src/lib.rs b/src/tools/miropt-test-tools/src/lib.rs
@@ -1,10 +1,16 @@
 use std::fs;
 use std::path::Path;
 
-pub struct MiroptTestFiles {
+pub struct MiroptTestFile {
     pub expected_file: std::path::PathBuf,
     pub from_file: String,
     pub to_file: Option<String>,
+}
+
+pub struct MiroptTest {
+    pub run_filecheck: bool,
+    pub suffix: String,
+    pub files: Vec<MiroptTestFile>,
     /// Vec of passes under test to be dumped
     pub passes: Vec<String>,
 }
@@ -14,11 +20,7 @@ pub enum PanicStrategy {
     Abort,
 }
 
-pub fn output_file_suffix(
-    testfile: &Path,
-    bit_width: u32,
-    panic_strategy: PanicStrategy,
-) -> String {
+fn output_file_suffix(testfile: &Path, bit_width: u32, panic_strategy: PanicStrategy) -> String {
     let mut each_bit_width = false;
     let mut each_panic_strategy = false;
     for line in fs::read_to_string(testfile).unwrap().lines() {
@@ -47,16 +49,22 @@ pub fn files_for_miropt_test(
     testfile: &std::path::Path,
     bit_width: u32,
     panic_strategy: PanicStrategy,
-) -> Vec<MiroptTestFiles> {
+) -> MiroptTest {
     let mut out = Vec::new();
     let test_file_contents = fs::read_to_string(&testfile).unwrap();
 
     let test_dir = testfile.parent().unwrap();
     let test_crate = testfile.file_stem().unwrap().to_str().unwrap().replace('-', "_");
 
     let suffix = output_file_suffix(testfile, bit_width, panic_strategy);
+    let mut run_filecheck = true;
+    let mut passes = Vec::new();
 
     for l in test_file_contents.lines() {
+        if l.starts_with("// skip-filecheck") {
+            run_filecheck = false;
+            continue;
+        }
         if l.starts_with("// EMIT_MIR ") {
             let test_name = l.trim_start_matches("// EMIT_MIR ").trim();
             let mut test_names = test_name.split(' ');
@@ -65,7 +73,6 @@ pub fn files_for_miropt_test(
             let mut expected_file;
             let from_file;
             let to_file;
-            let mut passes = Vec::new();
 
             if test_name.ends_with(".diff") {
                 let trimmed = test_name.trim_end_matches(".diff");
@@ -114,9 +121,9 @@ pub fn files_for_miropt_test(
             }
             let expected_file = test_dir.join(expected_file);
 
-            out.push(MiroptTestFiles { expected_file, from_file, to_file, passes });
+            out.push(MiroptTestFile { expected_file, from_file, to_file });
         }
     }
 
-    out
+    MiroptTest { run_filecheck, suffix, files: out, passes }
 }
diff --git a/src/tools/tidy/src/mir_opt_tests.rs b/src/tools/tidy/src/mir_opt_tests.rs
@@ -26,7 +26,8 @@ fn check_unused_files(path: &Path, bless: bool, bad: &mut bool) {
     for file in rs_files {
         for bw in [32, 64] {
             for ps in [PanicStrategy::Unwind, PanicStrategy::Abort] {
-                for output_file in miropt_test_tools::files_for_miropt_test(&file, bw, ps) {
+                let mir_opt_test = miropt_test_tools::files_for_miropt_test(&file, bw, ps);
+                for output_file in mir_opt_test.files {
                     output_files.remove(&output_file.expected_file);
                 }
             }

diff --git a/tests/mir-opt/README.md b/tests/mir-opt/README.md
@@ -49,3 +49,21 @@ This exists mainly for completeness and is rarely useful.
 ```
 // EMIT_MIR $file_name_of_some_mir_dump.before.mir
 ```
+
+# FileCheck directives
+
+The LLVM FileCheck tool is used to verify the contents of output MIR against `CHECK` directives
+present in the test file. This works on the runtime MIR, generated by `--emit=mir`, and not
+on the output of a individual passes.
+
+Use `// skip-filecheck` to prevent FileCheck from running.
+
+To check MIR for function `foo`, start with a `// CHECK-LABEL fn foo(` directive.
+
+`{{regex}}` syntax allows to match `regex`.
+
+`[[name:regex]]` syntax allows to bind `name` to a string matching `regex`, and refer to it
+as `[[name]]` in later directives, `regex` should be written not to match a leading space.
+Use `[[my_local:_.*]]` to name a local, and `[[my_bb:bb.*]]` to name a block.
+
+Documentation for FileCheck is available here: https://www.llvm.org/docs/CommandGuide/FileCheck.html
diff --git a/tests/mir-opt/address_of.address_of_reborrow.SimplifyCfg-initial.after.mir b/tests/mir-opt/address_of.address_of_reborrow.SimplifyCfg-initial.after.mir
@@ -1,36 +1,36 @@
 // MIR for `address_of_reborrow` after SimplifyCfg-initial
 
 | User Type Annotations
-| 0: user_ty: Canonical { value: Ty(*const ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:7:5: 7:18, inferred_ty: *const [i32; 10]
-| 1: user_ty: Canonical { value: Ty(*const dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:9:5: 9:25, inferred_ty: *const dyn std::marker::Send
-| 2: user_ty: Canonical { value: Ty(*const ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:13:12: 13:20, inferred_ty: *const [i32; 10]
-| 3: user_ty: Canonical { value: Ty(*const ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:13:12: 13:20, inferred_ty: *const [i32; 10]
-| 4: user_ty: Canonical { value: Ty(*const [i32; 10]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:14:12: 14:28, inferred_ty: *const [i32; 10]
-| 5: user_ty: Canonical { value: Ty(*const [i32; 10]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:14:12: 14:28, inferred_ty: *const [i32; 10]
-| 6: user_ty: Canonical { value: Ty(*const dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:15:12: 15:27, inferred_ty: *const dyn std::marker::Send
-| 7: user_ty: Canonical { value: Ty(*const dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:15:12: 15:27, inferred_ty: *const dyn std::marker::Send
-| 8: user_ty: Canonical { value: Ty(*const [i32]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:16:12: 16:24, inferred_ty: *const [i32]
-| 9: user_ty: Canonical { value: Ty(*const [i32]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:16:12: 16:24, inferred_ty: *const [i32]
-| 10: user_ty: Canonical { value: Ty(*const ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:18:5: 18:18, inferred_ty: *const [i32; 10]
-| 11: user_ty: Canonical { value: Ty(*const dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:20:5: 20:25, inferred_ty: *const dyn std::marker::Send
-| 12: user_ty: Canonical { value: Ty(*const ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:23:12: 23:20, inferred_ty: *const [i32; 10]
-| 13: user_ty: Canonical { value: Ty(*const ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:23:12: 23:20, inferred_ty: *const [i32; 10]
-| 14: user_ty: Canonical { value: Ty(*const [i32; 10]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:24:12: 24:28, inferred_ty: *const [i32; 10]
-| 15: user_ty: Canonical { value: Ty(*const [i32; 10]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:24:12: 24:28, inferred_ty: *const [i32; 10]
-| 16: user_ty: Canonical { value: Ty(*const dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:25:12: 25:27, inferred_ty: *const dyn std::marker::Send
-| 17: user_ty: Canonical { value: Ty(*const dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:25:12: 25:27, inferred_ty: *const dyn std::marker::Send
-| 18: user_ty: Canonical { value: Ty(*const [i32]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:26:12: 26:24, inferred_ty: *const [i32]
-| 19: user_ty: Canonical { value: Ty(*const [i32]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:26:12: 26:24, inferred_ty: *const [i32]
-| 20: user_ty: Canonical { value: Ty(*mut ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:28:5: 28:16, inferred_ty: *mut [i32; 10]
-| 21: user_ty: Canonical { value: Ty(*mut dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:30:5: 30:23, inferred_ty: *mut dyn std::marker::Send
-| 22: user_ty: Canonical { value: Ty(*mut ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:33:12: 33:18, inferred_ty: *mut [i32; 10]
-| 23: user_ty: Canonical { value: Ty(*mut ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:33:12: 33:18, inferred_ty: *mut [i32; 10]
-| 24: user_ty: Canonical { value: Ty(*mut [i32; 10]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:34:12: 34:26, inferred_ty: *mut [i32; 10]
-| 25: user_ty: Canonical { value: Ty(*mut [i32; 10]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:34:12: 34:26, inferred_ty: *mut [i32; 10]
-| 26: user_ty: Canonical { value: Ty(*mut dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:35:12: 35:25, inferred_ty: *mut dyn std::marker::Send
-| 27: user_ty: Canonical { value: Ty(*mut dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:35:12: 35:25, inferred_ty: *mut dyn std::marker::Send
-| 28: user_ty: Canonical { value: Ty(*mut [i32]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:36:12: 36:22, inferred_ty: *mut [i32]
-| 29: user_ty: Canonical { value: Ty(*mut [i32]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:36:12: 36:22, inferred_ty: *mut [i32]
+| 0: user_ty: Canonical { value: Ty(*const ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:8:5: 8:18, inferred_ty: *const [i32; 10]
+| 1: user_ty: Canonical { value: Ty(*const dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:10:5: 10:25, inferred_ty: *const dyn std::marker::Send
+| 2: user_ty: Canonical { value: Ty(*const ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:14:12: 14:20, inferred_ty: *const [i32; 10]
+| 3: user_ty: Canonical { value: Ty(*const ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:14:12: 14:20, inferred_ty: *const [i32; 10]
+| 4: user_ty: Canonical { value: Ty(*const [i32; 10]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:15:12: 15:28, inferred_ty: *const [i32; 10]
+| 5: user_ty: Canonical { value: Ty(*const [i32; 10]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:15:12: 15:28, inferred_ty: *const [i32; 10]
+| 6: user_ty: Canonical { value: Ty(*const dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:16:12: 16:27, inferred_ty: *const dyn std::marker::Send
+| 7: user_ty: Canonical { value: Ty(*const dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:16:12: 16:27, inferred_ty: *const dyn std::marker::Send
+| 8: user_ty: Canonical { value: Ty(*const [i32]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:17:12: 17:24, inferred_ty: *const [i32]
+| 9: user_ty: Canonical { value: Ty(*const [i32]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:17:12: 17:24, inferred_ty: *const [i32]
+| 10: user_ty: Canonical { value: Ty(*const ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:19:5: 19:18, inferred_ty: *const [i32; 10]
+| 11: user_ty: Canonical { value: Ty(*const dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:21:5: 21:25, inferred_ty: *const dyn std::marker::Send
+| 12: user_ty: Canonical { value: Ty(*const ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:24:12: 24:20, inferred_ty: *const [i32; 10]
+| 13: user_ty: Canonical { value: Ty(*const ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:24:12: 24:20, inferred_ty: *const [i32; 10]
+| 14: user_ty: Canonical { value: Ty(*const [i32; 10]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:25:12: 25:28, inferred_ty: *const [i32; 10]
+| 15: user_ty: Canonical { value: Ty(*const [i32; 10]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:25:12: 25:28, inferred_ty: *const [i32; 10]
+| 16: user_ty: Canonical { value: Ty(*const dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:26:12: 26:27, inferred_ty: *const dyn std::marker::Send
+| 17: user_ty: Canonical { value: Ty(*const dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:26:12: 26:27, inferred_ty: *const dyn std::marker::Send
+| 18: user_ty: Canonical { value: Ty(*const [i32]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:27:12: 27:24, inferred_ty: *const [i32]
+| 19: user_ty: Canonical { value: Ty(*const [i32]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:27:12: 27:24, inferred_ty: *const [i32]
+| 20: user_ty: Canonical { value: Ty(*mut ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:29:5: 29:16, inferred_ty: *mut [i32; 10]
+| 21: user_ty: Canonical { value: Ty(*mut dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:31:5: 31:23, inferred_ty: *mut dyn std::marker::Send
+| 22: user_ty: Canonical { value: Ty(*mut ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:34:12: 34:18, inferred_ty: *mut [i32; 10]
+| 23: user_ty: Canonical { value: Ty(*mut ^0), max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }, span: $DIR/address_of.rs:34:12: 34:18, inferred_ty: *mut [i32; 10]
+| 24: user_ty: Canonical { value: Ty(*mut [i32; 10]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:35:12: 35:26, inferred_ty: *mut [i32; 10]
+| 25: user_ty: Canonical { value: Ty(*mut [i32; 10]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:35:12: 35:26, inferred_ty: *mut [i32; 10]
+| 26: user_ty: Canonical { value: Ty(*mut dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:36:12: 36:25, inferred_ty: *mut dyn std::marker::Send
+| 27: user_ty: Canonical { value: Ty(*mut dyn std::marker::Send), max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }] }, span: $DIR/address_of.rs:36:12: 36:25, inferred_ty: *mut dyn std::marker::Send
+| 28: user_ty: Canonical { value: Ty(*mut [i32]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:37:12: 37:22, inferred_ty: *mut [i32]
+| 29: user_ty: Canonical { value: Ty(*mut [i32]), max_universe: U0, variables: [] }, span: $DIR/address_of.rs:37:12: 37:22, inferred_ty: *mut [i32]
 |
 fn address_of_reborrow() -> () {
     let mut _0: ();

diff --git a/tests/mir-opt/address_of.rs b/tests/mir-opt/address_of.rs
@@ -1,3 +1,4 @@
+// skip-filecheck
 // EMIT_MIR address_of.address_of_reborrow.SimplifyCfg-initial.after.mir
 
 fn address_of_reborrow() {

diff --git a/tests/mir-opt/array_index_is_temporary.rs b/tests/mir-opt/array_index_is_temporary.rs
@@ -1,3 +1,4 @@
+// unit-test: SimplifyCfg-elaborate-drops
 // EMIT_MIR_FOR_EACH_PANIC_STRATEGY
 // Retagging (from Stacked Borrows) relies on the array index being a fresh
 // temporary, so that side-effects cannot change it.
@@ -11,6 +12,12 @@ unsafe fn foo(z: *mut usize) -> u32 {
 
 // EMIT_MIR array_index_is_temporary.main.SimplifyCfg-elaborate-drops.after.mir
 fn main() {
+    // CHECK-LABEL: fn main(
+    // CHECK: debug x => [[x:_.*]];
+    // CHECK: debug y => [[y:_.*]];
+    // CHECK: [[y]] = const 1_usize;
+    // CHECK: [[tmp:_.*]] = [[y]];
+    // CHECK: [[x]][[[tmp]]] =
     let mut x = [42, 43, 44];
     let mut y = 1;
     let z: *mut usize = &mut y;