Warn on unused offset_of!() result

[ChayimFriedman2]

The usage of core::hint::must_use() means that we don't get a specialized message. I figured out that since there are plenty of other methods that just have #[must_use] with no message it'll be fine, but it is a bit unfortunate that the error mentions must_use and not offset_of!.

Fixes #111669.

[rustbot]

r? @davidtwco

(rustbot has picked a reviewer for you, use r? to override)

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[WaffleLapkin]

@ChayimFriedman2 you should probably rebase on master and then run ./x.py test tests/ui/offset-of to see which tests fail because of those changes and fix them (by adding let _ = s, I suppose).

[WaffleLapkin]

Kind of a shame we are missing those, but I'm not sure what to do with that...

[WaffleLapkin]

@bors r+

[bors]

📌 Commit 460c3a8f3019370ecf20876b24bc0ff4cb7372d6 has been approved by WaffleLapkin

It is now in the queue for this repository.

[bors]

⌛ Testing commit 460c3a8f3019370ecf20876b24bc0ff4cb7372d6 with merge 4c238050a5a58daca1fee25804464e9138506abf...

[bors]

💔 Test failed - checks-actions

[bors]

📌 Commit d3caa2ab349d9eb234b1f61f57db27cf98ccf117 has been approved by WaffleLapkin

It is now in the queue for this repository.

[est31]

@ChayimFriedman2 that's interesting. I have tried running the command I suggested above and ran into some issues due to cross compilation toolchains not being available. So I couldn't reproduce your issue, but not because it's working for me but because it's even more broken :).

[bors]

⌛ Testing commit d3caa2ab349d9eb234b1f61f57db27cf98ccf117 with merge 2e99d62427028bc9995f9bd056acdb1ccab80ab0...

[bors]

💔 Test failed - checks-actions

[ChayimFriedman2]

I don't understand. This is exactly the opposite of the previous failure. Something strange is happening here.

[ChayimFriedman2]

If I do what bors wants (which BTW is what --bless emits), then it fails locally. What should I do?

[oli-obk]

A few things:

• what's your host?
• do you have a config.toml? If so, what did you configure?
• is it only failing for the 32 bit target but works for the 64 bit target?

[ChayimFriedman2]

@oli-obk

what's your host?

x86_64-pc-windows-msvc

do you have a config.toml? If so, what did you configure?

It is configured to profile = "compiler".

is it only failing for the 32 bit target but works for the 64 bit target?

No, it fails for the 64 bit target (and doesn't check 32 bit therefore).

[oli-obk]

wait I'm confused. If you --bless, then the tests fail locally? Can you gist your entire log of that happening?

[ChayimFriedman2]

@oli-obk

Building bootstrap
    Finished dev [unoptimized] target(s) in 0.19s
Building stage0 library artifacts (x86_64-pc-windows-msvc)
    Finished release [optimized] target(s) in 0.54s
Building compiler artifacts (stage0 -> stage1, x86_64-pc-windows-msvc)
    Finished release [optimized] target(s) in 1.66s
Creating a sysroot for stage1 compiler (use `rustup toolchain link 'name' build/host/stage1`)
Building stage1 library artifacts (x86_64-pc-windows-msvc)
    Finished release [optimized] target(s) in 1.01s
Building stage0 tool compiletest (x86_64-pc-windows-msvc)
    Finished release [optimized] target(s) in 0.69s
Check compiletest suite=mir-opt mode=mir-opt (x86_64-pc-windows-msvc -> x86_64-pc-windows-msvc)

running 1 tests
F

failures:

---- [mir-opt] tests\mir-opt\const_prop\offset_of.rs stdout ----
29                StorageLive(_2);
30      -         _2 = OffsetOf(Alpha, [0]);
31      +         _2 = const 4_usize;
-                 _1 = must_use::<usize>(move _2) -> [return: bb1, unwind unreachable];
+                 _1 = must_use::<usize>(move _2) -> bb1;
33            }
34
35            bb1: {

38                StorageLive(_4);
39      -         _4 = OffsetOf(Alpha, [1]);
40      +         _4 = const 0_usize;
-                 _3 = must_use::<usize>(move _4) -> [return: bb2, unwind unreachable];
+                 _3 = must_use::<usize>(move _4) -> bb2;
42            }
43
44            bb2: {

47                StorageLive(_6);
48      -         _6 = OffsetOf(Alpha, [2, 0]);
49      +         _6 = const 2_usize;
-                 _5 = must_use::<usize>(move _6) -> [return: bb3, unwind unreachable];
+                 _5 = must_use::<usize>(move _6) -> bb3;
51            }
52
53            bb3: {

56                StorageLive(_8);
57      -         _8 = OffsetOf(Alpha, [2, 1]);
58      +         _8 = const 3_usize;
-                 _7 = must_use::<usize>(move _8) -> [return: bb4, unwind unreachable];
+                 _7 = must_use::<usize>(move _8) -> bb4;
60            }
61
62            bb4: {

thread '[mir-opt] tests\mir-opt\const_prop\offset_of.rs' panicked at 'Actual MIR output differs from expected MIR output C:\rust\tests\mir-opt\const_prop\offset_of.concrete.ConstProp.diff', src\tools\compiletest\src\runtest.rs:3652:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    [mir-opt] tests\mir-opt\const_prop\offset_of.rs

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 260 filtered out; finished in 891.43ms

[ChayimFriedman2]

That is, --bless adds the possibility of unwinding, but locally it wants it to be non-unwinding call (probably it determines that the function cannot unwind). On borts, #111684 (comment) wants unwinding, but previously #111684 (comment) wanted non-unwinding.

[oli-obk]

🤦 your test needs a // EMIT_MIR_FOR_EACH_PANIC_STRATEGY comment at the top of the file so that we end up with two diff files

[oli-obk]

@bors r=WaffleLapkin

[bors]

📌 Commit 592844c has been approved by WaffleLapkin

It is now in the queue for this repository.

[bors]

⌛ Testing commit 592844c with merge 006a26c...

[bors]

☀️ Test successful - checks-actions
Approved by: WaffleLapkin
Pushing 006a26c to master...

[rust-timer]

Finished benchmarking commit (006a26c): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.2%	[3.2%, 3.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.1%	[2.1%, 2.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 660.125s -> 659.628s (-0.08%)