Add a new lint UNCONSTRUCTABLE_PUB_STRUCT to detect unconstructable public structs

[mu001999]

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Add a new lint UNCONSTRUCTABLE_PUB_STRUCT to detect unconstructable public structs, based on the following observations:

1. A public struct with private field(s) cannot be directly constructed from external crates.
2. Associated functions with a receiver require an already constructed value of type Self.
3. Therefore, public structs with private fields and their associated functions that take a receiver can be included in the local crate's dead code analysis.
4. If a public struct with private fields cannot be constructed in any reachable code path, it could be considered dead.

And, the lint UNCONSTRUCTABLE_PUB_STRUCT won't affect the dead-code lint's result, but allow or deny dead-code lint may affect the result of UNCONSTRUCTABLE_PUB_STRUCT

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[juntyr]

Would the lint fire on token structs that are public, have private fields, have no public constructor method, but expose a limited number of pre-constructed objects, e.g. through a static that contains an optional token?

[mu001999]

a static that contains an optional token

won't fire like private types used in such places

[davidtwco]

Nominating for t-lang to decide whether we want this lint, then I'll review the implementation.

Also, s/unconstructible/unconstructable.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[Darksonn]

Does this trigger on structs that are intended only as marker types in generic parameters?

[mu001999]

Does this trigger on structs that are intended only as marker types in generic parameters?

Such types usually have intended private unit fields and this won't trigger on them. This will only trigger types with trivial fields but not be used or cannot be constructed.

[nikomatsakis]

Are there examples of code in the wild that is affected by this lint?

[scottmcm]

I worry about completeness here. If I have something like pub struct Foo(pub [u8]); it's not "constructible" in a sense, but it might be entirely expected anyway. What if there's something only created via slice_from_raw_parts and pointer casts to get &MyType?

---

Musing: what if this was signature-based, say? Could it be phrased as "why is this pub when it's not in a signature; maybe it should be pub(crate)?" or something?

[joshtriplett]

We discussed this in today's @rust-lang/lang meeting.

Was there any particular motivating use case that led to proposing this? Can you point to some code that motivated this?

We wondered about potential corner cases, notably structs that are only constructed in unsafe code. For instance, something using repr(C) or repr(transparent) that's constructed via transmute, or a dynamically sized type that requires unsafe code to construct. We're hoping those can be handled and won't produce false positives.

Once those are addressed, we'd like to see the (triaged) results of a crater run with this lint marked as deny-by-default, so we can get an idea of 1) how widespread this is and 2) whether this catches any issues.

[traviscross]

Please renominate for lang when these answers are available.

[bors]

☔ The latest upstream changes (presumably #150645) made this pull request unmergeable. Please resolve the merge conflicts.

[rust-bors]

☔ The latest upstream changes (presumably #150645) made this pull request unmergeable. Please resolve the merge conflicts.

[mu001999]

I think this PR needs further design regarding its interaction with the lint dead_code.

@rustbot blocked

[mu001999]

@bors try @rust-timer queue

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

Executing "/scripts/stage_2_test_set2.sh"
+ /scripts/stage_2_test_set2.sh
PR_CI_JOB set; skipping tidy
+ '[' 1 == 1 ']'
+ echo 'PR_CI_JOB set; skipping tidy'
+ SKIP_TIDY='--skip tidy'
+ ../x.py --stage 2 test --skip tidy --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest
##[group]Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.04s
##[endgroup]
downloading https://static.rust-lang.org/dist/2026-04-14/rustfmt-nightly-aarch64-unknown-linux-gnu.tar.xz
---
[BUILD] mini_core
error: struct `DynMetadata` is never constructed
##[error]  --> example/mini_core.rs:30:12
   |
30 | pub struct DynMetadata<Dyn: PointeeSized> {
   |            ^^^^^^^^^^^
   |
   = note: this `pub` struct is unconstructable externally and never constructed locally, so consider providing a pub constructor or removing this
   = note: `#[deny(unconstructable_pub_struct)]` (part of `#[deny(unused)]`) on by default

error: aborting due to 1 previous error

"/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/dist/rustc-clif" "-Zannotate-moves" "-Zrandomize-layout" "-Zunstable-options" "--check-cfg=cfg(bootstrap)" "-Zmacro-backtrace" "-Csplit-debuginfo=off" "-Clink-arg=-L/usr/lib/llvm-21/lib" "-Cllvm-args=-import-instr-limit=10" "-Clink-args=-Wl,-z,origin" "-Clink-args=-Wl,-rpath,$ORIGIN/../lib" "-Alinker-messages" "-L" "crate=/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/build/example" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/build/example" "-Cdebuginfo=2" "--target" "aarch64-unknown-linux-gnu" "-Cpanic=abort" "--check-cfg=cfg(jit)" "--edition=2024" "example/mini_core.rs" "--crate-type" "lib,dylib" exited with status ExitStatus(unix_wait_status(256))
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo run -Zwarnings --target aarch64-unknown-linux-gnu -Zbinary-dep-depinfo -j 4 -Zroot-dir=/checkout --locked --color=always --profile=release --manifest-path /checkout/compiler/rustc_codegen_cranelift/build_system/Cargo.toml -- test --download-dir /checkout/obj/build/cg_clif_download --out-dir /checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif --no-unstable-features --use-backend cranelift --sysroot llvm --skip-test testsuite.extended_sysroot [workdir=/checkout/compiler/rustc_codegen_cranelift]` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/test.rs:3955:25
Executed at: src/bootstrap/src/core/build_steps/test.rs:4000:26

--- BACKTRACE vvv
   0: std::backtrace_rs::backtrace::libunwind::trace
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/../../backtrace/src/backtrace/libunwind.rs:117:9
   1: std::backtrace_rs::backtrace::trace_unsynchronized::<<std::backtrace::Backtrace>::create::{closure#0}>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/../../backtrace/src/backtrace/mod.rs:66:14
   2: <std::backtrace::Backtrace>::create
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/backtrace.rs:331:13
   3: <bootstrap::utils::exec::DeferredCommand>::finish_process
             at /checkout/src/bootstrap/src/utils/exec.rs:939:17
   4: <bootstrap::utils::exec::DeferredCommand>::wait_for_output::<&bootstrap::utils::exec::ExecutionContext>
             at /checkout/src/bootstrap/src/utils/exec.rs:831:21
   5: <bootstrap::utils::exec::ExecutionContext>::run
             at /checkout/src/bootstrap/src/utils/exec.rs:741:45
   6: <bootstrap::utils::exec::BootstrapCommand>::run::<&bootstrap::core::builder::Builder>
             at /checkout/src/bootstrap/src/utils/exec.rs:339:27
   7: <bootstrap::core::build_steps::test::CodegenCranelift as bootstrap::core::builder::Step>::run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:4000:26
   8: <bootstrap::core::builder::Builder>::ensure::<bootstrap::core::build_steps::test::CodegenCranelift>
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1595:36
   9: <bootstrap::core::build_steps::test::CodegenCranelift as bootstrap::core::builder::Step>::make_run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:3941:17
  10: <bootstrap::core::builder::StepDescription>::maybe_run
             at /checkout/src/bootstrap/src/core/builder/mod.rs:476:13
  11: bootstrap::core::builder::cli_paths::match_paths_to_steps_and_run
             at /checkout/src/bootstrap/src/core/builder/cli_paths.rs:141:22
  12: <bootstrap::core::builder::Builder>::run_step_descriptions
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1138:9
  13: <bootstrap::core::builder::Builder>::execute_cli
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1117:14
  14: <bootstrap::Build>::build
             at /checkout/src/bootstrap/src/lib.rs:803:25
  15: bootstrap::main
             at /checkout/src/bootstrap/src/bin/main.rs:130:11
  16: <fn() as core::ops::function::FnOnce<()>>::call_once
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/core/src/ops/function.rs:250:5
  17: std::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/sys/backtrace.rs:166:18
  18: std::rt::lang_start::<()>::{closure#0}
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/rt.rs:206:18
  19: <&dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync as core::ops::function::FnOnce<()>>::call_once
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/core/src/ops/function.rs:287:21
  20: std::panicking::catch_unwind::do_call::<&dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync, i32>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/panicking.rs:581:40
  21: std::panicking::catch_unwind::<i32, &dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/panicking.rs:544:19
  22: std::panic::catch_unwind::<&dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync, i32>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/panic.rs:359:14
  23: std::rt::lang_start_internal::{closure#0}
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/rt.rs:175:24
  24: std::panicking::catch_unwind::do_call::<std::rt::lang_start_internal::{closure#0}, isize>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/panicking.rs:581:40
---
  31: __libc_start_main
  32: _start


Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `--stage 2 test --skip tidy --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Build completed unsuccessfully in 0:26:00
  local time: Thu May 14 17:09:10 UTC 2026
  network time: Thu, 14 May 2026 17:09:10 GMT
##[error]Process completed with exit code 1.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 51f5dfb (51f5dfbf846f8119ef576ad8c6b09b854ad81b99, parent: 480d8520dfc8f8d77869d7d3b2011026272c9528)

[rust-timer]

Finished benchmarking commit (51f5dfb): comparison URL.

Overall result: ❌ regressions - BENCHMARK(S) FAILED

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

❗ ❗ ❗ ❗ ❗
Warning ⚠️: The following benchmark(s) failed to build:

• Job failure
• issue-58319
• image-0.25.6
• cranelift-codegen-0.119.0

❗ ❗ ❗ ❗ ❗

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.2%, 0.4%]	18
Regressions ❌ (secondary)	0.4%	[0.2%, 0.4%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.2%, 0.4%]	18

Max RSS (memory usage)

Results (primary 2.4%, secondary -1.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.4%	[2.4%, 2.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.7%	[-1.9%, -1.5%]	2
All ❌✅ (primary)	2.4%	[2.4%, 2.4%]	1

Cycles

Results (secondary -7.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-7.3%	[-7.3%, -7.3%]	1
All ❌✅ (primary)	-	-	0

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 510.687s -> 510.851s (0.03%)
Artifact size: 398.06 MiB -> 398.11 MiB (0.01%)