Add CONST_ITEM_MUTATION lint

[Aaron1011]

Fixes#74053
Fixes#55721

This PR adds a new lint CONST_ITEM_MUTATION.
Given an item const FOO: SomeType = .., this lint fires on:

• Attempting to write directly to a field (FOO.field = some_val) or
  array entry (FOO.array_field[0] = val)
• Taking a mutable reference to the const item (&mut FOO), including
  through an autoderef FOO.some_mut_self_method()

The lint message explains that since each use of a constant creates a
new temporary, the original const item will not be modified.

[rust-highfive]

r? @oli-obk

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

[Aaron1011]

This seems quite verbose - I'm open to suggestions as to how to condense this information.

[leonardo-m]

Eventually in the next Rust edition this warning could become an error?

[tesuji]

I would like the "^" to be cover the name and type, like this:

Suggested change

	LL | const ARRAY: [u8; 1] = [25];
	| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	LL | const ARRAY: [u8; 1] = [25];
	| ^^^^^^^^^^^^^^^^^^^^

The reason is the const expression in RHS could be a big block.

[Aaron1011]

If we decide to do this, I think it should be done consistently (e.g. as a part of #75465), rather than special-casing this lint.

[oli-obk]

I think your lint will also catch

let x = &mutMY_STRUCT;

which seems to me to be equivalent to let x = &mut Default::default() or similar things that (arguably rarely) are legitimite use cases.

[Aaron1011]

That's intentional - I think let x = &mut MY_STRUCT looks extremely misleading. If it there's ever the need for it in practice (maybe macro-generated code?), users can add #[allow(const_item_mutation)].

However, this shouldn't become a hard error.

[oli-obk]

We have rules about new lints, I'm not sure this warrants a rustc lint. I'd be fine with adding the relevant information to MIR but adding the lint to clippy (cc rust-lang/rust-clippy#829). @rust-lang/clippy what do you think?

[Aaron1011]

I think directly assigning to fields of a const should be a rust lint - it's completely useless (other than consuming the RHS), and always indicates a bug. I think this should be considered a natural extension of forbidding using a constant directly in the LHS of an assignment.

Calling a &mut self method might be intentional and legitimate, if the method is called for reasons unrelated to the &mut self (e.g. mutating global state or performing I/O). However, I think the danger of a questionable false-positive is far outweighed by the risk of accepting code like:

constMY_VEC:Vec<u8> = Vec::new();MY_VEC.push(25);

which might come about as a result of several unrelated refactoring (changing a static to a const, and changing a method to take &mut self rather than use interior mutability).

I don't know what the official policy is w.r.t adding new lints to rustc vs clippy. IMHO, 'correctness' lints which detect obviously broken code belong in rustc, not clippy. Assuming that there are no false positives, I see no downsides other than breaking #![deny(warnings)] crates (which explicitly opted into such breakage).

[oli-obk]

Well... I'm not sure how strongly that rule is followed, but new lints in rustc need an RFC. Maybe an MCP is good enough here though. You can open an MCP here. It's not that much effort, just writing a few sentences of explanation and linking to the clippy and rustc issues and this PR should be enough. We'll then bring it up at the compiler team mtg

[Aaron1011]

@oli-obk MCP opened at rust-lang/compiler-team#345

[flip1995]

Clippy already has a lint which implements part of the proposed lint: Playground

I would be good with stop linting assignments to constants in the temporary_assignment lint and move it to a new lint.

There was an issue opened in Clippy before, asking for the temporary_assignment to get uplifted: rust-lang/rust-clippy#4497

[flip1995]

I'm ok with adding this lint to Clippy. We can implement it in the rust repo and then sync it back later.

---

If it gets implemented directly in rustc, it would be great to stop linting on const assignments in clippy::temporary_assignment already in this PR.

[Manishearth]

I concur with @flip1995

I'm not against adding it to rustc directly, however rustc typically has a more onerous process for adding lints, so it's up to you!

[Aaron1011]

@oli-obk: The MCP has been accepted at rust-lang/compiler-team#345 - what is the next step?

[oli-obk]

Now we review the impl and merge it. I won't be able to get to this until the 7th though, so if someone else grabs it in that time, I won't complain.

[bors]

☔ The latest upstream changes (presumably #74862) made this pull request unmergeable. Please resolve the merge conflicts.

[flip1995]

It would be great if you could assure in this PR, that the cases in src/tools/clippy/tests/ui/temporary_assignment.rs doesn't get linted twice by both Clippy and rustc.

[Aaron1011]

@lzutao: This does not actually fix #65822 - that issue deals with interior mutability, while this PR only deals with normal 'exterior' mutability (&mut references)

[tesuji]

Oh, my mistake. Probably another orthogonal lint but requires sign off from language team.

[bors]

☔ The latest upstream changes (presumably #75077) made this pull request unmergeable. Please resolve the merge conflicts.

[oli-obk]

This lgtm now. Just some nits, then we can merge it

[flip1995]

Clippy stderr diff:

temporary_assignment.rs

-error: assignment to temporary- --> $DIR/temporary_assignment.rs:47:5- |-LL | Struct { field: 0 }.field = 1;- | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^- |- = note: `-D clippy::temporary-assignment` implied by `-D warnings`--error: assignment to temporary- --> $DIR/temporary_assignment.rs:48:5- |-LL | / MultiStruct {-LL | | structure: Struct { field: 0 },-LL | | }-LL | | .structure-LL | | .field = 1;- | |______________^--error: assignment to temporary- --> $DIR/temporary_assignment.rs:53:5- |-LL | ArrayStruct { array: [0] }.array[0] = 1;- | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^--error: assignment to temporary- --> $DIR/temporary_assignment.rs:54:5- |-LL | (0, 0).0 = 1;- | ^^^^^^^^^^^^--error: assignment to temporary+error: attempting to modify a `const` item
--> $DIR/temporary_assignment.rs:56:5
|
LL | A.0 = 2;
| ^^^^^^^
+ |+ = note: `-D const-item-mutation` implied by `-D warnings`+ = note: each usage of a `const` item creates a new temporary - the original `const` item will not be modified+note: `const` item defined here+ --> $DIR/temporary_assignment.rs:36:1+ |+LL | const A: TupleStruct = TupleStruct(1);+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-error: assignment to temporary+error: attempting to modify a `const` item
--> $DIR/temporary_assignment.rs:57:5
|
LL | B.field = 2;
| ^^^^^^^^^^^
+ |+ = note: each usage of a `const` item creates a new temporary - the original `const` item will not be modified+note: `const` item defined here+ --> $DIR/temporary_assignment.rs:37:1+ |+LL | const B: Struct = Struct { field: 1 };+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-error: assignment to temporary+error: attempting to modify a `const` item
--> $DIR/temporary_assignment.rs:58:5
|
LL | C.structure.field = 2;
| ^^^^^^^^^^^^^^^^^^^^^
+ |+ = note: each usage of a `const` item creates a new temporary - the original `const` item will not be modified+note: `const` item defined here+ --> $DIR/temporary_assignment.rs:38:1+ |+LL | / const C: MultiStruct = MultiStruct {+LL | | structure: Struct { field: 1 },+LL | | };+ | |__^-error: assignment to temporary+error: attempting to modify a `const` item
--> $DIR/temporary_assignment.rs:59:5
|
LL | D.array[0] = 2;
| ^^^^^^^^^^^^^^
+ |+ = note: each usage of a `const` item creates a new temporary - the original `const` item will not be modified+note: `const` item defined here+ --> $DIR/temporary_assignment.rs:41:1+ |+LL | const D: ArrayStruct = ArrayStruct { array: [1] };+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-error: aborting due to 8 previous errors+error: aborting due to 4 previous errors

borrow_interior_mutable_const.rs

-error: a `const` item with interior mutability should not be borrowed- --> $DIR/borrow_interior_mutable_const.rs:65:5+error: taking a mutable reference to a `const` item+ --> $DIR/borrow_interior_mutable_const.rs:72:21
|
-LL | ATOMIC.store(1, Ordering::SeqCst); //~ ERROR interior mutability- | ^^^^^^- |- = note: `-D clippy::borrow-interior-mutable-const` implied by `-D warnings`- = help: assign this const to a local or static variable, and use the variable here--error: a `const` item with interior mutability should not be borrowed- --> $DIR/borrow_interior_mutable_const.rs:66:16- |-LL | assert_eq!(ATOMIC.load(Ordering::SeqCst), 5); //~ ERROR interior mutability- | ^^^^^^- |- = help: assign this const to a local or static variable, and use the variable here--error: a `const` item with interior mutability should not be borrowed- --> $DIR/borrow_interior_mutable_const.rs:69:22- |-LL | let _once_ref = &ONCE_INIT; //~ ERROR interior mutability- | ^^^^^^^^^- |- = help: assign this const to a local or static variable, and use the variable here--error: a `const` item with interior mutability should not be borrowed- --> $DIR/borrow_interior_mutable_const.rs:70:25- |-LL | let _once_ref_2 = &&ONCE_INIT; //~ ERROR interior mutability- | ^^^^^^^^^- |- = help: assign this const to a local or static variable, and use the variable here--error: a `const` item with interior mutability should not be borrowed- --> $DIR/borrow_interior_mutable_const.rs:71:27- |-LL | let _once_ref_4 = &&&&ONCE_INIT; //~ ERROR interior mutability- | ^^^^^^^^^- |- = help: assign this const to a local or static variable, and use the variable here--error: a `const` item with interior mutability should not be borrowed- --> $DIR/borrow_interior_mutable_const.rs:72:26- |
LL | let _once_mut = &mut ONCE_INIT; //~ ERROR interior mutability
- | ^^^^^^^^^+ | ^^^^^^^^^^^^^^
|
- = help: assign this const to a local or static variable, and use the variable here--error: a `const` item with interior mutability should not be borrowed- --> $DIR/borrow_interior_mutable_const.rs:83:14+ = note: `-D const-item-mutation` implied by `-D warnings`+ = note: each usage of a `const` item creates a new temporary+ = note: the mutable reference will refer to this temporary, not the original `const` item+note: `const` item defined here+ --> $DIR/borrow_interior_mutable_const.rs:19:1
|
-LL | let _ = &ATOMIC_TUPLE; //~ ERROR interior mutability- | ^^^^^^^^^^^^- |- = help: assign this const to a local or static variable, and use the variable here+LL | const ONCE_INIT: Once = Once::new();+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-error: a `const` item with interior mutability should not be borrowed- --> $DIR/borrow_interior_mutable_const.rs:84:14- |-LL | let _ = &ATOMIC_TUPLE.0; //~ ERROR interior mutability- | ^^^^^^^^^^^^- |- = help: assign this const to a local or static variable, and use the variable here--error: a `const` item with interior mutability should not be borrowed- --> $DIR/borrow_interior_mutable_const.rs:85:19- |-LL | let _ = &(&&&&ATOMIC_TUPLE).0; //~ ERROR interior mutability- | ^^^^^^^^^^^^- |- = help: assign this const to a local or static variable, and use the variable here--error: a `const` item with interior mutability should not be borrowed- --> $DIR/borrow_interior_mutable_const.rs:86:14- |-LL | let _ = &ATOMIC_TUPLE.0[0]; //~ ERROR interior mutability- | ^^^^^^^^^^^^- |- = help: assign this const to a local or static variable, and use the variable here--error: a `const` item with interior mutability should not be borrowed- --> $DIR/borrow_interior_mutable_const.rs:87:13- |-LL | let _ = ATOMIC_TUPLE.0[0].load(Ordering::SeqCst); //~ ERROR interior mutability- | ^^^^^^^^^^^^- |- = help: assign this const to a local or static variable, and use the variable here--error: a `const` item with interior mutability should not be borrowed- --> $DIR/borrow_interior_mutable_const.rs:93:13- |-LL | let _ = ATOMIC_TUPLE.0[0]; //~ ERROR interior mutability- | ^^^^^^^^^^^^- |- = help: assign this const to a local or static variable, and use the variable here--error: a `const` item with interior mutability should not be borrowed- --> $DIR/borrow_interior_mutable_const.rs:98:5- |-LL | CELL.set(2); //~ ERROR interior mutability- | ^^^^- |- = help: assign this const to a local or static variable, and use the variable here--error: a `const` item with interior mutability should not be borrowed- --> $DIR/borrow_interior_mutable_const.rs:99:16- |-LL | assert_eq!(CELL.get(), 6); //~ ERROR interior mutability- | ^^^^- |- = help: assign this const to a local or static variable, and use the variable here--error: a `const` item with interior mutability should not be borrowed- --> $DIR/borrow_interior_mutable_const.rs:112:5- |-LL | u64::ATOMIC.store(5, Ordering::SeqCst); //~ ERROR interior mutability- | ^^^^^^^^^^^- |- = help: assign this const to a local or static variable, and use the variable here--error: a `const` item with interior mutability should not be borrowed- --> $DIR/borrow_interior_mutable_const.rs:113:16- |-LL | assert_eq!(u64::ATOMIC.load(Ordering::SeqCst), 9); //~ ERROR interior mutability- | ^^^^^^^^^^^- |- = help: assign this const to a local or static variable, and use the variable here--error: aborting due to 16 previous errors+error: aborting due to previous error

[Aaron1011]

I don't understand why the CONST_ITEM_MUTATION lint is causing some Clippy lints to not run - does -D warnings prevent other lints from running after one causes an error?

[flip1995]

Maybe rustc lints are executed before Clippy lints and since these lints error (-D warnings), the Clippy lints are never executed.

The fix for this is, that the temporary_assignment lint should either be deprecated or only lint in those cases, where the CONST_ITEM_MUTATION lint doesn't trigger.

In the borrow_interior_mutable_const.rs test case the CONST_ITEM_MUTATION should just be allowed.

[Aaron1011]

@oli-obk@flip1995: I've updated Clippy.

[oli-obk]

@bors r+

[bors]

📌 Commit 4434e8c has been approved by oli-obk

[bors]

⌛ Testing commit 4434e8c with merge 8819721...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: oli-obk
Pushing 8819721 to master...