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Rollup merge of #77251 - dtolnay:drop, r=Aaron1011 

Bypass const_item_mutation if const's type has Drop impl

Follow-up to #75573. This PR disables the const_item_mutation lint in cases that the const has a Drop impl which observes the mutation.

```rust
struct Log { msg: &'static str }
const LOG: Log = Log { msg: "" };
impl Drop for Log {
    fn drop(&mut self) { println!("{}", self.msg); }
}

LOG.msg = "wow";  // prints "wow"
```

r? @Aaron1011
This commit is contained in:
Jonas Schievink 2020-10-03 00:31:12 +02:00 committed by GitHub
parent ccc020ab42 804d159c62
commit 6522868664
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GPG Key ID: 4AEE18F83AFDEB23
3 changed files with 102 additions and 14 deletions
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31
compiler/rustc_mir/src/transform/check_const_item_mutation.rs
View File

@ -31,6 +31,35 @@ impl<'a, 'tcx> ConstMutationChecker<'a, 'tcx> {
None
}
}
fn is_const_item_without_destructor(&self, local: Local) -> Option<DefId> {
let def_id = self.is_const_item(local)?;
let mut any_dtor = |_tcx, _def_id| Ok(());
// We avoid linting mutation of a const item if the const's type has a
// Drop impl. The Drop logic observes the mutation which was performed.
//
// pub struct Log { msg: &'static str }
// pub const LOG: Log = Log { msg: "" };
// impl Drop for Log {
// fn drop(&mut self) { println!("{}", self.msg); }
// }
//
// LOG.msg = "wow"; // prints "wow"
//
// FIXME(https://github.com/rust-lang/rust/issues/77425):
// Drop this exception once there is a stable attribute to suppress the
// const item mutation lint for a single specific const only. Something
// equivalent to:
//
// #[const_mutation_allowed]
// pub const LOG: Log = Log { msg: "" };
match self.tcx.calculate_dtor(def_id, &mut any_dtor) {
Some(_) => None,
None => Some(def_id),
}
}
fn lint_const_item_usage(
&self,
const_item: DefId,
@ -59,7 +88,7 @@ impl<'a, 'tcx> Visitor<'tcx> for ConstMutationChecker<'a, 'tcx> {
// Assigning directly to a constant (e.g. `FOO = true;`) is a hard error,
// so emitting a lint would be redundant.
if !lhs.projection.is_empty() {
if let Some(def_id) = self.is_const_item(lhs.local) {
if let Some(def_id) = self.is_const_item_without_destructor(lhs.local) {
// Don't lint on writes through a pointer
// (e.g. `unsafe { *FOO = 0; *BAR.field = 1; }`)
if !matches!(lhs.projection.last(), Some(PlaceElem::Deref)) {

21
src/test/ui/lint/lint-const-item-mutation.rs
View File

@ -9,9 +9,26 @@ impl MyStruct {
fn use_mut(&mut self) {}
}
struct Mutable {
msg: &'static str,
}
impl Drop for Mutable {
fn drop(&mut self) {
println!("{}", self.msg);
}
}
struct Mutable2 { // this one has drop glue but not a Drop impl
msg: &'static str,
other: String,
}
const ARRAY: [u8; 1] = [25];
const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw_ptr: 2 as *mut u8 };
const RAW_PTR: *mut u8 = 1 as *mut u8;
const MUTABLE: Mutable = Mutable { msg: "" };
const MUTABLE2: Mutable2 = Mutable2 { msg: "", other: String::new() };
const VEC: Vec<i32> = Vec::new();
fn main() {
ARRAY[0] = 5; //~ WARN attempting to modify
@ -29,4 +46,8 @@ fn main() {
*RAW_PTR = 0;
*MY_STRUCT.raw_ptr = 0;
}
MUTABLE.msg = "wow"; // no warning, because Drop observes the mutation
MUTABLE2.msg = "wow"; //~ WARN attempting to modify
VEC.push(0); //~ WARN taking a mutable reference to a `const` item
}

64
src/test/ui/lint/lint-const-item-mutation.stderr
View File

@ -1,5 +1,5 @@
warning: attempting to modify a `const` item
--> $DIR/lint-const-item-mutation.rs:17:5
--> $DIR/lint-const-item-mutation.rs:34:5
|
LL | ARRAY[0] = 5;
| ^^^^^^^^^^^^
@ -7,39 +7,39 @@ LL | ARRAY[0] = 5;
= note: `#[warn(const_item_mutation)]` on by default
= 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/lint-const-item-mutation.rs:12:1
--> $DIR/lint-const-item-mutation.rs:26:1
|
LL | const ARRAY: [u8; 1] = [25];
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: attempting to modify a `const` item
--> $DIR/lint-const-item-mutation.rs:18:5
--> $DIR/lint-const-item-mutation.rs:35:5
|
LL | MY_STRUCT.field = false;
| ^^^^^^^^^^^^^^^^^^^^^^^
|
= 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/lint-const-item-mutation.rs:13:1
--> $DIR/lint-const-item-mutation.rs:27:1
|
LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw_ptr: 2 as *mut u8 };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: attempting to modify a `const` item
--> $DIR/lint-const-item-mutation.rs:19:5
--> $DIR/lint-const-item-mutation.rs:36:5
|
LL | MY_STRUCT.inner_array[0] = 'b';
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= 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/lint-const-item-mutation.rs:13:1
--> $DIR/lint-const-item-mutation.rs:27:1
|
LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw_ptr: 2 as *mut u8 };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: taking a mutable reference to a `const` item
--> $DIR/lint-const-item-mutation.rs:20:5
--> $DIR/lint-const-item-mutation.rs:37:5
|
LL | MY_STRUCT.use_mut();
| ^^^^^^^^^^^^^^^^^^^
@ -52,13 +52,13 @@ note: mutable reference created due to call to this method
LL | fn use_mut(&mut self) {}
| ^^^^^^^^^^^^^^^^^^^^^
note: `const` item defined here
--> $DIR/lint-const-item-mutation.rs:13:1
--> $DIR/lint-const-item-mutation.rs:27:1
|
LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw_ptr: 2 as *mut u8 };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: taking a mutable reference to a `const` item
--> $DIR/lint-const-item-mutation.rs:21:5
--> $DIR/lint-const-item-mutation.rs:38:5
|
LL | &mut MY_STRUCT;
| ^^^^^^^^^^^^^^
@ -66,13 +66,13 @@ LL | &mut MY_STRUCT;
= 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/lint-const-item-mutation.rs:13:1
--> $DIR/lint-const-item-mutation.rs:27:1
|
LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw_ptr: 2 as *mut u8 };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: taking a mutable reference to a `const` item
--> $DIR/lint-const-item-mutation.rs:22:5
--> $DIR/lint-const-item-mutation.rs:39:5
|
LL | (&mut MY_STRUCT).use_mut();
| ^^^^^^^^^^^^^^^^
@ -80,10 +80,48 @@ LL | (&mut MY_STRUCT).use_mut();
= 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/lint-const-item-mutation.rs:13:1
--> $DIR/lint-const-item-mutation.rs:27:1
|
LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw_ptr: 2 as *mut u8 };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: 6 warnings emitted
warning: attempting to modify a `const` item
--> $DIR/lint-const-item-mutation.rs:51:5
|
LL | MUTABLE2.msg = "wow";
| ^^^^^^^^^^^^^^^^^^^^
|
= 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/lint-const-item-mutation.rs:30:1
|
LL | const MUTABLE2: Mutable2 = Mutable2 { msg: "", other: String::new() };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: taking a mutable reference to a `const` item
--> $DIR/lint-const-item-mutation.rs:52:5
|
LL | VEC.push(0);
| ^^^^^^^^^^^
|
= 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: mutable reference created due to call to this method
--> $SRC_DIR/alloc/src/vec.rs:LL:COL
|
LL | / pub fn push(&mut self, value: T) {
LL | | // This will panic or abort if we would allocate > isize::MAX bytes
LL | | // or if the length increment would overflow for zero-sized types.
LL | | if self.len == self.buf.capacity() {
... |
LL | | }
LL | | }
| |_____^
note: `const` item defined here
--> $DIR/lint-const-item-mutation.rs:31:1
|
LL | const VEC: Vec<i32> = Vec::new();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: 8 warnings emitted