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Deny-by-default never type lints #146167

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2744060
deny never type lints by default
WaffleLapkin Aug 30, 2025
f025827
bless ui tests
WaffleLapkin Aug 30, 2025
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4 changes: 2 additions & 2 deletions compiler/rustc_lint_defs/src/builtin.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -4100,7 +4100,7 @@ declare_lint! {
/// [`!`]: https://doc.rust-lang.org/core/primitive.never.html
/// [`()`]: https://doc.rust-lang.org/core/primitive.unit.html
pub NEVER_TYPE_FALLBACK_FLOWING_INTO_UNSAFE,
Warn,
Deny,
"never type fallback affecting unsafe function calls",
@future_incompatible = FutureIncompatibleInfo {
reason: FutureIncompatibilityReason::EditionAndFutureReleaseSemanticsChange(Edition::Edition2024),
Expand Down Expand Up @@ -4155,7 +4155,7 @@ declare_lint! {
///
/// See [Tracking Issue for making `!` fall back to `!`](https://github.com/rust-lang/rust/issues/123748).
pub DEPENDENCY_ON_UNIT_NEVER_TYPE_FALLBACK,
Warn,
Deny,
"never type fallback affecting unsafe function calls",
@future_incompatible = FutureIncompatibleInfo {
reason: FutureIncompatibilityReason::EditionAndFutureReleaseError(Edition::Edition2024),
Expand Down
22 changes: 10 additions & 12 deletions tests/ui/editions/never-type-fallback-breaking.e2021.fixed
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Original file line number Diff line number Diff line change
Expand Up @@ -3,9 +3,7 @@
//@[e2021] edition: 2021
//@[e2024] edition: 2024
//
//@[e2021] run-pass
//@[e2021] run-rustfix
//@[e2024] check-fail

fn main() {
m();
Expand All @@ -16,8 +14,8 @@ fn main() {
}

fn m() {
//[e2021]~^ WARN this function depends on never type fallback being `()`
//[e2021]~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
//[e2021]~^ error: this function depends on never type fallback being `()`
//[e2021]~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
let x: () = match true {
true => Default::default(),
//[e2024]~^ error: the trait bound `!: Default` is not satisfied
Expand All @@ -28,8 +26,8 @@ fn m() {
}

fn q() -> Option<()> {
//[e2021]~^ WARN this function depends on never type fallback being `()`
//[e2021]~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
//[e2021]~^ error: this function depends on never type fallback being `()`
//[e2021]~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
fn deserialize<T: Default>() -> Option<T> {
Some(T::default())
}
Expand All @@ -45,8 +43,8 @@ fn help<'a: 'a, T: Into<()>, U>(_: U) -> Result<T, ()> {
Err(())
}
fn meow() -> Result<(), ()> {
//[e2021]~^ WARN this function depends on never type fallback being `()`
//[e2021]~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
//[e2021]~^ error: this function depends on never type fallback being `()`
//[e2021]~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
help::<(), _>(1)?;
//[e2024]~^ error: the trait bound `(): From<!>` is not satisfied
Ok(())
Expand All @@ -57,8 +55,8 @@ pub fn takes_apit<T>(_y: impl Fn() -> T) -> Result<T, ()> {
}

pub fn fallback_return() -> Result<(), ()> {
//[e2021]~^ WARN this function depends on never type fallback being `()`
//[e2021]~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
//[e2021]~^ error: this function depends on never type fallback being `()`
//[e2021]~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
takes_apit::<()>(|| Default::default())?;
//[e2024]~^ error: the trait bound `!: Default` is not satisfied
Ok(())
Expand All @@ -71,8 +69,8 @@ fn mk<T>() -> Result<T, ()> {
fn takes_apit2(_x: impl Default) {}

fn fully_apit() -> Result<(), ()> {
//[e2021]~^ WARN this function depends on never type fallback being `()`
//[e2021]~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
//[e2021]~^ error: this function depends on never type fallback being `()`
//[e2021]~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
takes_apit2(mk::<()>()?);
//[e2024]~^ error: the trait bound `!: Default` is not satisfied
Ok(())
Expand Down
74 changes: 37 additions & 37 deletions tests/ui/editions/never-type-fallback-breaking.e2021.stderr
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
warning: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:18:1
error: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:16:1
|
LL | fn m() {
| ^^^^^^
Expand All @@ -8,18 +8,18 @@ LL | fn m() {
= note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2024/never-type-fallback.html>
= help: specify the types explicitly
note: in edition 2024, the requirement `!: Default` will fail
--> $DIR/never-type-fallback-breaking.rs:22:17
--> $DIR/never-type-fallback-breaking.rs:20:17
|
LL | true => Default::default(),
| ^^^^^^^^^^^^^^^^^^
= note: `#[warn(dependency_on_unit_never_type_fallback)]` (part of `#[warn(rust_2024_compatibility)]`) on by default
= note: `#[deny(dependency_on_unit_never_type_fallback)]` (part of `#[deny(rust_2024_compatibility)]`) on by default
help: use `()` annotations to avoid fallback changes
|
LL | let x: () = match true {
| ++++

warning: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:30:1
error: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:28:1
|
LL | fn q() -> Option<()> {
| ^^^^^^^^^^^^^^^^^^^^
Expand All @@ -28,7 +28,7 @@ LL | fn q() -> Option<()> {
= note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2024/never-type-fallback.html>
= help: specify the types explicitly
note: in edition 2024, the requirement `!: Default` will fail
--> $DIR/never-type-fallback-breaking.rs:37:5
--> $DIR/never-type-fallback-breaking.rs:35:5
|
LL | deserialize()?;
| ^^^^^^^^^^^^^
Expand All @@ -37,8 +37,8 @@ help: use `()` annotations to avoid fallback changes
LL | deserialize::<()>()?;
| ++++++

warning: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:47:1
error: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:45:1
|
LL | fn meow() -> Result<(), ()> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
Expand All @@ -47,7 +47,7 @@ LL | fn meow() -> Result<(), ()> {
= note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2024/never-type-fallback.html>
= help: specify the types explicitly
note: in edition 2024, the requirement `(): From<!>` will fail
--> $DIR/never-type-fallback-breaking.rs:50:5
--> $DIR/never-type-fallback-breaking.rs:48:5
|
LL | help(1)?;
| ^^^^^^^
Expand All @@ -56,8 +56,8 @@ help: use `()` annotations to avoid fallback changes
LL | help::<(), _>(1)?;
| +++++++++

warning: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:59:1
error: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:57:1
|
LL | pub fn fallback_return() -> Result<(), ()> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Expand All @@ -66,7 +66,7 @@ LL | pub fn fallback_return() -> Result<(), ()> {
= note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2024/never-type-fallback.html>
= help: specify the types explicitly
note: in edition 2024, the requirement `!: Default` will fail
--> $DIR/never-type-fallback-breaking.rs:62:19
--> $DIR/never-type-fallback-breaking.rs:60:19
|
LL | takes_apit(|| Default::default())?;
| ^^^^^^^^^^^^^^^^^^
Expand All @@ -75,8 +75,8 @@ help: use `()` annotations to avoid fallback changes
LL | takes_apit::<()>(|| Default::default())?;
| ++++++

warning: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:73:1
error: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:71:1
|
LL | fn fully_apit() -> Result<(), ()> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Expand All @@ -85,7 +85,7 @@ LL | fn fully_apit() -> Result<(), ()> {
= note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2024/never-type-fallback.html>
= help: specify the types explicitly
note: in edition 2024, the requirement `!: Default` will fail
--> $DIR/never-type-fallback-breaking.rs:76:17
--> $DIR/never-type-fallback-breaking.rs:74:17
|
LL | takes_apit2(mk()?);
| ^^^^^
Expand All @@ -94,11 +94,11 @@ help: use `()` annotations to avoid fallback changes
LL | takes_apit2(mk::<()>()?);
| ++++++

warning: 5 warnings emitted
error: aborting due to 5 previous errors

Future incompatibility report: Future breakage diagnostic:
warning: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:18:1
error: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:16:1
|
LL | fn m() {
| ^^^^^^
Expand All @@ -107,19 +107,19 @@ LL | fn m() {
= note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2024/never-type-fallback.html>
= help: specify the types explicitly
note: in edition 2024, the requirement `!: Default` will fail
--> $DIR/never-type-fallback-breaking.rs:22:17
--> $DIR/never-type-fallback-breaking.rs:20:17
|
LL | true => Default::default(),
| ^^^^^^^^^^^^^^^^^^
= note: `#[warn(dependency_on_unit_never_type_fallback)]` (part of `#[warn(rust_2024_compatibility)]`) on by default
= note: `#[deny(dependency_on_unit_never_type_fallback)]` (part of `#[deny(rust_2024_compatibility)]`) on by default
help: use `()` annotations to avoid fallback changes
|
LL | let x: () = match true {
| ++++

Future breakage diagnostic:
warning: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:30:1
error: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:28:1
|
LL | fn q() -> Option<()> {
| ^^^^^^^^^^^^^^^^^^^^
Expand All @@ -128,19 +128,19 @@ LL | fn q() -> Option<()> {
= note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2024/never-type-fallback.html>
= help: specify the types explicitly
note: in edition 2024, the requirement `!: Default` will fail
--> $DIR/never-type-fallback-breaking.rs:37:5
--> $DIR/never-type-fallback-breaking.rs:35:5
|
LL | deserialize()?;
| ^^^^^^^^^^^^^
= note: `#[warn(dependency_on_unit_never_type_fallback)]` (part of `#[warn(rust_2024_compatibility)]`) on by default
= note: `#[deny(dependency_on_unit_never_type_fallback)]` (part of `#[deny(rust_2024_compatibility)]`) on by default
help: use `()` annotations to avoid fallback changes
|
LL | deserialize::<()>()?;
| ++++++

Future breakage diagnostic:
warning: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:47:1
error: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:45:1
|
LL | fn meow() -> Result<(), ()> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
Expand All @@ -149,19 +149,19 @@ LL | fn meow() -> Result<(), ()> {
= note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2024/never-type-fallback.html>
= help: specify the types explicitly
note: in edition 2024, the requirement `(): From<!>` will fail
--> $DIR/never-type-fallback-breaking.rs:50:5
--> $DIR/never-type-fallback-breaking.rs:48:5
|
LL | help(1)?;
| ^^^^^^^
= note: `#[warn(dependency_on_unit_never_type_fallback)]` (part of `#[warn(rust_2024_compatibility)]`) on by default
= note: `#[deny(dependency_on_unit_never_type_fallback)]` (part of `#[deny(rust_2024_compatibility)]`) on by default
help: use `()` annotations to avoid fallback changes
|
LL | help::<(), _>(1)?;
| +++++++++

Future breakage diagnostic:
warning: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:59:1
error: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:57:1
|
LL | pub fn fallback_return() -> Result<(), ()> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Expand All @@ -170,19 +170,19 @@ LL | pub fn fallback_return() -> Result<(), ()> {
= note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2024/never-type-fallback.html>
= help: specify the types explicitly
note: in edition 2024, the requirement `!: Default` will fail
--> $DIR/never-type-fallback-breaking.rs:62:19
--> $DIR/never-type-fallback-breaking.rs:60:19
|
LL | takes_apit(|| Default::default())?;
| ^^^^^^^^^^^^^^^^^^
= note: `#[warn(dependency_on_unit_never_type_fallback)]` (part of `#[warn(rust_2024_compatibility)]`) on by default
= note: `#[deny(dependency_on_unit_never_type_fallback)]` (part of `#[deny(rust_2024_compatibility)]`) on by default
help: use `()` annotations to avoid fallback changes
|
LL | takes_apit::<()>(|| Default::default())?;
| ++++++

Future breakage diagnostic:
warning: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:73:1
error: this function depends on never type fallback being `()`
--> $DIR/never-type-fallback-breaking.rs:71:1
|
LL | fn fully_apit() -> Result<(), ()> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Expand All @@ -191,11 +191,11 @@ LL | fn fully_apit() -> Result<(), ()> {
= note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2024/never-type-fallback.html>
= help: specify the types explicitly
note: in edition 2024, the requirement `!: Default` will fail
--> $DIR/never-type-fallback-breaking.rs:76:17
--> $DIR/never-type-fallback-breaking.rs:74:17
|
LL | takes_apit2(mk()?);
| ^^^^^
= note: `#[warn(dependency_on_unit_never_type_fallback)]` (part of `#[warn(rust_2024_compatibility)]`) on by default
= note: `#[deny(dependency_on_unit_never_type_fallback)]` (part of `#[deny(rust_2024_compatibility)]`) on by default
help: use `()` annotations to avoid fallback changes
|
LL | takes_apit2(mk::<()>()?);
Expand Down
16 changes: 8 additions & 8 deletions tests/ui/editions/never-type-fallback-breaking.e2024.stderr
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
error[E0277]: the trait bound `!: Default` is not satisfied
--> $DIR/never-type-fallback-breaking.rs:22:17
--> $DIR/never-type-fallback-breaking.rs:20:17
|
LL | true => Default::default(),
| ^^^^^^^^^^^^^^^^^^ the trait `Default` is not implemented for `!`
Expand All @@ -8,21 +8,21 @@ LL | true => Default::default(),
= help: you might have intended to use the type `()` here instead

error[E0277]: the trait bound `!: Default` is not satisfied
--> $DIR/never-type-fallback-breaking.rs:37:5
--> $DIR/never-type-fallback-breaking.rs:35:5
|
LL | deserialize()?;
| ^^^^^^^^^^^^^ the trait `Default` is not implemented for `!`
|
= note: this error might have been caused by changes to Rust's type-inference algorithm (see issue #48950 <https://github.com/rust-lang/rust/issues/48950> for more information)
= help: you might have intended to use the type `()` here instead
note: required by a bound in `deserialize`
--> $DIR/never-type-fallback-breaking.rs:33:23
--> $DIR/never-type-fallback-breaking.rs:31:23
|
LL | fn deserialize<T: Default>() -> Option<T> {
| ^^^^^^^ required by this bound in `deserialize`

error[E0277]: the trait bound `(): From<!>` is not satisfied
--> $DIR/never-type-fallback-breaking.rs:50:5
--> $DIR/never-type-fallback-breaking.rs:48:5
|
LL | help(1)?;
| ^^^^^^^ the trait `From<!>` is not implemented for `()`
Expand All @@ -39,13 +39,13 @@ LL | help(1)?;
and 4 others
= note: required for `!` to implement `Into<()>`
note: required by a bound in `help`
--> $DIR/never-type-fallback-breaking.rs:44:20
--> $DIR/never-type-fallback-breaking.rs:42:20
|
LL | fn help<'a: 'a, T: Into<()>, U>(_: U) -> Result<T, ()> {
| ^^^^^^^^ required by this bound in `help`

error[E0277]: the trait bound `!: Default` is not satisfied
--> $DIR/never-type-fallback-breaking.rs:62:19
--> $DIR/never-type-fallback-breaking.rs:60:19
|
LL | takes_apit(|| Default::default())?;
| ^^^^^^^^^^^^^^^^^^ the trait `Default` is not implemented for `!`
Expand All @@ -54,7 +54,7 @@ LL | takes_apit(|| Default::default())?;
= help: you might have intended to use the type `()` here instead

error[E0277]: the trait bound `!: Default` is not satisfied
--> $DIR/never-type-fallback-breaking.rs:76:17
--> $DIR/never-type-fallback-breaking.rs:74:17
|
LL | takes_apit2(mk()?);
| ----------- ^^^^^ the trait `Default` is not implemented for `!`
Expand All @@ -64,7 +64,7 @@ LL | takes_apit2(mk()?);
= note: this error might have been caused by changes to Rust's type-inference algorithm (see issue #48950 <https://github.com/rust-lang/rust/issues/48950> for more information)
= help: you might have intended to use the type `()` here instead
note: required by a bound in `takes_apit2`
--> $DIR/never-type-fallback-breaking.rs:71:25
--> $DIR/never-type-fallback-breaking.rs:69:25
|
LL | fn takes_apit2(_x: impl Default) {}
| ^^^^^^^ required by this bound in `takes_apit2`
Expand Down
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