Auto merge of #117094 - Nadrieril:warn-lint-on-arm, r=cjgillot

Warn users who set `non_exhaustive_omitted_patterns` lint level on a match arm

Before https://github.com/rust-lang/rust/pull/116734, the recommended usage of the [`non_exhaustive_omitted_patterns` lint](https://github.com/rust-lang/rust/issues/89554) was:
```rust
match Bar::A {
    Bar::A => {},
    #[warn(non_exhaustive_omitted_patterns)]
    _ => {},
}
```
After https://github.com/rust-lang/rust/pull/116734 this no longer makes sense, and recommended usage is now:
```rust
#[warn(non_exhaustive_omitted_patterns)]
match Bar::A {
    Bar::A => {},
    _ => {},
}
```

As you can guess, this silently breaks all uses of the lint that used the previous form. This is a problem in particular because `syn` recommends usage of this lint to its users in the old way. This PR emits a warning when the previous form is used so users can update.

r? `@cjgillot`

3 files changed, 159 insertions, 0 deletions

diff --git a/tests/ui/rfcs/rfc-2008-non-exhaustive/omitted-patterns-dont-lint-on-arm.lint.stderr b/tests/ui/rfcs/rfc-2008-non-exhaustive/omitted-patterns-dont-lint-on-arm.lint.stderr
new file mode 100644
index 00000000000..9192c66143c
--- /dev/null
+++ b/tests/ui/rfcs/rfc-2008-non-exhaustive/omitted-patterns-dont-lint-on-arm.lint.stderr
@@ -0,0 +1,75 @@
+error: some variants are not matched explicitly
+  --> $DIR/omitted-patterns-dont-lint-on-arm.rs:15:11
+   |
+LL |     match val {
+   |           ^^^ pattern `NonExhaustiveEnum::Struct { .. }` not covered
+   |
+   = help: ensure that all variants are matched explicitly by adding the suggested match arms
+   = note: the matched value is of type `NonExhaustiveEnum` and the `non_exhaustive_omitted_patterns` attribute was found
+note: the lint level is defined here
+  --> $DIR/omitted-patterns-dont-lint-on-arm.rs:14:12
+   |
+LL |     #[deny(non_exhaustive_omitted_patterns)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: some variants are not matched explicitly
+  --> $DIR/omitted-patterns-dont-lint-on-arm.rs:23:11
+   |
+LL |     match val {
+   |           ^^^ pattern `NonExhaustiveEnum::Struct { .. }` not covered
+   |
+   = help: ensure that all variants are matched explicitly by adding the suggested match arms
+   = note: the matched value is of type `NonExhaustiveEnum` and the `non_exhaustive_omitted_patterns` attribute was found
+note: the lint level is defined here
+  --> $DIR/omitted-patterns-dont-lint-on-arm.rs:22:27
+   |
+LL |     #[cfg_attr(lint, deny(non_exhaustive_omitted_patterns))]
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: the lint level must be set on the whole match
+  --> $DIR/omitted-patterns-dont-lint-on-arm.rs:34:9
+   |
+LL |         #[deny(non_exhaustive_omitted_patterns)]
+   |                ------------------------------- remove this attribute
+LL |         _ => {}
+   |         ^
+   |
+   = help: it no longer has any effect to set the lint level on an individual match arm
+help: set the lint level on the whole match
+   |
+LL +     #[deny(non_exhaustive_omitted_patterns)]
+LL |     match val {
+   |
+
+warning: the lint level must be set on the whole match
+  --> $DIR/omitted-patterns-dont-lint-on-arm.rs:42:9
+   |
+LL |         #[cfg_attr(lint, deny(non_exhaustive_omitted_patterns))]
+   |                               ------------------------------- remove this attribute
+LL |         _ => {}
+   |         ^
+   |
+   = help: it no longer has any effect to set the lint level on an individual match arm
+help: set the lint level on the whole match
+   |
+LL +     #[deny(non_exhaustive_omitted_patterns)]
+LL |     match val {
+   |
+
+warning: the lint level must be set on the whole match
+  --> $DIR/omitted-patterns-dont-lint-on-arm.rs:50:9
+   |
+LL |         #[cfg_attr(lint, warn(non_exhaustive_omitted_patterns))]
+   |                               ------------------------------- remove this attribute
+LL |         _ => {}
+   |         ^
+   |
+   = help: it no longer has any effect to set the lint level on an individual match arm
+help: set the lint level on the whole match
+   |
+LL +     #[warn(non_exhaustive_omitted_patterns)]
+LL |     match val {
+   |
+
+error: aborting due to 2 previous errors; 3 warnings emitted
+
diff --git a/tests/ui/rfcs/rfc-2008-non-exhaustive/omitted-patterns-dont-lint-on-arm.normal.stderr b/tests/ui/rfcs/rfc-2008-non-exhaustive/omitted-patterns-dont-lint-on-arm.normal.stderr
new file mode 100644
index 00000000000..46093eb9fb3
--- /dev/null
+++ b/tests/ui/rfcs/rfc-2008-non-exhaustive/omitted-patterns-dont-lint-on-arm.normal.stderr
@@ -0,0 +1,31 @@
+error: some variants are not matched explicitly
+  --> $DIR/omitted-patterns-dont-lint-on-arm.rs:15:11
+   |
+LL |     match val {
+   |           ^^^ pattern `NonExhaustiveEnum::Struct { .. }` not covered
+   |
+   = help: ensure that all variants are matched explicitly by adding the suggested match arms
+   = note: the matched value is of type `NonExhaustiveEnum` and the `non_exhaustive_omitted_patterns` attribute was found
+note: the lint level is defined here
+  --> $DIR/omitted-patterns-dont-lint-on-arm.rs:14:12
+   |
+LL |     #[deny(non_exhaustive_omitted_patterns)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: the lint level must be set on the whole match
+  --> $DIR/omitted-patterns-dont-lint-on-arm.rs:34:9
+   |
+LL |         #[deny(non_exhaustive_omitted_patterns)]
+   |                ------------------------------- remove this attribute
+LL |         _ => {}
+   |         ^
+   |
+   = help: it no longer has any effect to set the lint level on an individual match arm
+help: set the lint level on the whole match
+   |
+LL +     #[deny(non_exhaustive_omitted_patterns)]
+LL |     match val {
+   |
+
+error: aborting due to previous error; 1 warning emitted
+
diff --git a/tests/ui/rfcs/rfc-2008-non-exhaustive/omitted-patterns-dont-lint-on-arm.rs b/tests/ui/rfcs/rfc-2008-non-exhaustive/omitted-patterns-dont-lint-on-arm.rs
new file mode 100644
index 00000000000..33f9f56a5bb
--- /dev/null
+++ b/tests/ui/rfcs/rfc-2008-non-exhaustive/omitted-patterns-dont-lint-on-arm.rs
@@ -0,0 +1,53 @@
+// revisions: normal lint
+// Test that putting the lint level on a match arm emits a warning, as this was previously
+// meaningful and is no longer.
+#![feature(non_exhaustive_omitted_patterns_lint)]
+
+// aux-build:enums.rs
+extern crate enums;
+
+use enums::NonExhaustiveEnum;
+
+fn main() {
+    let val = NonExhaustiveEnum::Unit;
+
+    #[deny(non_exhaustive_omitted_patterns)]
+    match val {
+        //~^ ERROR some variants are not matched explicitly
+        NonExhaustiveEnum::Unit => {}
+        NonExhaustiveEnum::Tuple(_) => {}
+        _ => {}
+    }
+
+    #[cfg_attr(lint, deny(non_exhaustive_omitted_patterns))]
+    match val {
+        //[lint]~^ ERROR some variants are not matched explicitly
+        NonExhaustiveEnum::Unit => {}
+        NonExhaustiveEnum::Tuple(_) => {}
+        _ => {}
+    }
+
+    match val {
+        NonExhaustiveEnum::Unit => {}
+        NonExhaustiveEnum::Tuple(_) => {}
+        #[deny(non_exhaustive_omitted_patterns)]
+        _ => {}
+    }
+    //~^^ WARN lint level must be set on the whole match
+
+    match val {
+        NonExhaustiveEnum::Unit => {}
+        NonExhaustiveEnum::Tuple(_) => {}
+        #[cfg_attr(lint, deny(non_exhaustive_omitted_patterns))]
+        _ => {}
+    }
+    //[lint]~^^ WARN lint level must be set on the whole match
+
+    match val {
+        NonExhaustiveEnum::Unit => {}
+        NonExhaustiveEnum::Tuple(_) => {}
+        #[cfg_attr(lint, warn(non_exhaustive_omitted_patterns))]
+        _ => {}
+    }
+    //[lint]~^^ WARN lint level must be set on the whole match
+}