Turn pointer_structural_match forward compat lint into normal lint

This commit removes the pointer_structural_match forward compat lint
from the forward compatibility warning mechanism, as there is no concrete
plan for removal of matching on function pointers.

It also turns the lint from allow-by-default to warn-by-default,
as the behaviour might be surprising to users and they should specifically
opt into the dangers.
As the lint is recognized on older compilers, users can easily allow it
after having made the descision on whether they want to keep it in their
code or not.

Author: est31

compiler/rustc_lint_defs/src/builtin.rs

@@ -2277,19 +2277,16 @@ declare_lint! {
     ///
     /// ### Explanation
     ///
-    /// Previous versions of Rust allowed function pointers and wide raw pointers in patterns.
-    /// While these work in many cases as expected by users, it is possible that due to
-    /// optimizations pointers are "not equal to themselves" or pointers to different functions
-    /// compare as equal during runtime. This is because LLVM optimizations can deduplicate
-    /// functions if their bodies are the same, thus also making pointers to these functions point
-    /// to the same location. Additionally functions may get duplicated if they are instantiated
+    /// Use of function pointers and wide raw pointers in patterns works in many cases as
+    /// expected by users. However, it is possible that due to optimizations pointers are "not equal
+    /// to themselves" or pointers to different functions compare as equal during runtime.
+    /// This is because LLVM optimizations can deduplicate functions if their bodies are the same,
+    /// thus also making pointers to these functions point to the same location.
+    /// Additionally, functions may get duplicated if they are instantiated
     /// in different crates and not deduplicated again via LTO.
     pub POINTER_STRUCTURAL_MATCH,
-    Allow,
-    "pointers are not structural-match",
-    @future_incompatible = FutureIncompatibleInfo {
-        reference: "issue #62411 <https://github.com/rust-lang/rust/issues/70861>",
-    };
+    Warn,
+    "pointers are not structural-match"
 }
 
 declare_lint! {

library/core/src/marker.rs

@@ -243,6 +243,7 @@ pub trait StructuralPartialEq {
 ///
 /// const CFN: Wrap<fn(&())> = Wrap(higher_order);
 ///
+/// # #[allow(pointer_structural_match)]
 /// fn main() {
 ///     match CFN {
 ///         CFN => {}

tests/ui/consts/const_in_pattern/issue-44333.rs

@@ -1,7 +1,5 @@
 // run-pass
 
-#![warn(pointer_structural_match)]
-
 type Func = fn(usize, usize) -> usize;
 
 fn foo(a: usize, b: usize) -> usize { a + b }
@@ -17,9 +15,7 @@ const BAR: Func = bar;
 fn main() {
     match test(std::env::consts::ARCH.len()) {
         FOO => println!("foo"), //~ WARN pointers in patterns behave unpredictably
-        //~^ WARN will become a hard error
         BAR => println!("bar"), //~ WARN pointers in patterns behave unpredictably
-        //~^ WARN will become a hard error
         _ => unreachable!(),
     }
 }

tests/ui/consts/const_in_pattern/issue-44333.stderr

@@ -1,25 +1,16 @@
 warning: function pointers and unsized pointers in patterns behave unpredictably and should not be relied upon. See https://github.com/rust-lang/rust/issues/70861 for details.
-  --> $DIR/issue-44333.rs:19:9
+  --> $DIR/issue-44333.rs:17:9
    |
 LL |         FOO => println!("foo"),
    |         ^^^
    |
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #62411 <https://github.com/rust-lang/rust/issues/70861>
-note: the lint level is defined here
-  --> $DIR/issue-44333.rs:3:9
-   |
-LL | #![warn(pointer_structural_match)]
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: `#[warn(pointer_structural_match)]` on by default
 
 warning: function pointers and unsized pointers in patterns behave unpredictably and should not be relied upon. See https://github.com/rust-lang/rust/issues/70861 for details.
-  --> $DIR/issue-44333.rs:21:9
+  --> $DIR/issue-44333.rs:18:9
    |
 LL |         BAR => println!("bar"),
    |         ^^^
-   |
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #62411 <https://github.com/rust-lang/rust/issues/70861>
 
 warning: 2 warnings emitted
 

tests/ui/consts/issue-34784.rs

@@ -1,6 +1,6 @@
 // run-pass
 
-#![warn(pointer_structural_match)]
+
 #![allow(dead_code)]
 const C: *const u8 = &0;
 

tests/ui/pattern/usefulness/consts-opaque.rs

@@ -2,7 +2,7 @@
 // unnecessary warnings because const_to_pat.rs converts a constant pattern to a wildcard when the
 // constant is not allowed as a pattern. This is an edge case so we may not care to fix it.
 // See also https://github.com/rust-lang/rust/issues/78057
-
+#![allow(pointer_structural_match)]
 #![deny(unreachable_patterns)]
 
 #[derive(PartialEq)]

tests/ui/rfc-1445-restrict-constants-in-patterns/allow-hide-behind-direct-unsafe-ptr-embedded.rs

@@ -3,7 +3,7 @@
 
 // run-pass
 
-#![warn(pointer_structural_match)]
+#![deny(pointer_structural_match)]
 
 struct NoDerive(#[allow(unused_tuple_struct_fields)] i32);
 

tests/ui/rfc-1445-restrict-constants-in-patterns/allow-hide-behind-direct-unsafe-ptr-param.rs

@@ -3,7 +3,7 @@
 
 // run-pass
 
-#![warn(pointer_structural_match)]
+#![deny(pointer_structural_match)]
 
 struct NoDerive(#[allow(unused_tuple_struct_fields)] i32);
 

tests/ui/rfc-1445-restrict-constants-in-patterns/allow-hide-behind-indirect-unsafe-ptr-embedded.rs

@@ -3,7 +3,7 @@
 
 // run-pass
 
-#![warn(pointer_structural_match)]
+#![deny(pointer_structural_match)]
 
 struct NoDerive(#[allow(unused_tuple_struct_fields)] i32);
 

tests/ui/rfc-1445-restrict-constants-in-patterns/allow-hide-behind-indirect-unsafe-ptr-param.rs

@@ -3,7 +3,7 @@
 
 // run-pass
 
-#![warn(pointer_structural_match)]
+#![deny(pointer_structural_match)]
 
 struct NoDerive(#[allow(unused_tuple_struct_fields)] i32);