Rollup merge of #52207 - RalfJung:unsafety-errors, r=estebank

improve error message shown for unsafe operations

Add a short explanation saying why undefined behavior could arise. In particular, the error many people got for "creating a pointer to a packed field requires unsafe block" was not worded great -- it lead to people just adding the unsafe block without considering if what they are doing follows the rules.

I am not sure if a "note" is the right thing, but that was the easiest thing to add...

Inspired by @gnzlbg at https://github.com/rust-lang/rust/issues/46043#issuecomment-381544673

src/test/compile-fail/foreign-unsafe-fn-called.rs

@@ -17,5 +17,5 @@ mod test {
 
 fn main() {
     test::free();
-    //~^ ERROR call to unsafe function requires unsafe function or block
+    //~^ ERROR call to unsafe function is unsafe
 }

src/test/compile-fail/init-unsafe.rs

@@ -12,7 +12,7 @@
 
 use std::intrinsics::{init};
 
-// Test that the `forget` and `init` intrinsics are really unsafe
+// Test that the `init` intrinsic is really unsafe
 pub fn main() {
-    let stuff = init::<isize>(); //~ ERROR call to unsafe function requires unsafe
+    let stuff = init::<isize>(); //~ ERROR call to unsafe function is unsafe
 }

src/test/compile-fail/issue-27060.rs

@@ -1,43 +0,0 @@
-// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-#[repr(packed)]
-pub struct Good {
-    data: &'static u32,
-    data2: [&'static u32; 2],
-    aligned: [u8; 32],
-}
-
-#[repr(packed)]
-pub struct JustArray {
-    array: [u32]
-}
-
-#[deny(safe_packed_borrows)]
-fn main() {
-    let good = Good {
-        data: &0,
-        data2: [&0, &0],
-        aligned: [0; 32]
-    };
-
-    unsafe {
-        let _ = &good.data; // ok
-        let _ = &good.data2[0]; // ok
-    }
-
-    let _ = &good.data; //~ ERROR borrow of packed field requires unsafe
-                        //~| hard error
-    let _ = &good.data2[0]; //~ ERROR borrow of packed field requires unsafe
-                            //~| hard error
-    let _ = &*good.data; // ok, behind a pointer
-    let _ = &good.aligned; // ok, has align 1
-    let _ = &good.aligned[2]; // ok, has align 1
-}

src/test/compile-fail/issue-43733.rs

@@ -27,12 +27,12 @@ fn __getit() -> std::option::Option<
     &'static std::cell::UnsafeCell<
         std::option::Option<Foo>>>
 {
-    __KEY.get() //~ ERROR call to unsafe function requires unsafe
+    __KEY.get() //~ ERROR call to unsafe function is unsafe
 }
 
 static FOO: std::thread::LocalKey<Foo> =
     std::thread::LocalKey::new(__getit, Default::default);
-//~^ ERROR call to unsafe function requires unsafe
+//~^ ERROR call to unsafe function is unsafe
 
 fn main() {
     FOO.with(|foo| println!("{}", foo.borrow()));

src/test/compile-fail/issue-45087-unreachable-unsafe.rs

@@ -11,5 +11,5 @@
 fn main() {
     return;
     *(1 as *mut u32) = 42;
-    //~^ ERROR dereference of raw pointer requires unsafe
+    //~^ ERROR dereference of raw pointer is unsafe
 }

src/test/compile-fail/issue-45729-unsafe-in-generator.rs

@@ -13,7 +13,7 @@
 fn main() {
     let _ = || {
         *(1 as *mut u32) = 42;
-        //~^ ERROR dereference of raw pointer requires unsafe
+        //~^ ERROR dereference of raw pointer is unsafe
         yield;
     };
 }

src/test/compile-fail/issue-47412.rs

@@ -19,13 +19,13 @@ fn union_field() {
     union Union { unit: (), void: Void }
     let u = Union { unit: () };
     match u.void {}
-    //~^ ERROR access to union field requires unsafe function or block
+    //~^ ERROR access to union field is unsafe
 }
 
 fn raw_ptr_deref() {
     let ptr = std::ptr::null::<Void>();
     match *ptr {}
-    //~^ ERROR dereference of raw pointer requires unsafe function or block
+    //~^ ERROR dereference of raw pointer is unsafe
 }
 
 fn main() {}

src/test/compile-fail/safe-extern-statics-mut.rs

@@ -18,8 +18,8 @@ extern {
 }
 
 fn main() {
-    let b = B; //~ ERROR use of mutable static requires unsafe function or block
-    let rb = &B; //~ ERROR use of mutable static requires unsafe function or block
-    let xb = XB; //~ ERROR use of mutable static requires unsafe function or block
-    let xrb = &XB; //~ ERROR use of mutable static requires unsafe function or block
+    let b = B; //~ ERROR use of mutable static is unsafe
+    let rb = &B; //~ ERROR use of mutable static is unsafe
+    let xb = XB; //~ ERROR use of mutable static is unsafe
+    let xrb = &XB; //~ ERROR use of mutable static is unsafe
 }

src/test/compile-fail/safe-extern-statics.rs

@@ -20,12 +20,12 @@ extern {
 }
 
 fn main() {
-    let a = A; //~ ERROR use of extern static requires unsafe function or block
+    let a = A; //~ ERROR use of extern static is unsafe
                //~^ WARN this was previously accepted by the compiler
-    let ra = &A; //~ ERROR use of extern static requires unsafe function or block
+    let ra = &A; //~ ERROR use of extern static is unsafe
                  //~^ WARN this was previously accepted by the compiler
-    let xa = XA; //~ ERROR use of extern static requires unsafe function or block
+    let xa = XA; //~ ERROR use of extern static is unsafe
                  //~^ WARN this was previously accepted by the compiler
-    let xra = &XA; //~ ERROR use of extern static requires unsafe function or block
+    let xra = &XA; //~ ERROR use of extern static is unsafe
                    //~^ WARN this was previously accepted by the compiler
 }

src/test/compile-fail/union/union-unsafe.rs

@@ -28,7 +28,7 @@ union U4<T: Copy> {
 
 fn generic_noncopy<T: Default>() {
     let mut u3 = U3 { a: T::default() };
-    u3.a = T::default(); //~ ERROR assignment to non-`Copy` union field requires unsafe
+    u3.a = T::default(); //~ ERROR assignment to non-`Copy` union field is unsafe
 }
 
 fn generic_copy<T: Copy + Default>() {
@@ -40,16 +40,16 @@ fn generic_copy<T: Copy + Default>() {
 
 fn main() {
     let mut u1 = U1 { a: 10 }; // OK
-    let a = u1.a; //~ ERROR access to union field requires unsafe
+    let a = u1.a; //~ ERROR access to union field is unsafe
     u1.a = 11; // OK
-    let U1 { a } = u1; //~ ERROR access to union field requires unsafe
-    if let U1 { a: 12 } = u1 {} //~ ERROR access to union field requires unsafe
+    let U1 { a } = u1; //~ ERROR access to union field is unsafe
+    if let U1 { a: 12 } = u1 {} //~ ERROR access to union field is unsafe
     // let U1 { .. } = u1; // OK
 
     let mut u2 = U2 { a: String::from("old") }; // OK
-    u2.a = String::from("new"); //~ ERROR assignment to non-`Copy` union field requires unsafe
+    u2.a = String::from("new"); //~ ERROR assignment to non-`Copy` union field is unsafe
     let mut u3 = U3 { a: 0 }; // OK
     u3.a = 1; // OK
     let mut u3 = U3 { a: String::from("old") }; // OK
-    u3.a = String::from("new"); //~ ERROR assignment to non-`Copy` union field requires unsafe
+    u3.a = String::from("new"); //~ ERROR assignment to non-`Copy` union field is unsafe
 }

src/test/compile-fail/unsafe-fn-assign-deref-ptr.rs

@@ -11,7 +11,7 @@
 
 
 fn f(p: *mut u8) {
-    *p = 0; //~ ERROR dereference of raw pointer requires unsafe function or block
+    *p = 0; //~ ERROR dereference of raw pointer is unsafe
     return;
 }
 

src/test/compile-fail/unsafe-fn-called-from-safe.rs

@@ -12,5 +12,5 @@
 unsafe fn f() { return; }
 
 fn main() {
-    f(); //~ ERROR call to unsafe function requires unsafe function or block
+    f(); //~ ERROR call to unsafe function is unsafe
 }

src/test/compile-fail/unsafe-fn-deref-ptr.rs

@@ -10,7 +10,7 @@
 
 
 fn f(p: *const u8) -> u8 {
-    return *p; //~ ERROR dereference of raw pointer requires unsafe function or block
+    return *p; //~ ERROR dereference of raw pointer is unsafe
 }
 
 fn main() {

src/test/compile-fail/unsafe-fn-used-as-value.rs

@@ -13,5 +13,5 @@ unsafe fn f() { return; }
 
 fn main() {
     let x = f;
-    x();    //~ ERROR call to unsafe function requires unsafe function or block
+    x();    //~ ERROR call to unsafe function is unsafe
 }

src/test/compile-fail/unsafe-move-val-init.rs

@@ -16,5 +16,5 @@ use std::intrinsics;
 // as unsafe.
 fn main() {
     intrinsics::move_val_init(1 as *mut u32, 1);
-    //~^ ERROR dereference of raw pointer requires unsafe function or block
+    //~^ ERROR dereference of raw pointer is unsafe
 }