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

20 files changed, 69 insertions, 34 deletions

diff --git a/src/test/compile-fail/foreign-unsafe-fn-called.rs b/src/test/compile-fail/foreign-unsafe-fn-called.rs
index 88fe9cd44a3..5fad4c3677c 100644
--- a/src/test/compile-fail/foreign-unsafe-fn-called.rs
+++ b/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
 }
diff --git a/src/test/compile-fail/forget-init-unsafe.rs b/src/test/compile-fail/init-unsafe.rs
index 48c9fda31e8..9e599cebd7a 100644
--- a/src/test/compile-fail/forget-init-unsafe.rs
+++ b/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
 }
diff --git a/src/test/compile-fail/issue-43733.rs b/src/test/compile-fail/issue-43733.rs
index 90ccc589b4e..7b1364ff41c 100644
--- a/src/test/compile-fail/issue-43733.rs
+++ b/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()));
diff --git a/src/test/compile-fail/issue-45087-unreachable-unsafe.rs b/src/test/compile-fail/issue-45087-unreachable-unsafe.rs
index eeb66fa0e2c..5af0bf6be56 100644
--- a/src/test/compile-fail/issue-45087-unreachable-unsafe.rs
+++ b/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
 }
diff --git a/src/test/compile-fail/issue-45729-unsafe-in-generator.rs b/src/test/compile-fail/issue-45729-unsafe-in-generator.rs
index 489e91797f3..b42ced07583 100644
--- a/src/test/compile-fail/issue-45729-unsafe-in-generator.rs
+++ b/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;
     };
 }
diff --git a/src/test/compile-fail/issue-47412.rs b/src/test/compile-fail/issue-47412.rs
index 7481befcb79..683ef876f4e 100644
--- a/src/test/compile-fail/issue-47412.rs
+++ b/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() {}
diff --git a/src/test/compile-fail/safe-extern-statics-mut.rs b/src/test/compile-fail/safe-extern-statics-mut.rs
index ed8d5900776..35ea6dd52c4 100644
--- a/src/test/compile-fail/safe-extern-statics-mut.rs
+++ b/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
 }
diff --git a/src/test/compile-fail/safe-extern-statics.rs b/src/test/compile-fail/safe-extern-statics.rs
index 4d939f33c46..83aa4b3316d 100644
--- a/src/test/compile-fail/safe-extern-statics.rs
+++ b/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
 }
diff --git a/src/test/compile-fail/union/union-unsafe.rs b/src/test/compile-fail/union/union-unsafe.rs
index e57d65dcb89..d993816e6f4 100644
--- a/src/test/compile-fail/union/union-unsafe.rs
+++ b/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
 }
diff --git a/src/test/compile-fail/unsafe-fn-assign-deref-ptr.rs b/src/test/compile-fail/unsafe-fn-assign-deref-ptr.rs
index f30da250f6a..baf2002a94f 100644
--- a/src/test/compile-fail/unsafe-fn-assign-deref-ptr.rs
+++ b/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;
 }
 
diff --git a/src/test/compile-fail/unsafe-fn-called-from-safe.rs b/src/test/compile-fail/unsafe-fn-called-from-safe.rs
index 15bcad95cb2..46f28da43d0 100644
--- a/src/test/compile-fail/unsafe-fn-called-from-safe.rs
+++ b/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
 }
diff --git a/src/test/compile-fail/unsafe-fn-deref-ptr.rs b/src/test/compile-fail/unsafe-fn-deref-ptr.rs
index bf87df71fd8..8e3ce8ff9b5 100644
--- a/src/test/compile-fail/unsafe-fn-deref-ptr.rs
+++ b/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() {
diff --git a/src/test/compile-fail/unsafe-fn-used-as-value.rs b/src/test/compile-fail/unsafe-fn-used-as-value.rs
index 8e2d82b8fdb..f09a0c7107a 100644
--- a/src/test/compile-fail/unsafe-fn-used-as-value.rs
+++ b/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
 }
diff --git a/src/test/compile-fail/unsafe-move-val-init.rs b/src/test/compile-fail/unsafe-move-val-init.rs
index 84a8c84a0db..b4e425e7bbf 100644
--- a/src/test/compile-fail/unsafe-move-val-init.rs
+++ b/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
 }
diff --git a/src/test/ui/error-codes/E0133.stderr b/src/test/ui/error-codes/E0133.stderr
index dc57a627444..9be80f8f21b 100644
--- a/src/test/ui/error-codes/E0133.stderr
+++ b/src/test/ui/error-codes/E0133.stderr
@@ -1,8 +1,10 @@
-error[E0133]: call to unsafe function requires unsafe function or block
+error[E0133]: call to unsafe function is unsafe and requires unsafe function or block
   --> $DIR/E0133.rs:14:5
    |
 LL |     f();
    |     ^^^ call to unsafe function
+   |
+   = note: consult the function's documentation for information on how to avoid undefined behavior
 
 error: aborting due to previous error
 
diff --git a/src/test/compile-fail/issue-27060.rs b/src/test/ui/issue-27060.rs
index 37369d551fc..f88c2137e77 100644
--- a/src/test/compile-fail/issue-27060.rs
+++ b/src/test/ui/issue-27060.rs
@@ -33,9 +33,9 @@ fn main() {
         let _ = &good.data2[0]; // ok
     }
 
-    let _ = &good.data; //~ ERROR borrow of packed field requires unsafe
+    let _ = &good.data; //~ ERROR borrow of packed field is unsafe
                         //~| hard error
-    let _ = &good.data2[0]; //~ ERROR borrow of packed field requires unsafe
+    let _ = &good.data2[0]; //~ ERROR borrow of packed field is unsafe
                             //~| hard error
     let _ = &*good.data; // ok, behind a pointer
     let _ = &good.aligned; // ok, has align 1
diff --git a/src/test/ui/issue-27060.stderr b/src/test/ui/issue-27060.stderr
new file mode 100644
index 00000000000..bd01f75d8fb
--- /dev/null
+++ b/src/test/ui/issue-27060.stderr
@@ -0,0 +1,27 @@
+error: borrow of packed field is unsafe and requires unsafe function or block (error E0133)
+  --> $DIR/issue-27060.rs:36:13
+   |
+LL |     let _ = &good.data; //~ ERROR borrow of packed field is unsafe
+   |             ^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/issue-27060.rs:23:8
+   |
+LL | #[deny(safe_packed_borrows)]
+   |        ^^^^^^^^^^^^^^^^^^^
+   = 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 #46043 <https://github.com/rust-lang/rust/issues/46043>
+   = note: fields of packed structs might be misaligned: dereferencing a misaligned pointer or even just creating a misaligned reference is undefined behavior
+
+error: borrow of packed field is unsafe and requires unsafe function or block (error E0133)
+  --> $DIR/issue-27060.rs:38:13
+   |
+LL |     let _ = &good.data2[0]; //~ ERROR borrow of packed field is unsafe
+   |             ^^^^^^^^^^^^^^
+   |
+   = 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 #46043 <https://github.com/rust-lang/rust/issues/46043>
+   = note: fields of packed structs might be misaligned: dereferencing a misaligned pointer or even just creating a misaligned reference is undefined behavior
+
+error: aborting due to 2 previous errors
+
diff --git a/src/test/ui/issue-28776.stderr b/src/test/ui/issue-28776.stderr
index 3b468a88205..aef0d9cd1d8 100644
--- a/src/test/ui/issue-28776.stderr
+++ b/src/test/ui/issue-28776.stderr
@@ -1,8 +1,10 @@
-error[E0133]: call to unsafe function requires unsafe function or block
+error[E0133]: call to unsafe function is unsafe and requires unsafe function or block
   --> $DIR/issue-28776.rs:14:5
    |
 LL |     (&ptr::write)(1 as *mut _, 42);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
+   |
+   = note: consult the function's documentation for information on how to avoid undefined behavior
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/trait-safety-fn-body.stderr b/src/test/ui/trait-safety-fn-body.stderr
index 432df438222..0b7b6e61678 100644
--- a/src/test/ui/trait-safety-fn-body.stderr
+++ b/src/test/ui/trait-safety-fn-body.stderr
@@ -1,8 +1,10 @@
-error[E0133]: dereference of raw pointer requires unsafe function or block
+error[E0133]: dereference of raw pointer is unsafe and requires unsafe function or block
   --> $DIR/trait-safety-fn-body.rs:21:9
    |
 LL |         *self += 1;
    |         ^^^^^^^^^^ dereference of raw pointer
+   |
+   = note: raw pointers may be NULL, dangling or unaligned; they can violate aliasing rules and cause data races: all of these are undefined behavior
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/unsafe-const-fn.stderr b/src/test/ui/unsafe-const-fn.stderr
index 270b90ec3fc..d4b3ed687e5 100644
--- a/src/test/ui/unsafe-const-fn.stderr
+++ b/src/test/ui/unsafe-const-fn.stderr
@@ -1,8 +1,10 @@
-error[E0133]: call to unsafe function requires unsafe function or block
+error[E0133]: call to unsafe function is unsafe and requires unsafe function or block
   --> $DIR/unsafe-const-fn.rs:19:18
    |
 LL | const VAL: u32 = dummy(0xFFFF);
    |                  ^^^^^^^^^^^^^ call to unsafe function
+   |
+   = note: consult the function's documentation for information on how to avoid undefined behavior
 
 error: aborting due to previous error