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/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
 

src/test/ui/issue-27060.rs

@@ -0,0 +1,43 @@
+// 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 is unsafe
+                        //~| hard error
+    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
+    let _ = &good.aligned[2]; // ok, has align 1
+}

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
+

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
 

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
 

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