improve error message shown for unsafe operations: explain why undefined behavior could arise

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

6 files changed, 82 insertions, 4 deletions

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/ui/issue-27060.rs b/src/test/ui/issue-27060.rs
new file mode 100644
index 00000000000..f88c2137e77
--- /dev/null
+++ b/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
+}
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