fix: don't panic on volatile access to null

According to
https://discourse.llvm.org/t/rfc-volatile-access-to-non-dereferenceable-memory-may-be-well-defined/86303/4,
LLVM allows volatile operations on null and handles it correctly. This
should be allowed in Rust as well, because I/O memory may be hard-coded
to address 0 in some cases, like the AVR chip ATtiny1626.

A test case that ensured a failure when passing null to volatile was
removed, since it's now valid.

Due to the addition of `maybe_is_aligned` to `ub_checks`,
`maybe_is_aligned_and_not_null` was refactored to use it.

docs: revise restrictions on volatile operations

A distinction between usage on Rust memory vs. non-Rust memory was
introduced. Documentation was reworded to explain what that means, and
make explicit that:

- No trapping can occur from volatile operations;
- On Rust memory, all safety rules must be respected;
- On Rust memory, the primary difference from regular access is that
  volatile always involves a memory dereference;
- On Rust memory, the only data affected by an operation is the one
  pointed to in the argument(s) of the function;
- On Rust memory, provenance follows the same rules as non-volatile
  access;
- On non-Rust memory, any address known to not contain Rust memory is
  valid (including 0 and usize::MAX);
- On non-Rust memory, no Rust memory may be affected (it is implicit
  that any other non-Rust memory may be affected, though, even if not
  referenced by the pointer). This should be relevant when, for example,
  reading register A causes a flag to change in register B, or writing
  to A causes B to change in some way. Everything affected mustn't be
  inside an allocation.
- On non-Rust memory, provenance is irrelevant and a pointer with none
  can be used in a valid way.

fix: don't lint null as UB for volatile

Also remove a now-unneeded `allow` line.

fix: additional wording nits

4 files changed, 18 insertions, 57 deletions

diff --git a/tests/ui/lint/invalid_null_args.rs b/tests/ui/lint/invalid_null_args.rs
index f40f06a0d36..ee29d622ad7 100644
--- a/tests/ui/lint/invalid_null_args.rs
+++ b/tests/ui/lint/invalid_null_args.rs
@@ -58,10 +58,9 @@ unsafe fn null_ptr() {
     let _a: A = ptr::read_unaligned(ptr::null_mut());
     //~^ ERROR calling this function with a null pointer is undefined behavior
 
+    // These two should *not* fire the lint.
     let _a: A = ptr::read_volatile(ptr::null());
-    //~^ ERROR calling this function with a null pointer is undefined behavior
     let _a: A = ptr::read_volatile(ptr::null_mut());
-    //~^ ERROR calling this function with a null pointer is undefined behavior
 
     let _a: A = ptr::replace(ptr::null_mut(), v);
     //~^ ERROR calling this function with a null pointer is undefined behavior
@@ -82,8 +81,8 @@ unsafe fn null_ptr() {
     ptr::write_unaligned(ptr::null_mut(), v);
     //~^ ERROR calling this function with a null pointer is undefined behavior
 
+    // This one should *not* fire the lint.
     ptr::write_volatile(ptr::null_mut(), v);
-    //~^ ERROR calling this function with a null pointer is undefined behavior
 
     ptr::write_bytes::<usize>(ptr::null_mut(), 42, 0);
     //~^ ERROR calling this function with a null pointer is undefined behavior
diff --git a/tests/ui/lint/invalid_null_args.stderr b/tests/ui/lint/invalid_null_args.stderr
index 11c6270cfb7..028bd7051dc 100644
--- a/tests/ui/lint/invalid_null_args.stderr
+++ b/tests/ui/lint/invalid_null_args.stderr
@@ -164,27 +164,7 @@ LL |     let _a: A = ptr::read_unaligned(ptr::null_mut());
    = help: for more information, visit <https://doc.rust-lang.org/std/ptr/index.html> and <https://doc.rust-lang.org/reference/behavior-considered-undefined.html>
 
 error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused
-  --> $DIR/invalid_null_args.rs:61:17
-   |
-LL |     let _a: A = ptr::read_volatile(ptr::null());
-   |                 ^^^^^^^^^^^^^^^^^^^-----------^
-   |                                    |
-   |                                    null pointer originates from here
-   |
-   = help: for more information, visit <https://doc.rust-lang.org/std/ptr/index.html> and <https://doc.rust-lang.org/reference/behavior-considered-undefined.html>
-
-error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused
-  --> $DIR/invalid_null_args.rs:63:17
-   |
-LL |     let _a: A = ptr::read_volatile(ptr::null_mut());
-   |                 ^^^^^^^^^^^^^^^^^^^---------------^
-   |                                    |
-   |                                    null pointer originates from here
-   |
-   = help: for more information, visit <https://doc.rust-lang.org/std/ptr/index.html> and <https://doc.rust-lang.org/reference/behavior-considered-undefined.html>
-
-error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused
-  --> $DIR/invalid_null_args.rs:66:17
+  --> $DIR/invalid_null_args.rs:65:17
    |
 LL |     let _a: A = ptr::replace(ptr::null_mut(), v);
    |                 ^^^^^^^^^^^^^---------------^^^^
@@ -194,7 +174,7 @@ LL |     let _a: A = ptr::replace(ptr::null_mut(), v);
    = help: for more information, visit <https://doc.rust-lang.org/std/ptr/index.html> and <https://doc.rust-lang.org/reference/behavior-considered-undefined.html>
 
 error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused
-  --> $DIR/invalid_null_args.rs:69:5
+  --> $DIR/invalid_null_args.rs:68:5
    |
 LL |     ptr::swap::<A>(ptr::null_mut(), &mut v);
    |     ^^^^^^^^^^^^^^^---------------^^^^^^^^^
@@ -204,7 +184,7 @@ LL |     ptr::swap::<A>(ptr::null_mut(), &mut v);
    = help: for more information, visit <https://doc.rust-lang.org/std/ptr/index.html> and <https://doc.rust-lang.org/reference/behavior-considered-undefined.html>
 
 error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused
-  --> $DIR/invalid_null_args.rs:71:5
+  --> $DIR/invalid_null_args.rs:70:5
    |
 LL |     ptr::swap::<A>(&mut v, ptr::null_mut());
    |     ^^^^^^^^^^^^^^^^^^^^^^^---------------^
@@ -214,7 +194,7 @@ LL |     ptr::swap::<A>(&mut v, ptr::null_mut());
    = help: for more information, visit <https://doc.rust-lang.org/std/ptr/index.html> and <https://doc.rust-lang.org/reference/behavior-considered-undefined.html>
 
 error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused
-  --> $DIR/invalid_null_args.rs:74:5
+  --> $DIR/invalid_null_args.rs:73:5
    |
 LL |     ptr::swap_nonoverlapping::<A>(ptr::null_mut(), &mut v, 0);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^---------------^^^^^^^^^^^^
@@ -224,7 +204,7 @@ LL |     ptr::swap_nonoverlapping::<A>(ptr::null_mut(), &mut v, 0);
    = help: for more information, visit <https://doc.rust-lang.org/std/ptr/index.html> and <https://doc.rust-lang.org/reference/behavior-considered-undefined.html>
 
 error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused
-  --> $DIR/invalid_null_args.rs:76:5
+  --> $DIR/invalid_null_args.rs:75:5
    |
 LL |     ptr::swap_nonoverlapping::<A>(&mut v, ptr::null_mut(), 0);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^---------------^^^^
@@ -234,7 +214,7 @@ LL |     ptr::swap_nonoverlapping::<A>(&mut v, ptr::null_mut(), 0);
    = help: for more information, visit <https://doc.rust-lang.org/std/ptr/index.html> and <https://doc.rust-lang.org/reference/behavior-considered-undefined.html>
 
 error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused
-  --> $DIR/invalid_null_args.rs:79:5
+  --> $DIR/invalid_null_args.rs:78:5
    |
 LL |     ptr::write(ptr::null_mut(), v);
    |     ^^^^^^^^^^^---------------^^^^
@@ -244,7 +224,7 @@ LL |     ptr::write(ptr::null_mut(), v);
    = help: for more information, visit <https://doc.rust-lang.org/std/ptr/index.html> and <https://doc.rust-lang.org/reference/behavior-considered-undefined.html>
 
 error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused
-  --> $DIR/invalid_null_args.rs:82:5
+  --> $DIR/invalid_null_args.rs:81:5
    |
 LL |     ptr::write_unaligned(ptr::null_mut(), v);
    |     ^^^^^^^^^^^^^^^^^^^^^---------------^^^^
@@ -254,17 +234,7 @@ LL |     ptr::write_unaligned(ptr::null_mut(), v);
    = help: for more information, visit <https://doc.rust-lang.org/std/ptr/index.html> and <https://doc.rust-lang.org/reference/behavior-considered-undefined.html>
 
 error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused
-  --> $DIR/invalid_null_args.rs:85:5
-   |
-LL |     ptr::write_volatile(ptr::null_mut(), v);
-   |     ^^^^^^^^^^^^^^^^^^^^---------------^^^^
-   |                         |
-   |                         null pointer originates from here
-   |
-   = help: for more information, visit <https://doc.rust-lang.org/std/ptr/index.html> and <https://doc.rust-lang.org/reference/behavior-considered-undefined.html>
-
-error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused
-  --> $DIR/invalid_null_args.rs:88:5
+  --> $DIR/invalid_null_args.rs:87:5
    |
 LL |     ptr::write_bytes::<usize>(ptr::null_mut(), 42, 0);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^---------------^^^^^^^^
@@ -274,7 +244,7 @@ LL |     ptr::write_bytes::<usize>(ptr::null_mut(), 42, 0);
    = help: for more information, visit <https://doc.rust-lang.org/std/ptr/index.html> and <https://doc.rust-lang.org/reference/behavior-considered-undefined.html>
 
 error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused
-  --> $DIR/invalid_null_args.rs:93:18
+  --> $DIR/invalid_null_args.rs:92:18
    |
 LL |     let _a: u8 = ptr::read(const_ptr);
    |                  ^^^^^^^^^^^^^^^^^^^^
@@ -287,7 +257,7 @@ LL |     let null_ptr = ptr::null_mut();
    |                    ^^^^^^^^^^^^^^^
 
 error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused
-  --> $DIR/invalid_null_args.rs:100:5
+  --> $DIR/invalid_null_args.rs:99:5
    |
 LL |     std::slice::from_raw_parts::<()>(ptr::null(), 0);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-----------^^^^
@@ -297,7 +267,7 @@ LL |     std::slice::from_raw_parts::<()>(ptr::null(), 0);
    = help: for more information, visit <https://doc.rust-lang.org/std/ptr/index.html> and <https://doc.rust-lang.org/reference/behavior-considered-undefined.html>
 
 error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused
-  --> $DIR/invalid_null_args.rs:102:5
+  --> $DIR/invalid_null_args.rs:101:5
    |
 LL |     std::slice::from_raw_parts::<Zst>(ptr::null(), 0);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-----------^^^^
@@ -307,7 +277,7 @@ LL |     std::slice::from_raw_parts::<Zst>(ptr::null(), 0);
    = help: for more information, visit <https://doc.rust-lang.org/std/ptr/index.html> and <https://doc.rust-lang.org/reference/behavior-considered-undefined.html>
 
 error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused
-  --> $DIR/invalid_null_args.rs:104:5
+  --> $DIR/invalid_null_args.rs:103:5
    |
 LL |     std::slice::from_raw_parts_mut::<()>(ptr::null_mut(), 0);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^---------------^^^^
@@ -317,7 +287,7 @@ LL |     std::slice::from_raw_parts_mut::<()>(ptr::null_mut(), 0);
    = help: for more information, visit <https://doc.rust-lang.org/std/ptr/index.html> and <https://doc.rust-lang.org/reference/behavior-considered-undefined.html>
 
 error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused
-  --> $DIR/invalid_null_args.rs:106:5
+  --> $DIR/invalid_null_args.rs:105:5
    |
 LL |     std::slice::from_raw_parts_mut::<Zst>(ptr::null_mut(), 0);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^---------------^^^^
@@ -326,5 +296,5 @@ LL |     std::slice::from_raw_parts_mut::<Zst>(ptr::null_mut(), 0);
    |
    = help: for more information, visit <https://doc.rust-lang.org/std/ptr/index.html> and <https://doc.rust-lang.org/reference/behavior-considered-undefined.html>
 
-error: aborting due to 31 previous errors
+error: aborting due to 28 previous errors
 
diff --git a/tests/ui/precondition-checks/read_volatile.rs b/tests/ui/precondition-checks/read_volatile.rs
index ada8932c398..d858e6b939a 100644
--- a/tests/ui/precondition-checks/read_volatile.rs
+++ b/tests/ui/precondition-checks/read_volatile.rs
@@ -1,9 +1,7 @@
 //@ run-fail
 //@ compile-flags: -Copt-level=3 -Cdebug-assertions=no -Zub-checks=yes
 //@ error-pattern: unsafe precondition(s) violated: ptr::read_volatile requires
-//@ revisions: null misaligned
-
-#![allow(invalid_null_arguments)]
+//@ revisions: misaligned
 
 use std::ptr;
 
@@ -11,8 +9,6 @@ fn main() {
     let src = [0u16; 2];
     let src = src.as_ptr();
     unsafe {
-        #[cfg(null)]
-        ptr::read_volatile(ptr::null::<u8>());
         #[cfg(misaligned)]
         ptr::read_volatile(src.byte_add(1));
     }
diff --git a/tests/ui/precondition-checks/write_volatile.rs b/tests/ui/precondition-checks/write_volatile.rs
index 0d5ecb014b3..ddc882be432 100644
--- a/tests/ui/precondition-checks/write_volatile.rs
+++ b/tests/ui/precondition-checks/write_volatile.rs
@@ -1,9 +1,7 @@
 //@ run-fail
 //@ compile-flags: -Copt-level=3 -Cdebug-assertions=no -Zub-checks=yes
 //@ error-pattern: unsafe precondition(s) violated: ptr::write_volatile requires
-//@ revisions: null misaligned
-
-#![allow(invalid_null_arguments)]
+//@ revisions: misaligned
 
 use std::ptr;
 
@@ -11,8 +9,6 @@ fn main() {
     let mut dst = [0u16; 2];
     let mut dst = dst.as_mut_ptr();
     unsafe {
-        #[cfg(null)]
-        ptr::write_volatile(ptr::null_mut::<u8>(), 1u8);
         #[cfg(misaligned)]
         ptr::write_volatile(dst.byte_add(1), 1u16);
     }