give some context in error messages

[RalfJung]

Some examples for how different errors look now

Unsupported operation:

error: unsupported operation: Miri does not support threading
  --> /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/sys/unix/thread.rs:68:19
   |
68 |         let ret = libc::pthread_create(&mut native, &attr, thread_start, &*p as *const _ as *mut _);
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Miri does not support threading
   |
   = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support

Unsupported operation that works without isolation:

error: unsupported operation: `clock_gettime` not available when isolation is enabled
   --> /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/sys/unix/time.rs:349:22
    |
349 |         cvt(unsafe { libc::clock_gettime(clock, &mut t.t) }).unwrap();
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `clock_gettime` not available when isolation is enabled
    |
    = help: pass the flag `-Zmiri-disable-isolation` to disable isolation

Program abort:

error: program stopped: the evaluated program aborted execution
   --> /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/panicking.rs:530:18
    |
530 |         unsafe { intrinsics::abort() }
    |                  ^^^^^^^^^^^^^^^^^^^ the evaluated program aborted execution
    |

UB:

error: Undefined Behavior: type validation failed: encountered 2, but expected a boolean
 --> tests/compile-fail/validity/invalid_bool.rs:2:23
  |
2 |     let _b = unsafe { std::mem::transmute::<u8, bool>(2) }; //~ ERROR encountered 2, but expected a boolean
  |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 2, but expected a boolean
  |
  = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior

Experimental UB:

error: Undefined Behavior: not granting access to tag <1562> because incompatible item is protected: [Unique for <1567> (call 1189)]
  --> tests/compile-fail/stacked_borrows/aliasing_mut1.rs:3:1
   |
3  | pub fn safe(_x: &mut i32, _y: &mut i32) {} //~ ERROR protect
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not granting access to tag <1562> because incompatible item is protected: [Unique for <1567> (call 1189)]
   |
   = help: this indicates a potential bug in the program: it performed an invalid operation, but the rules it violated are still experimental

Fixes #417

[oli-obk]

PR lgtm, just a general question, if we are explaining things to users, should we link them to appropriate issues or so where they can get more info?

[RalfJung]

I mean long-term it would be really cool for every "this is UB" message to link to the part of the Reference that says why this, specifically, is UB -- but that will need some more work I think.

Right now, I am not sure what things we could specifically link to?

[oli-obk]

Maybe the ucg repo?

[RalfJung]

For most UB, that doesn't really have a lot to say, I feel. But we could link to the general reference UB page:

error: Undefined Behavior: type validation failed: encountered 2, but expected a boolean
 --> tests/compile-fail/validity/invalid_bool.rs:2:23
  |
2 |     let _b = unsafe { std::mem::transmute::<u8, bool>(2) }; //~ ERROR encountered 2, but expected a boolean
  |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 2, but expected a boolean
  |
  = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
  = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

And for Stacked Borrows:

error: Undefined Behavior: not granting access to tag <1562> because incompatible item is protected: [Unique for <1567> (call 1189)]
  --> tests/compile-fail/stacked_borrows/aliasing_mut1.rs:3:1
   |
3  | pub fn safe(_x: &mut i32, _y: &mut i32) {} //~ ERROR protect
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not granting access to tag <1562> because incompatible item is protected: [Unique for <1567> (call 1189)]
   |
   = help: this indicates a potential bug in the program: it performed an invalid operation, but the rules it violated are still experimental
   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

[oli-obk]

@bors r+

[bors]

📌 Commit 6e302b8 has been approved by oli-obk

[bors]

⌛ Testing commit 6e302b8 with merge 9fe39ba...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: oli-obk
Pushing 9fe39ba to master...

[shepmaster]

Those messages are great!

[RalfJung]

It took almost 2 years but finally we are there. ;)