Span for asm error too large

[sophiajt]

Looking around the compile-fail test suite, I noticed this example:

jturner-23759:rust jturner$ ./build/x86_64-apple-darwin/stage1/bin/rustc src/test/compile-fail/asm-out-assign-imm.rs 
error[E0384]: re-assignment of immutable variable `x`
  --> src/test/compile-fail/asm-out-assign-imm.rs:24:9
   |
21 |     x = 1; //~ NOTE first assignment
   |     ----- first assignment to `x`
...
24 |         asm!("mov $1, $0" : "=r"(x) : "r"(5));
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ re-assignment of immutable variable
src/test/compile-fail/asm-out-assign-imm.rs:24:9: 24:47 note: in this expansion of asm!

The span on the asm line seems a bit much. Ideally, I think we'd show them where the assignment is happening so they don't have to look for it. Something like:

24 |         asm!("mov $1, $0" : "=r"(x) : "r"(5));
   |                             ^^^^^^^ re-assignment of immutable variable
src/test/compile-fail/asm-out-assign-imm.rs:24:9: 24:47 note: in this expansion of asm!

Or even:

24 |         asm!("mov $1, $0" : "=r"(x) : "r"(5));
   |                                  ^ re-assignment of immutable variable
src/test/compile-fail/asm-out-assign-imm.rs:24:9: 24:47 note: in this expansion of asm!

[nagisa]

In the compiler inline assembly is one big opaque blob. I doubt its worth much spending effort on such unstable feature yet.

[sophiajt]

@nagisa - I was wondering. It does open up questions about how to make macros give better errors but perhaps there is a better forum for that design question.