Add a span to CompilerBuiltinsCannotCall

[tgross35]

Currently, this error emit a diagnostic with no context like:

error: `compiler_builtins` cannot call functions through upstream monomorphizations; encountered invalid call from `<math::libm::support::hex_float::Hexf<i32> as core::fmt::LowerHex>::fmt` to `core::fmt::num::<impl core::fmt::LowerHex for i32>::fmt`

With this change, it at least usually points to the problematic function:

error: `compiler_builtins` cannot call functions through upstream monomorphizations; encountered invalid call from `<math::libm::support::hex_float::Hexf<i32> as core::fmt::LowerHex>::fmt` to `core::fmt::num::<impl core::fmt::LowerHex for i32>::fmt`
   --> src/../libm/src/math/support/hex_float.rs:270:5
    |
270 |     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |

[rustbot]

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[rustbot]

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

[compiler-errors]

r=me after fixing this commit (pls just amend it, don't push another one) and CI is green

[tgross35]

Hang on - sometimes this is giving a bad error where it points at two trait functions in core rather than whatever the problem in compiler-builtins is

error: `compiler_builtins` cannot call functions through upstream monomorphizations; encountered invalid call from `core::fmt::num::<impl core::fmt::Debug for i128>::fmt` to `core::fmt::num::<impl core::fmt::LowerHex for i128>::fmt`
   --> /Users/tmgross/Documents/projects/rust/library/core/src/fmt/num.rs:516:1
    |
516 | / impl_Debug! {
517 | |     i8 i16 i32 i64 i128 isize
518 | |     u8 u16 u32 u64 u128 usize
519 | | }
    | |_^
    |
    = note: this error originates in the macro `impl_Debug` (in Nightly builds, run with -Z macro-backtrace for more info)

I'm not exactly sure what the code in this area is doing, but based on the results it looks like maybe:

• compiler-builtins is calling i128::Debug somewhere, this is getting inlined
• i128::Debug is calling i128::LowerHex, which is also getting lined
• The error is getting raised because this call to LowerHex is (via inlining) within compiler-builtins, but it's showing up as a problem in core because fcx is for i128::Debug

@compiler-errors any idea if the span in compiler-builtins is readily available here?

(not the biggest deal since it's an internal error, but I haven't been very successful hunting these errors)

[tgross35]

@bors r=compiler-errors rollup

[bors]

📌 Commit 7a2db88 has been approved by compiler-errors

It is now in the queue for this repository.