Tracking issue for future-incompatibility lint `float_literal_f32_fallback`

[folkertdev]

Lint on cases where {float} falls back to f32 instead of the default f64.

If there is only a single instance of a trait, like From<f32> for f32, it is used to guide type inference. That enables expressions like f32::from(0.0). With the addition of From<f16> for f32 this mechanism breaks, and the example would stop to compile.

To prevent this breakage, a workaround was introduced in the type checker. However, we'd like to remove this workaround in the future. Hence, start emitting a warning whenever this workaround is used.

This is a tracking issue for the RFC "XXX" (rust-lang/rfcs#NNN).

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Steps

• Implement the RFC (cc @rust-lang/XXX -- can anyone write up mentoring
  instructions?)
• Adjust documentation (see instructions on rustc-dev-guide)
• Style updates for any new syntax (nightly-style-procedure)
  • Style team decision on new formatting
  • Formatting for new syntax has been added to the Style Guide
  • (non-blocking) Formatting has been implemented in rustfmt
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

XXX --- list all the "unresolved questions" found in the RFC to ensure they are
not forgotten

Implementation history