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).

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

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