Emit `note` when calling `rustc` without specifying an edition

[estebank]

Proposal

Cargo currently follows the edition specified in Cargo.toml. When doing cargo init, the latest stable edition is selected to create a new crate. rustc, to maintain backwards compatibility, doesn't change the implicit edition when unspecified from the original "2015". But nowadays, users are unlikely to be following documentation targeting such an old edition, and some of the differences can be confusing when following instructions result on a failing build. Improving diagnostics for behavior that diverges between editions is one mitigation, but we should also be nudging people to be explicit on which edition they meant. The brunt of the effect toolchain behavior is mainly forced on people just starting to learn Rust, and it can lead to confusion.

Implementation-wise, the changes are minimal if we go with a plain unsilenceable note: rust-lang/rust#158102

Mentors or Reviewers

@estebank

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member who is knowledgeable in the area can second by writing @rustbot second or kickoff a team FCP with @rfcbot fcp $RESOLUTION.
  • Refer to Proposals, Approvals and Stabilization docs for when a second is sufficient, or when a full team FCP is required.
• Once an MCP is seconded, the Final Comment Period begins.
  • Final Comment Period lasts for 10 days after all outstanding concerns are solved.
  • Outstanding concerns will block the Final Comment Period from finishing. Once all concerns are resolved, the 10 day countdown is restarted.
  • If no concerns are raised after 10 days since the resolution of the last outstanding concern, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

[rustbot]

Important

This issue is not meant to be used for technical discussion. There is a Zulip stream for that.
Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

Concerns or objections can formally be registered here by adding a comment.

@rustbot concern reason-for-concern
<description of the concern>

Concerns can be lifted with:

@rustbot resolve reason-for-concern

See documentation at https://forge.rust-lang.org

cc @rust-lang/compiler

[jieyouxu]

Given that this is a easily reversible note
@rustbot second