link to the rustc-dev-guide on cycle errors

[jyn514]

right now, new contributors to the compiler can be easily confused by the cycle error - queries are not common in other production compilers (that i know of) and i think it would make it a lot easier to understand if we linked to the docs: https://rustc-dev-guide.rust-lang.org/overview.html#queries / https://rustc-dev-guide.rust-lang.org/query.html

the error reporting has changed a bit since i last looked at it, but i think

rust/compiler/rustc_query_system/src/error.rs

Lines 46 to 60 in a7838d8

	#[derive(Diagnostic)]
	#[diag(query_system_cycle, code = "E0391")]
	pub struct Cycle {
	#[primary_span]
	pub span: Span,
	pub stack_bottom: String,
	#[subdiagnostic]
	pub cycle_stack: Vec<CycleStack>,
	#[subdiagnostic]
	pub stack_count: StackCount,
	#[subdiagnostic]
	pub alias: Option<Alias>,
	#[subdiagnostic]
	pub cycle_usage: Option<CycleUsage>,
	}

is the right place.

cc @jieyouxu

[zaneduffield]

Hi @jyn514, is it normal to link to developer docs in compiler error reporting? Maybe it would be suitable to have this information in the output of rustc --explain E0391? The current output only shows an example of a cyclic trait dependency.

[jyn514]

@zaneduffield the cycle error is unlike other errors in that it's much more likely to trigger it by accident while working on the compiler itself.

i don't mind adding info to --explain but i think we should do that in addition to an inline link, not instead of.

[RickleAndMortimer]

Hey everyone, I have a patch that adds the links in the --explain error code and compiler error. Is it alright if I submit a PR or are we still deciding on a approach currently?

[jyn514]

@RickleAndMortimer a PR would be great, thanks!