Suggest only Span without source changes when source code is unavailable

[xizheyin]

In order to be able to reproduce in ui tests the errors that rustc generates suggestions in std sources (essentially external macros), i.e. #142403, this PR make emit_suggestion_default suggest only the base information when source is not available and does not show the code change.

Now it's something like

rust/tests/ui/wf/wf-impl-self-type.stderr

Lines 8 to 10 in 9ba00e0

	note: required by an implicit `Sized` bound in `Option`
	--> $SRC_DIR/core/src/option.rs:LL:COL

cc #139316 (comment)

r? @jieyouxu

[rustbot]

jieyouxu is currently at their maximum review capacity.
They may take a while to respond.

[rustbot]

Some changes occurred in src/tools/compiletest

cc @jieyouxu

[xizheyin]

Looks like jieyou is busy

r? compiler

[jieyouxu]

This feels very sus. How does this handle stable vs beta vs nightly vs in-tree std sources?

I feel like there's a reason why ui tests don't get to access std sources, which is to prevent different diagnostics depending on availability of std sources, and which std sources?

[xizheyin]

How does this handle stable vs beta vs nightly vs in-tree std sources?

Is it possible to normalize the content of the source code and just let us know that the source code is displayed in stderr?

I'm curious what kind of impact different versions (stable/nightly/beta/in-tree) would have, I don't know enough about CI. Any examples of this?

[xizheyin]

Path remapped is introduced in #105500 may cc @oli-obk
Indeed it's not that simple, some of the existing test surprisingly shows something like

note: required by a bound in `Option`
  --> $SRC_DIR/core/src/option.rs:LL:COL

, just without the source code...

[xizheyin]

I've written up this change in the PR description above, and it should have a similar mechanism to span_note for now.

[estebank]

I don't think we should be suggesting changes to code the user doesn't own, specially for stdlib. For a suggestion here it should be pointing at the parameters instead (which might be difficult to accomplish).

[xizheyin]

Yes, this is exactly the problem we need to solve.

This pr is to solve the problem that UI test can't be reproduced to make suggestions in std. Being able to reproduce compiler bugs will make review easier.

[xizheyin]

For further explanation, I moved the comments here.

When fixing the problems listed in #142403, we found that the bug could not be reproduced in ui test. After investigation, I found that span suggestion and span note are not realized in the same way, so I submitted this pr to make std visible in ui tests.

In daily use, the source of std should be visible, and execution will fall to the else branch, so these will not be displayed.

This new test is a bug recorded in #142403, only for verifying this code change, I will fix it in later pr.