Warning or error messages set via a lint group attribute
(e.g. `#[deny(warnings)]`) should still make it clear which individual
lint (by name) was triggered, similarly to how we include "on by
default" language for default lints. This—and, while we're here, the
existing "on by default" language—can be tucked into a note rather than
cluttering the main error message. This occasions the slightest of
refactorings (we now have to get the diagnostic-builder with the main
message first, before matching on the lint source).
This is in the matter of #36846.
When dealing with multiline spans that span few lines, show the complete
span instead of restricting to the first character of the first line.
For example, instead of:
```
% ./rustc foo.rs
error[E0277]: the trait bound `{integer}: std::ops::Add<()>` is not satisfied
--> foo.rs:13:9
|
13 | foo(1 + bar(x,
| ^ trait `{integer}: std::ops::Add<()>` not satisfied
|
```
show
```
% ./rustc foo.rs
error[E0277]: the trait bound `{integer}: std::ops::Add<()>` is not satisfied
--> foo.rs:13:9
|
13 | foo(1 + bar(x,
| ________^ starting here...
14 | | y),
| |_____________^ ...ending here: trait `{integer}: std::ops::Add<()>` not satisfied
|
```
