Add a struct called Source that holds the function, file and line
of a location in the program's source code.
When HandleOptions.AddSource is true, the ReplaceAttr function will
get an Attr whose key is SourceKey and whose value is a *Source.
We use *Source instead of Source to save an allocation. The pointer
and the value each cause one allocation up front: the pointer when it
is created, and the value when it is assigned to the `any` field of a
slog.Value (handle.go:283). If a ReplaceAttr function wanted to modify
a Source value, it would have to create a new slog.Value to return,
causing a second allocation, but the function can modify a *Source in
place.
TextHandler displays a Source as "file:line".
JSONHandler displays a Source as a group of its non-zero fields.
This replaces the previous design, where source location was always a
string with the format "file:line". The new design gives users more
control over how to output and consume source locations.
Fixes #59280.
Change-Id: I84475abd5ed83fc354b50e34325c7b246cf327c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/486376
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com> Reviewed-by: Alan Donovan <adonovan@google.com>