Tools using go/types sometimes need to implement special handling for
certain errors produced by the type-checker. They can offer suggested
fixes, expand the error position to surrounding syntax, highlight
related syntax (for example in the case of a declaration cycle), link to
additional documentation, group errors by category, or correlate errors
with signals produced by other static analysis tools.

All these require a notion of error identity. Tools need to be able to
reliably determine the nature of an error without re-implementing type
checking logic or parsing error messages. This CL is a first-pass at
adding such an identifier to types.Error: a (for the moment unexported)
field containing one of many declared errorCode constants.

A wide variety of error code constants are defined, and assigned to type
checker errors according to their 'functional equivalence', meaning that
they should be ideally be stable under refactoring.

With few exceptions, each error code is documented with an example that
produces it. This is enforced by tests.

When error codes are exported they will represent quite a large API
surface. For this reason, as well as the likelihood that error codes
will change at the start, both the code field and the codes themselves
are initially unexported. gopls will read these fields using reflection
during this experimental phase. Others can obviously do the same,
provided they accept the lack of forward compatibility.

For #42290

Change-Id: I15e3c2bffd2046c20297b1857057d421f633098a
Reviewed-on: https://go-review.googlesource.com/c/go/+/264179
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>

In CL 264179, some reorganization of error codes was deferred in order
to minimize diffs between patch-sets.

This CL reorganizes the error codes as discussed. It is a pure
reordering, with no material changes other than the changing of internal
const values.

For #42290

Change-Id: I0e9b421a92e96b19e53039652f8de898c5255290
Reviewed-on: https://go-review.googlesource.com/c/go/+/266637
Run-TryBot: Robert Findley <rfindley@google.com>
Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

Tools often need to associate errors not with a single position, but
with a span of source code. For example, gopls currently estimates
diagnostic spans using heuristics to expand the positions reported by
the type checker to surrounding source code. Unfortunately this is often
inaccurate.

This CL lays the groundwork to solve this within go/types by adding a
start and end position to type checker errors. This is an experimental
API, both because we are uncertain of the ideal representation for these
spans and because their initial positioning is naive. In most cases this
CL simply expands errors to the surrounding ast.Node being typechecked,
if available. This might not be the best error span to present to the
user. For these reasons the API is unexported -- gopls can read these
positions using reflection, allowing us to gain experience and improve
them during the next development cycle.

For #42290

Change-Id: I39a04d70ea2bb2134b4d4c937f32b2ddb4456430
Reviewed-on: https://go-review.googlesource.com/c/go/+/265250
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>

This CL adds a copy of the internal error codes added to go/types in
https://golang.org/cl/264179. In a subsequent CL, we will use these to
extract the unexported error code from go/types errors via reflection.

A generated String method is added to provide a human-readable code for
the user.

For golang/go#42290

Change-Id: I280c9b111598426dce4eef38b9979919ed590068
Reviewed-on: https://go-review.googlesource.com/c/tools/+/267939
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

In Go 1.16 error codes as well as start and end positions are added for
go/types errors. This information is temporarily stored in unexported
fields, until we're more confident in the correctness of both the API
and the underlying data.

Read this information using reflection and, if available, use it to set
the corresponding field in compiler diagnostics. This establishes a
positive feedback loop: in most cases this should improve the gopls
diagnostic, and wherever it doesn't we can make a note and fall back on
the old heuristics for that error code.

For golang/go#42290

Change-Id: I37475189cbd14a0a5bcfde163f599c9a7b957372
Reviewed-on: https://go-review.googlesource.com/c/tools/+/268539
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
Trust: Robert Findley <rfindley@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

In Go 1.16 error codes as well as start and end positions are added for
go/types errors. This information is temporarily stored in unexported
fields, until we're more confident in the correctness of both the API
and the underlying data.

Read this information using reflection and, if available, use it to set
the corresponding field in compiler diagnostics. This establishes a
positive feedback loop: in most cases this should improve the gopls
diagnostic, and wherever it doesn't we can make a note and fall back on
the old heuristics for that error code.

For golang/go#42290

Change-Id: I37475189cbd14a0a5bcfde163f599c9a7b957372
Reviewed-on: https://go-review.googlesource.com/c/tools/+/268539
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
Trust: Robert Findley <rfindley@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>