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Robert Griesemer ddb5a42b25 cmd/compile/internal/types2: avoid "declared but not used" errors for invalid code

Agressively mark all LHS variables in assignments as used if there
is any error in the (entire) assignment. This reduces the number of
spurious "declared but not used" errors in programs that are invalid
in the first place. This behavior is closer to the behavior of the
compiler's original type checker (types1) and lets us remove lines
of the form "_ = variable" just to satisfy test cases. It also makes
more important errors visible by not crowding them out.

Remove the Checker.useLHS function and use Checker.use instead:
useLHS didn't evaluate top-level variables, but we actually want
them to be evaluated in an error scenario so that they are getting
used (and thus we don't get the "declared but not used" error).

Fixes #42937.

Change-Id: Idda460f6b81c66735bf9fd597c54188949bf12b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/351730
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

2021-09-23 19:41:45 +00:00

README.vendor

Vendoring in std and cmd
========================

The Go command maintains copies of external packages needed by the
standard library in the src/vendor and src/cmd/vendor directories.

In GOPATH mode, imports of vendored packages are resolved to these
directories following normal vendor directory logic
(see golang.org/s/go15vendor).

In module mode, std and cmd are modules (defined in src/go.mod and
src/cmd/go.mod). When a package outside std or cmd is imported
by a package inside std or cmd, the import path is interpreted
as if it had a "vendor/" prefix. For example, within "crypto/tls",
an import of "golang.org/x/crypto/cryptobyte" resolves to
"vendor/golang.org/x/crypto/cryptobyte". When a package with the
same path is imported from a package outside std or cmd, it will
be resolved normally. Consequently, a binary may be built with two
copies of a package at different versions if the package is
imported normally and vendored by the standard library.

Vendored packages are internally renamed with a "vendor/" prefix
to preserve the invariant that all packages have distinct paths.
This is necessary to avoid compiler and linker conflicts. Adding
a "vendor/" prefix also maintains the invariant that standard
library packages begin with a dotless path element.

The module requirements of std and cmd do not influence version
selection in other modules. They are only considered when running
module commands like 'go get' and 'go mod vendor' from a directory
in GOROOT/src.

Maintaining vendor directories
==============================

Before updating vendor directories, ensure that module mode is enabled.
Make sure GO111MODULE=off is not set ('on' or 'auto' should work).

Requirements may be added, updated, and removed with 'go get'.
The vendor directory may be updated with 'go mod vendor'.
A typical sequence might be:

    cd src
    go get -d golang.org/x/net@latest
    go mod tidy
    go mod vendor

Use caution when passing '-u' to 'go get'. The '-u' flag updates
modules providing all transitively imported packages, not only
the module providing the target package.

Note that 'go mod vendor' only copies packages that are transitively
imported by packages in the current module. If a new package is needed,
it should be imported before running 'go mod vendor'.