cmd/compile: be more specific in cannot assign errors

"cannot assign to" compiler errors are very laconic: they never
explain why the lhs cannot be assigned to (with one exception, when
assigning to a struct field in a map).

This change makes them a little more specific, in two more cases: when
assigning to a string, or to a const; by giving a very brief reason
why the lhs cannot be assigned to.

Change-Id: I244cca7fc3c3814e00e0ccadeec62f747c293979
Reviewed-on: https://go-review.googlesource.com/c/go/+/255199
Trust: Alberto Donizetti <alb.donizetti@gmail.com>
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
2 files changed
tree: 07fb78617a9a03ca21cec914efb7d851629be3cf
  1. .gitattributes
  2. .github/
  3. .gitignore
  4. AUTHORS
  5. CONTRIBUTING.md
  6. CONTRIBUTORS
  7. LICENSE
  8. PATENTS
  9. README.md
  10. SECURITY.md
  11. api/
  12. doc/
  13. favicon.ico
  14. lib/
  15. misc/
  16. robots.txt
  17. src/
  18. test/
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