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
Trust: Alberto Donizetti <>
Run-TryBot: Alberto Donizetti <>
TryBot-Result: Go Bot <>
Reviewed-by: Matthew Dempsky <>
2 files changed
tree: 07fb78617a9a03ca21cec914efb7d851629be3cf
  1. .github/
  2. api/
  3. doc/
  4. lib/
  5. misc/
  6. src/
  7. test/
  8. .gitattributes
  9. .gitignore
  13. favicon.ico
  17. robots.txt

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