cmd/compile: consistently report "declared but not used" errors - needs release notes #49214
Description
Problem
Per the spec:
A compiler may make it illegal to declare a variable inside a function body if the variable is never used.
a compiler may report errors for unused variables. The pre-1.18 compiler fails to do so if a variable is never used but set inside a function literal. Example:
package main
func main() {
p := true
func() {
p = true
}()
}There is no use of p in this program but until now the compiler didn't report an error. See also issues #8560 and #46004.
The new 1.18 compiler front-end uses a go/types-based type checker (types2) which correctly reports an error in cases like these; so does go/types, and so does go/vet.
Proposal
The compiler should consistently report such errors from now on. This does not affect existing code that passes go/vet. There is a chance that code that used to compile (such as the example above) won't compile anymore; it is also likely that such code was not correct in the first place.
For existing code that won't compile anymore there are several work-arounds:
- Fix the code if it was in fact incorrect (this may be as simple as removing the unused variable, or using the correct variable).
- If the code was correct, address the error by introducing a use if the variable is still desired (which is as simple as
_ = xfor an unused variablex). - Compile using the 1.17 compiler by setting the -G=0 compiler flag. This doesn't require code changes.
Implementation
If the proposal is accepted, nothing needs to be done; this is already the new behavior of the compiler.
If the proposal is not accepted, the compiler needs to be adjusted such that any access of a variable in a function literal is considered a "use".