x/tools/go/analysis/passes/modernize: reflectypefor leads to unused local var

[adonovan]

The reflecttypefor modernizer transforms code such as var zero T; return TypeOf(zero) to use TypeFor[T](), leaving the zero variable unused, which is an illegal program. The analyzer should check whether it eliminated the last use of the variable and if so eliminate the variable itself. This seems like a refactoring operator that would find use across our refactoring tools, perhaps some of which exhibit similar bugs.

Thanks to @cuishuang for pointing it out.

[gabyhelp]

Related Issues

• x/tools/go/analysis/passes/unusedwrite: detect dead stores to local vars #67109
• x/tools/gopls: provide a suggested fix for unused variables #48975 (closed)
• x/tools/gopls/internal/analysis/modernize: forvar: eliminate k := k; v := v in one fell swoop #74849 (closed)

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