Fix: Shadowing of output variables in methods in Dafny-to-Rust #6278
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| return preventShadowing ? v.GetOrCreateCompileName(currentIdGenerator) : v.CompileNameShadowable; | ||
| var canShadow = !preventShadowing && | ||
| !(enclosingMethod != null && enclosingMethod.Outs.Any(outVar => outVar.Name == v.Name)); | ||
| return canShadow ? v.CompileNameShadowable : v.GetOrCreateCompileName(currentIdGenerator); |
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Is it acceptable to shadow a name declared in the body of the enclosing method, not necessarily among the out-parameters?
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Good question. Yes, because there is no way in Dafny to refer to outer variables, so there won't be an ambiguity in the generated code.
ssomayyajula
approved these changes
Jun 24, 2025
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Fixes #6261
The root cause of the bug was that shadowed variables are never accessible in Dafny anymore except for one case: Output variables, through the return statement.
Indeed, a return statement with an expression assigns the result of the expression to the (unique) output variable, and then returns. Even in the presence of shadowing when another local variable overrides the output variable, the (outer) output variable is correctly assigned in the compiler.
I had not considered this case when considering enabling shadowing.
What was changed?
How has this been tested?
By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.