• For certain classes of changes to a Dafny program, prevent unexpected changes in verification behavior.
• Add command line options to assist in debugging verification performance.
• Critical fixes to the IDE and greatly improved responsiveness of non-verification IDE features.
• The C# back-end supports traits as type parameters on datatypes.

Verification

• feat: Prevent changes in the verification behavior of a proof, when any of these types of changes are made to Dafny user code:

  • Changes to declarations not referenced by the method being verified
  • Changes to the name of any declaration
  • Changes to the order of top-level declarations

• feat: Assist in debugging the verification performance of a proof by adding the /vcsSplitOnEveryAssert CLI option and {:vcs_split_on_every_assert} attribute (see boogie-org/boogie#465), and report the outcome and duration of splits when they occur in /verificationLogger:trx content.

• feat: Add a /verificationLogger:csv CLI option that emits the same status and timing information as /verificationLogger:trx, but in an easier-to-parse format, along with Z3 resource counts for more repeatable tracking of verification difficulty.

• fix: Resolve unsoundness issue (#1619).

• fix: Don't silently succeed if the solver crashes (boogie-org/boogie#488).

IDE

• feat: Verification status reporting shows which proof is being verified, which can help debug slow to verify proofs.

• feat: Publish parsing and resolution diagnostics before verification has completed. Verification diagnostics from previous runs are migrated.

• feat: Enable 'go to definition', 'hover' and 'signature help' features before verification has completed.

• feat: Improve the hover feature to work for a wider scope of Dafny constructs, including function and method parameters, forall, exists and let expressions, and set and map comprehensions.

• feat: Add an experimental verification caching feature, which enables automatically determining which proofs need to verify again after making changes.

• feat: Display related resolution errors using nested diagnostics instead of independent diagnostics.

• fix: Clean up process resources if IDE closed or restarted.

• fix: Do not let the Dafny compilation status bar get in a stuck state.

UX

• feat: Improve error reporting when providing incorrectly typed arguments in a function call.
• feat: Improve error reporting when using type tests.

C#

• feat: Support variant type parameters on datatype definitions, which enables using traits as type arguments (#1499).
• feat: Support for downcasting both custom datatypes and functions (#1645, #1755).
• fix: Resolve various instances where Dafny would produce invalid C# code (#1607, #1761, and #1762).

Various improvements

• fix: DafnyLanguageServer.dll and Dafny.dll depended on two different versions of Newtonsoft.Json, which could cause crashes in development environments.
• fix: (error reporting) Types with the same name but from different modules are now disambiguated in error messages.
• fix: (error reporting) Messages about arguments / parameters type mismatch are clearer and include the parameter name if available.
• fix: (robustness) Exceptions during parsing, if any, won't crash the language server anymore.
• fix: The elephant operator (:-) has a clearer error message and no longer reject generic methods on its right-hand side.

Breaking changes

• The verifier in Dafny 3.4 is now more efficient for many programs, and making changes to Dafny programs is less likely to cause verification to take longer or timeout. However, it is still possible for some correct programs to take longer to verify than on Dafny 3.3, or for verification to fail. For users with such programs who are not yet ready to modify them to pass the 3.4 verifier, we offer the command line option /mimicVerificationOf:3.3 to keep the Dafny 3.4 verification behavior consistent with 3.3.

• In Dafny 3.3, comprehensions quantified over subset types did not validate the constraint of the subset type, which could result in crashes at run-time. In 3.4, subset types are disabled in set comprehensions in compiled contexts, unless the subset constraint is itself compilable.

  Before, the following code would pass Dafny and be compiled without error, but would crash at run-time:

  type RefinedData = x: Data | ghostFunction(x)
  method Main() {
    var s: set<Data> = ...
    var t = set x: RefinedData | x in s;
    forall x in t {
      if !ghostFunction(x) {
        var crash := 1/0;
      }
    }
  }

  In Dafny 3.4, the same code triggers a resolution error of the form:

  Error: RefinedData is a subset type and its constraint is not compilable, hence it cannot yet be used as the type of a bound variable in set comprehension. The next error will explain why the constraint is not compilable.
  Error: ghost constants are allowed only in specification contexts
  

• Changes in type inference may cause some programs to need manual type annotations. For example, in the nested pattern in the following program

  datatype X<+T> = X(x: T)
  datatype Y<T> = Y(y: T)
  
  function method M(): (r: X<Y<nat>>) {
      var d: X<Y<int>> := X(Y(3));
      match d
      case X(Y(i)) => X(Y(i))
  }

  the type of the Y constructor needs the type to be given explicitly X(Y<nat>.Y(i). As a variation of that program

  datatype X<+T> = X(x: T)
  datatype Y<T> = Y(y: T)
  
  trait Tr {}
  class Cl extends Tr {
      constructor () {}
  }
  
  method M() returns (r: X<Y<Cl>>) {
      var cl := new Cl();
      var d: X<Y<Tr>> := X(Y(cl));
      match d
      case X(Y(tr)) => r := X(Y(tr));
  }

  the program can be specified with an explicit cast X(Y(tr as Cl)).