Nightly Release v-2023-12-15-0759

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• ViperServer release v-2023-12-15-0713
• Z3 4.8.7
• Boogie release latest

Nightly Release v-2023-12-06-0725

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• ViperServer release v-2023-12-06-0713
• Z3 4.8.7
• Boogie release latest

Nightly Release v-2023-09-27-0729

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• ViperServer release v-2023-09-27-0716
• Z3 4.8.7
• Boogie release latest

Nightly Release v-2023-08-26-2125

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• ViperServer release v-2023-08-26-0719
• Z3 4.8.7
• Boogie release latest

v4.2.2 (July 2023 release)

Release 2023.7

Date 31/07/23

Changes in Viper Language

• Improved language flexibility (Silver#685): The syntax of declarations and assignments is now more permissive, allowing, for example, multiple declarations at once:

  field n: Ref, m: Ref  // Declare multiple fields at once
  var a: Ref, b: Ref, c: Int  // Declare multiple locals at once
  

  Additionally, method calls and new statements can now have fields or newly-declared local variables on their left hand side:

  var f: Ref := new(n)                   // Declare and initialise locals with `new`
  a, f.n := get_refs()                   // Assign to field from method call
  f.n := new(n)                          // Assign to field with `new`
  

  The Viper AST remains unchanged; the newly-supported syntax is desugared into equivalent code that uses the existing Viper AST.
• Annotations (Silver#666): The Viper language now has support for annotations on statements, expressions, and top-level declarations. The syntax for an annotation is @key("value", "value2", ...), i.e., they have a key which is preceded by an @ sign and a sequence of values written as comma-separated string literals. A single expression/statement/declaration can have multiple annotations. If there are two annotations of the same key, their value sequences are concatenated. In the AST, annotations are represented in the info field of the annotated AST node using an AnnotationInfo object.
  Currently, there are two supported annotations: @weight("n") on quantifiers, where n is a positive integer, sets the weight of the quantifier in the SMT solver.
  The second annotation is exclusive to the Symbolic Execution backend (see below).
  Unknown annotations are ignored.
• Domain axioms may now refer to non-domain functions that do not have any preconditions (Silver#698).
• Support for chained comparisons (Silver#713): expressions like e1 < e2 <= e3 > e4 can now be parsed and will be desugared to (e1 < e2) && (e2 <= e3) && (e3 > e4). The AST nodes for comparison operations remain unchanged.
• Two important changes in the termination plugin:
  • The termination plugin now enforces that functions that refer to themselves (or to other functions that call the original function in a mutually-recursive way) in their own postconditions have a decreases clause (Silver#711). That is, for such functions, one must either prove termination using a decreases clause, or explicitly state that termination should be assumed by writing decreases *.
    This change addresses frequent issues where users wrote functions that are not well-defined using such postconditions and subsequently reported seemingly unsound behavior (e.g., (Silver#525) and (Silver#668)).
  • The termination plugin now automatically imports the default definitions of well-founded orders if decreases-clauses are used but no such definitions are present in the program (Silver#710). That is, it is not longer necessary (but still possible) to write an import statement like import <decreases/int.vpr> when using a termination measure of type integer.
• The Rational type, an alias for Perm, is deprecated. Viper issues a warning whenever the type is used.

Viper API Changes

• Introduced a new API for frontends to interact with Viper (Silver#732). Frontends can create instances of ViperFrontendAPI and interact exclusively through those (instead of instances of Verifier or SilFrontend), which saves some boilerplate code and lets Viper manage plugins.

Other Viper-level Improvements

• Improved error messages for parse errors (Silver#702) and type errors ((Silver#684), (Silver#724))
• Fixed parsing and pretty-printing of scopes (Silver#704), which were previously ommitted under some circumstances by both the pretty-printer and the parser, which could make otherwise valid Viper code invalid.
• Introduced caching to improve performance of common lookups on the AST ((Silver#659) and (Silver#719))
• Viper now reports inferred quantifiers using a QuantifierInstantiationsMessage (Silver#653)
• Introduced various new consistency checks for invalid code that used to crash the backends or lead to unexpected behavior:
  • perm-expressions are no longer allowed in function postconditions (Silver#681)
  • wildcard is no longer allowed as part of compound expressions (Silver#700)
  • empty ADTs are rejected (Silver#696)
  • predicate argument expressions must be pure (Silver#721)
• Labelled old-expressions may now be used as triggers (Silver#695)
• Various small fixes for crashes or other incorrect behavior.

Backend-specific Upgrades/Changes

Symbolic Execution Backend (Silicon)

• The previously experimental mode --moreCompleteExhale is no longer experimental, but is now a supported mode that performs exhales and heap lookups in a way that is usually more complete (but possibly slower). However, Silicon's previous exhale mode remains the default. (Silicon#682)
  • The new flag --exhaleMode=n, where n is 0, 1, or 2, can now be used to select the exhale mode. 0 is the default, 1 is the previous moreCompleteExhale mode.
  • The new exhaleMode 2 uses the default exhale mode 0 throughout the program, until it encounters a verification error. When it does, it retries proving the failing assertion using the (more complete) mode 1. Since mode 0 is usually faster than mode 1, mode 2 often results in the best mix of performance and completeness.
  • Additionally, one can now override the exhale mode selected via command line for individual methods using an annotation @exhaleMode("n") on the method (Silicon#724)
• Via the option --prover=Z3-API, Silicon can now use Z3 as a library via its Java API ((Silicon#633) and (Silicon#692)). Note that this requires a native version of libz3java to be present on the path Java uses to look for native libraries (which can be set, e.g., via the environment variableLD_LIBRARY_PATH on Linux or DYLD_LIBRARY_PATH on MacOS).
  Depending on the OS and the Viper program, this option can lead to a significant speedup.
• Silicon now emits warnings in cases where it cannot use the triggers specified by the user (Silicon#687)
• old-expressions in triggers are now interpreted correctly (Silicon#710); previously, they were sometimes ignored.
• Changed default options used for Z3; the new options have similar performance and completeness characteristics with the supported Z3 version 4.8.7, but perform better with newer versions (tested in particular with 4.12.1) (Silicon#694)
• Silicon no longer creates a tmp directory containing .smt2 files that log its prover interactions by default (Silicon#709). The flag --disableTempDirectory no longer exists. Instead, one can use the new flag --enableTempDirectory or the flag --proverLogFile.
• Improved heuristics for handling of quantified permissions, which improves performance in some cases (Silicon#704)
• Fixed some incorrect (too short) timeouts, which could lead to unstable verification results in some cases (Silicon#705)
• Added a new command line flag --reportReasonUnknown, which instructs Silicon to report the reason the SMT solver names for not being able to prove a property (per error) (Silicon#701). This flag can be useful to find out whether an error occurred due to non-linear arithmetic or timeouts.
• Fixed five unsoundnesses:
  • Incorrect handling of old-expressions in postconditions of methods called inside a package statement (Silicon#699)
  • Incorrect handling of fold statements for predicates that are used inside a quantified permission in the current method (Silicon#696)
  • Incorrect behavior for quantifiers whose bodies have unreachable branches (Silicon#690)
  • Incorrectly terminating verification after using a refute (Silicon#741)
  • Exhale mode 1 now correctly checks permission amounts in function preconditions ([Silicon#682](https://github.com/viperproject/silicon/pul...

4.2.1-RC

Based on

• ViperServer release 23.07-RC2
• Z3 4.8.7
• Boogie release latest

4.2.0-RC

Based on

• ViperServer release 23.07-RC
• Z3 4.8.7
• Boogie release latest

Nightly Release v-2023-08-05-1220

Based on

• ViperServer release v-2023-08-05-0715
• Z3 4.8.7
• Boogie release latest

Nightly Release v-2023-08-02-0730

Based on

• ViperServer release v-2023-08-02-0718
• Z3 4.8.7
• Boogie release latest

Nightly Release v-2023-07-18-0828

Based on

• ViperServer release v-2023-07-17-1747
• Z3 4.8.7
• Boogie release latest