Trusted Manifesto

[Walter Bright via Digitalmars-d]

Andrei and I have learned a lot from the @trusted discussions. It's clear the 
way we were approaching the problem was inadequate. So we came up with a 
proposal based on the ideas and criticisms of the participants in the 
references. It involves no language changes, but offers usage guidelines that we 
believe are workable. Tell us what you think!
----------------------------------------------------

Trusted Manifesto
-----------------

Memory safety in D has the usual definition: a memory-safe program never reads
uninitialized memory and never reads or writes memory with a type incompatible
with the type it was written. The aim of D's
@safe, @trusted, and @system attributes is to provide as much
mechanically verified to be safe code as possible.


Functions
---------

Function signatures inform what must be true about a function. If
a function is marked as @safe, it must contain only @safe code. This
safety must be mechanically checkable.

Sometimes, an unsafe operation is needed even in a function
that is overall safe. For example,
here's a function that returns an upper case version of its
input string:

   string toUpper(string s) @safe
   {
      char[] r = new char[s.length];
      foreach (i, c; s)
	r[i] = toUpper(c);
      return cast(string)r; // <== unsafe operation
   }

The compiler rejects this function because it's declared as safe but contains an
unsafe operation.
Review of the whole function shows that the operation, in this instance, is safe.
One way to deal with that is to do what is called an "escape", meaning
telling the compiler "I know what I'm doing, so allow this operation":

   string toUpper(string s) @safe
   {
      char[] r = new char[s.length];
      foreach (i, c; s)
	r[i] = toUpper(c);

      static string trustedCast(char[] r) @trusted
      {
         return cast(string)r;
      }

      return trustedCast(r);
   }

This will pass typechecking. However, the only reason it is safe is because the 
context
in which trustedCast() is called makes it safe. In other words, a manual review 
of the surrounding
context is necessary, which violates the @safe promise that its contents are 
mechanically
checkable.

I.e. the trustedCast() function is an "escape" that injects unsafety into its 
surrounding
contents.

This leads to:

RULE 1: @trusted code accessible from @safe code must expose a safe interface to 
unsafe operations.

@trusted must not be used to inject unsafety into surrounding context that is 
marked @safe.
@safe code must be mechanically verifiable to be safe, and subverting that is 
not acceptable.

COROLLARY 1: @trusted functions should be as small as possible to encapsulate 
the unsafe operation
without injecting unsafety into @safe code.

In the case of toUpper(), it is necessary to review the entire function to 
verify that the cast is safe,
so it is properly written:

   string toUpper(string s) @trusted
   {
      char[] r = new char[s.length];
      foreach (i, c; s)
	r[i] = toUpper(c);
      return cast(string)r;
   }

Use of local @trusted functions with safe interfaces is encouraged to minimize 
the amount of safety code review required.


Generic Functions
-----------------

Generic functions are templates that accept compile time parameters in the form 
of types, values or aliases to other functions.
Whether the function is @safe or @system is not checkable until the template 
function is instantiated with explicit
arguments. If the template function is marked as @safe, then it can only be 
instantiated with arguments that expose
safe operations.

If the template function is marked @safe, then RULE 1 applies.

But that reduces the genericity of the function. The compiler is able to deduce 
whether a template function is @safe or @system
when it is instantiated. For maximum utility, we need a way to specify that:

     This template function is @safe if the generic and non-generic operations 
it uses are @safe as well,
     otherwise it is @system.

Consider a function to make an immutable array copy of a range:

   immutable(ElementType!Range)[] toArray(Range)(Range r)
   {
      alias ElementType!Range E;
      alias Unqual!E U;
      U[] a = new U[r.length];
      foreach (i, e; r)
	a[i] = e;
      return cast(immutable)a; // <== unsafe operation
   }

Being a template function without specified attributes, the compiler will infer 
the attributes.
But with the unsafe cast, toArray() will always be inferred to be @system. But 
the rest
of the code is safe. If toArray is marked as @trusted,

   immutable(ElementType!Range)[] toArray(Range)(Range r) @trusted
   {
      alias ElementType!Range E;
      alias Unqual!E U;
      U[] a = new U[r.length];
      foreach (i, e; r)
	a[i] = e;
      return cast(immutable)a; // <== unsafe operation
   }

then if the range primitives (front, empty, popFront) exposed by the argument to r
happen to be @system, then those are invalidly assumed to be trustable. Every usage
of toArray() would need to be reviewed for safety, which is impractical.

What is needed is a way to isolate the unsafe operation, and enable the compiler to
infer the rest. In other words, a local exemption from overall safety deduction 
is needed.

Introducing the 'trusted' template to be put in std.conv:

   @trusted auto trusted(alias fun)() { return fun(); }

and used:

   immutable(ElementType!Range)[] toArray(Range)(Range r)
   {
      alias ElementType!Range E;
      alias Unqual!E U;
      U[] a = new U[r.length];
      foreach (i, e; r)
	a[i] = e;

      import std.conv : trusted;
      auto result = trusted!(() => cast(immutable)a);
      return result;
   }

Use of the trusted escape requires the programmer to review the context to 
determine if
it really is safe. The compiler will infer safety from the rest of the operations.

RULE 2: Usage of escapes are only allowable in functions for which safety is 
inferred,
and never when calling into as-of-yet not defined generic functions.

But how can it be verified that toArray() is safe otherwise?

RULE 3: An @safe unittest must be used to verify safety when escapes are used.

   @safe unittest
   {
      ... TODO: test toArray() ...
   }

A unittest may also be constructed that verifies via static assert that if a type
with @system operations is passed to the function, that the function is inferred
as @system.

The programmer must still verify that the usage of escapes that leak
unsafety into the surrounding context is safe.

RULE 4: Escape unsafety must not inject unsafety beyond the template function it 
is used in.

Alternatives
------------

1. if(0) block

Provide an @trusted local function that fully encapsulates the unsafe code and 
its context,
providing a safe interface. For the operations on template parameters that may 
or may not be
safe inside the local function, represent them in an if(0) block of code:

   if (0) // safety inference
   {
     Unqual!T tmp = cast(Unqual!T)item;
     emplaceRef!(Unqual!T)(tmp, cast(Unqual!T)item);
   }

   @trusted void emplace()
   {
     auto bigData = _data.arr.ptr[0 .. len + 1];

     emplaceRef!(Unqual!T)(bigData[len], cast(Unqual!T)item);

     //We do this at the end, in case of exceptions
     _data.arr = bigData;
   }
   emplace();

Although this works, it requires duplication of code in a rather careful, 
tedious, and essentially
unmaintainable manner. It also simply looks wrong, although it could be made 
more palatable by
enclosing it in a template.

2. isSafe!T template

Such a template would test that all operations on type T are @safe. The template 
function could
then be marked @trusted. The troubles with this are (a) it is all or nothing 
with T, i.e. if a
template function only used an @safe subset of T, it still would not be accepted 
and (b) it does
not do proper inference of the safety of a template function.

3. @system escape

@system would be used for escaping unsafe code in an @trusted function, or in an 
un-attributed function
it would instruct compiler to not use the escaped code when deducing 
trustworthiness. Unsafe code in an @trusted function
not so marked would generate an error. While this works, it would essentially 
break every @trusted function
already in existence. It is a somewhat nicer syntax than the std.conv.trusted 
template, but the backwards compatibility
issue makes it unworkable. It offers a technical advantage over std.conv.trusted 
in that @system will not be
allowed in @safe functions, while not allowing std.conv.trusted escapes in @safe 
function would be by convention.

References
----------

https://github.com/D-Programming-Language/phobos/pull/2966

http://forum.dlang.org/post/mb0uvr$2fdb$1@digitalmars.com

Acknowledgements
----------------

Everyone who participated in the references!