DIP83

Title:	Configurable Assert Diagnostics
DIP:	83
Version:	1
Status:	Draft
Created:	2015-10-01
Last Modified:	2015-10-2
Author:	Per Nordlöw
Links:

Abstract

Allow for assert's to do pretty printing of its failing expression when flagged for in call to compiler. Printing is configurable via specific sets of (template) function overloads.

Rationale

Failing asserts in unittests currently given no hints about the reason why a test failed. To aid the developer in debugging the failing assert

• binary expressions such as assert(x == y) should, for instance, print the values of x and y and
• unary expressions such as assert(!x) should, for instance, print the value of x.

This extra, so called, pretty printing can be enable by developer by changing the D compilation flags (DFLAGS) to calls to rdmd, dub, scons etc upon failure with existing behavior of -unittest for a specific failing module. This will be more convenient than explicitly adding the prints of the left-hand-side expression lhs and right-hand-side expression rhs directly before the call to the failing assert.

Description

This DIP proposes to add library-level-configurable diagnostics to failing calls to assert(expr) typically called from within unittest-blocks.

This diagnostics is activated only when DMD is called with a specific command line flag, say -unittest=verbose.

If DMD is called with this flag it will rewrite all assert expressions (AssertExpr in dmd source) such as

assert(lhs BINOP rhs)

with

assertBinOp(__FILE__, __LINE__, __COLUMN__, "BINOP")(lhs, rhs)

where

void assertBinOp(string file, uint line, uint column, string binop)(L, R)(lazy L lhs, lazy R rhs);

and similarly for unary expressions replace

assert(UOP lhs)

with

assertUnOp(__FILE__, __LINE__, __COLUMN__, UNOP)(lhs, rhs)

where

void assertUnOp(string file, uint line, uint column, string unop)(L, R)(lazy L lhs, lazy R rhs);

The default implementation of assertBinOp (preferrably defined somewhere in druntime) would be D code that mimics the current behaviour of assert(expression) by throwing an AssertExpression if cast(bool)mixin(lhs ~ UNOP ~ rhs) is false (dont know about the behaviour of nothrow/@nogc discussed above though). Something like

import core.exception : AssertError;
void assertBinOp(string file, uint line, uint column, string binOp)(L, R)(lazy L lhs, lazy R rhs, string message)
{
    if (cast(bool)mixin("lhs" ~ binOp ~ "rhs"))
    {
        throw AssertError("Failing assertion: " ~ lhs ~ " " ~ binOp ~ " " ~ rhs);
    }
}

The Phobos-developer can then do what he likes with the information he needs in the extra arguments in specific templated overloads of assertBinOp.

This specific behaviour could be extendable by adding (typically templated) overloads of assertBinOp for specific sets of types (concepts).

In this way D would get the extendability we want in testing-frameworks such as std.experimental.testing (https://github.com/D-Programming-Language/phobos/pull/3207) without adding a new assert-overload-set and without sacrifycing default memory usage in DMD/Phobos unittests.

Further, this solution enables the possibility to provide fancy diagnostics behaviour in assertBinOp for failing array/range or aggregate (struct or class) comparisons. This diagnostics could also have different pretty printing backends such as HTML.

For example a failing

assert([1,2,3] == [1,2,4]);

could pretty-print

([1,2,3][2] is 3) != ([1,2,4][2] is 4)

or for aggregates a failing

struct A { int x, y; }
auto a = A(1,2);
auto b = A(1,3);
assert(a == b);

could pretty-print

(a.y is 2) != (b.y is 3)

Parts of the solution list at

https://issues.dlang.org/show_bug.cgi?id=5547#c3

including rewriting/expansion of AssertExpr could probably reused.

Copyright

This document has been placed in the Public Domain.