Custom asserts: a one-call assertion helper and self-labelled failures #915

Background

Writing a custom assertion today is a three-part contract, spelled out under "Best practices" in docs/custom-asserts.md:

function assert_positive_number() {
  local actual="$1"

  if [ "$actual" -le 0 ]; then
    bashunit::assertion_failed "positive number" "$actual"
    return
  fi

  bashunit::assertion_passed
}

There are two ways to get that wrong, and the framework catches neither:

• Forget the return after bashunit::assertion_failed, and both the failed and the passed counter bump for a single assertion.
• Forget bashunit::assertion_passed, and the test registers zero assertions, so it silently lands as risky (src/runner.sh:1411) or as a failure under --fail-on-risky.

Separately, a custom assertion cannot name itself in its own failure block. bashunit::assert::fail_with (src/assert.sh:52) already takes a label as $1, but the public bashunit::assertion_failed (src/bashunit.sh:7) hard-codes it to "", so the output always shows the test name.

Proposal

1. bashunit::assert_that <expected> <actual> <cmd> [args...]

Runs the command and marks the assertion passed or failed accordingly. No eval, no return dance, Bash 3.0 safe:

function bashunit::assert_that() {
  bashunit::assert::should_skip && return 0

  local expected=$1
  local actual=$2
  shift 2

  if "$@"; then
    bashunit::state::add_assertions_passed
    return 0
  fi

  bashunit::assert::fail_with "" "$expected" "but got " "$actual"
  return 1
}

The example above collapses to one line, with both counters impossible to desync:

function assert_positive_number() {
  bashunit::assert_that "positive number" "$1" test "$1" -gt 0
}

2. Optional label on bashunit::assertion_failed

Accept a fourth argument and forward it into fail_with's existing $1, so a custom assertion can label its own failure. Backward compatible.

3. Docs

Update docs/custom-asserts.md with the new form, and mention --boot / BASHUNIT_BOOTSTRAP as the way to load a shared custom-assert file once. Every current example sources it from set_up, which is per test file.

Notes

Entirely additive. The existing three-part form keeps working, so no deprecation is needed.

Part of a custom-asserts DX set, each sized as its own PR:

• Custom asserts: a one-call assertion helper and self-labelled failures #915 one-call helper, self-labelled failures, docs
• Custom asserts: a supported way to test that an assertion fails #916 testing a custom assertion without reaching into internals
• Custom asserts: a composed assertion should count and report once #917 composed assertions count and report once
• Custom asserts: list a project's own assertions in bashunit doc #918 listing project assertions in bashunit doc