Add PlaceholderList alert module

[Copilot]

Adds a generic PlaceholderList module for queries where a single alert element relates to a variable number of placeholder elements — collapsing N input rows per element into one alert row with a formatted list message.

New: src/qtil/list/PlaceholderList.qll

PlaceholderListSig<Element, Placeholder> — config signature:

• predicate problems(Element e, string msg, Placeholder p, string pStr) — one row per placeholder
• default int maxResults() — visible placeholder cap (default 5)
• default string orderBy(Placeholder p) — sort key (default p.toString())

PlaceholderList<Element, Placeholder, Config> — produces a fixed-arity query predicate problems(...) with 5 placeholder pairs. Expands the single $@ in the input message:

Placeholders	Message expansion
1	$@
2	$@ and $@
3	$@, $@, and $@
4	$@, $@, $@, and $@
5	$@, $@, $@, $@, and $@
>max	$@, $@, $@, $@, and $@ and N more

Unused placeholder slots are padded with the first placeholder and "".

Example usage

module MyConfig implements PlaceholderListSig<Function, Variable> {
  predicate problems(Function f, string msg, Variable v, string vStr) {
    v = f.getAParameter() and
    msg = f.getName() + " has parameter $@." and
    vStr = v.getName()
  }
}

import PlaceholderList<Function, Variable, MyConfig>
// Produces: "foo has parameter $@, $@, $@, $@, and $@." with 5 variable links

Updated: src/qtil/Qtil.qll

Added import qtil.list.PlaceholderList to the main module.

Updated: README.md

Added a PlaceholderList description under the Lists section.

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[MichaelRFairhurst]

Add a description as well to README.md, and delete .gitignore

[MichaelRFairhurst]

Increase the maximum to 5

[Copilot]

Done in cf18211 — default and cap are now 5; the output query predicate problems now has 5 placeholder pairs.

[MichaelRFairhurst]

this line is unnecessary

[Copilot]

Removed from getNthPlaceholder (cf18211). Note: the equivalent line in countPlaceholders must stay — count aggregation doesn't provide binding for the outer string msg variable, unlike rank[n].

[MichaelRFairhurst]

this line is unnecessary

[Copilot]

Removed in cf18211.

[MichaelRFairhurst]

split this into placeholderExpansion (returning $@, $@ and $@, ...) and moreString (returning "" or " and n more".

expansion() can return placeholderExpansion(...) + moreString(...)

[Copilot]

Done in cf18211 — split into placeholderExpansion(n), moreString(moreCount), and expansion(showCount, moreCount) = placeholderExpansion + moreString.

[MichaelRFairhurst]

reuse showCount here

[Copilot]

Done in cf18211 — expandedMsg now calls showCount(e, origMsg) directly and uses it to compute mc.

[MichaelRFairhurst]

Instead of a qnit test, have this be a basic query test.

You can have multiple test queries by defining different query predicates, e.g.

query predicate testMaxTwo = PlaceholderList<...>::problems/4;

And then the resulting set of query predicates will output the expected:

testMaxTwo
| my element has $@ | foo | "" | "" |
| my other element has $@ and $@ | foo | bar | "" |
| my last element has $@, $@, and 1 more | foo | bar | baz
testMaxThree
| my element has $@ | foo | "" | "" |
....

[Copilot]

Done in cf18211 — replaced the Qnit-based test with two query predicate aliases (testDefault and testMaxTwo) and a matching .expected file showing the actual output rows.

[Copilot]

Add a description as well to README.md, and delete .gitignore

Done in cf18211 — added a PlaceholderList section to the Lists part of README.md and deleted .gitignore.