Pledge diagnostics

One page per rule. Each diagnostic links here from the IDE and the build log. Severities are the defaults; .editorconfig can change them (dotnet_diagnostic.PLEDGE004.severity = error).

ID	Severity	Rule	Fix
PLEDGE001	Error	Contract call is not at the top of the method	Move it to the top
PLEDGE002	Error	Result<T>() used incorrectly	Use the method's result type
PLEDGE003	Error	Old(...) used outside a postcondition
PLEDGE004	Warning	Impure expression in a contract	Mark the callee [Pure]
PLEDGE005	Warning	Pledge generator is not active
PLEDGE006	Info	Contracts are not checked here: no call sites, a delegate, or a call through an interface or base method	Call the contracted method directly
PLEDGE007	Warning	Old(...) of a mutable reference	Copy with .ToArray() or with { }
PLEDGE008	Warning	Constant argument violates a precondition
PLEDGE009	Warning	Contract on an unsupported member

PLEDGE005, PLEDGE009 and PLEDGE006's no-call-sites form come from the source generator, the rest from analyzers in the same assembly. All ship in the Pledge package.