Merge #20255: util: Add Assume() identity function

faa05854f8 util: Remove probably misleading TODO (MarcoFalke)
fac5efe730 util: Add Assume() identity function (MarcoFalke)
fa861569dc util: Allow Assert(...) to be used in all contexts (practicalswift)

Pull request description:

  This is needed for #20138. Please refer to the added documentation for motivation.

ACKs for top commit:
  practicalswift:
    cr ACK faa05854f8
  jnewbery:
    utACK faa05854f8
  hebasto:
    ACK faa05854f8, I have reviewed the code and it looks OK, I agree it can be merged.

Tree-SHA512: 72165fbd898b92ab9a79b070993fa1faa86c2e3545b6645e72c652bda295d5107bc298d0482bf3aaf0926fc0c3e6418a445c0e073b08568c44231f547f76a688

doc/developer-notes.md

@@ -276,6 +276,33 @@ configure option adds `-DDEBUG_LOCKORDER` to the compiler flags. This inserts
 run-time checks to keep track of which locks are held and adds warnings to the
 `debug.log` file if inconsistencies are detected.
 
+### Assertions and Checks
+
+The util file `src/util/check.h` offers helpers to protect against coding and
+internal logic bugs. They must never be used to validate user, network or any
+other input.
+
+* `assert` or `Assert` should be used to document assumptions when any
+  violation would mean that it is not safe to continue program execution. The
+  code is always compiled with assertions enabled.
+   - For example, a nullptr dereference or any other logic bug in validation
+     code means the program code is faulty and must terminate immediately.
+* `CHECK_NONFATAL` should be used for recoverable internal logic bugs. On
+  failure, it will throw an exception, which can be caught to recover from the
+  error.
+   - For example, a nullptr dereference or any other logic bug in RPC code
+     means that the RPC code is faulty and can not be executed. However, the
+     logic bug can be shown to the user and the program can continue to run.
+* `Assume` should be used to document assumptions when program execution can
+  safely continue even if the assumption is violated. In debug builds it
+  behaves like `Assert`/`assert` to notify developers and testers about
+  nonfatal errors. In production it doesn't warn or log anything, though the
+  expression is always evaluated.
+   - For example it can be assumed that a variable is only initialized once,
+     but a failed assumption does not result in a fatal bug. A failed
+     assumption may or may not result in a slightly degraded user experience,
+     but it is safe to continue program execution.
+
 ### Valgrind suppressions file
 
 Valgrind is a programming tool for memory debugging, memory leak detection, and