diff --git a/src/SUMMARY.md b/src/SUMMARY.md
index c136d3716..af02b427f 100644
--- a/src/SUMMARY.md
+++ b/src/SUMMARY.md
@@ -4,7 +4,7 @@
 
 [About this guide](./about-this-guide.md)
 
----
+
 
 # Building and debugging `rustc`
 
@@ -236,7 +236,113 @@
 - [Sanitizers support](./sanitizers.md)
 - [Debugging support in the Rust compiler](./debugging-support-in-rustc.md)
 
----
+# Testing
+
+- [Prologue](./__testing/prologue.md)
+- [How to use this section](./__testing/how-to-use.md)
+- [Tutorials](./__testing/tutorials/intro.md)
+    - [Writing a `assembly-llvm` test]()
+    - [Writing a `codegen-llvm` test]()
+    - [Writing a `codegen-units` test]()
+    - [Writing a `coverage-run` test]()
+    - [Writing a `coverage-map` test]()
+    - [Writing a `crashes` test]()
+    - [Writing a `debuginfo` test]()
+    - [Writing a `mir-opt` test]()
+    - [Writing a `incremental` test]()
+    - [Writing a `pretty` test]()
+    - [Writing a `run-make` test]()
+    - [Writing a `run-make-cargo` test]()
+    - [Writing a `ui` test]()
+    - [Writing a `ui-fulldeps` test]()
+    - [Normalizing an unwanted string in a `ui` test]()
+    - [Only running a `ui` test under the nightly channel]()
+    - [Forcing a `rustc` to pretend to be stable in a `run-make` test]()
+- [How-to guides](./__testing/how-to-guides/intro.md)
+- [Reference](./__testing/reference/intro.md)
+    - [Local development]()
+    - [CI]()
+    - [compiletest]()
+        - [Interactions between bootstrap and compiletest]()
+        - [Interactions between rustc and compiletest]()
+        - [compiletest and staging]()
+        - [Test modes and test suites]()
+            - [The `assembly` test mode]()
+                - [The `assembly-llvm` test suite]()
+            - [The `codegen` test mode]()
+                - [The `codegen-llvm` test suite]()
+            - [The `codegen-units` test mode and test suite]()
+            - [The `crashes` test mode and test suite]()
+            - [The `coverage-map` and `coverage-run` test modes, and the `coverage` test suite]()
+            - [The `debuginfo` test mode and test suite]()
+            - [The `mir-opt` test mode and test suite]()
+            - [The `incremental` test mode and test suite]()
+            - [The `pretty` test mode and test suite]()
+            - [The `run-make` test mode]()
+                - [The `run-make` test suite]()
+                - [The `run-make-cargo` test suite]()
+            - [The `rustdoc` test mode]()
+                - [The `rustdoc-js` test suite]()
+                - [The `rustdoc-gui` test suite]()
+                - [The `rustdoc-json` test suite]()
+            - [The `ui` test mode]()
+                - [The `ui` test suite]()
+                - [The `ui-fulldeps` test suite]()
+                - [The `rustdoc-ui` test suite]()
+        - [Output snapshots]()
+            - [Error annotations]()
+        - [Normalizations]()
+        - [Controlling outcome expectations]()
+        - [Directives](./__testing/reference/directives.md)
+        - [run-make test infrastructure]()
+            - [run-make `rmake.rs` test recipes]()
+            - [`run-make-support` library]()
+        - [Remote testing and runners]()
+        - [Debuginfo test annotations]()
+        - [Pass modes]()
+        - [Compare modes]()
+        - [Clang-based tests]()
+        - [External dependencies]()
+            - [`nodejs`]()
+            - [`npm`]()
+            - [Debuggers]()
+            - [LLVM FileCheck]()
+    - [Auxiliary test support tools]()
+        - [`remote-test-client` and `remote-test-server`]()
+        - [jsondocck]()
+        - [jsondoclint]()
+        - [rustdoc-gui-tool]()
+        - [mir-opt-test-tool]()
+- [Explanations](./__testing/explanations/intro.md)
+    - [Qualities of a good test]()
+    - [Writing good FileCheck assertions]()
+    - [Why does my `ui` test `//@ build-pass` but not `//@ check-pass`?](./__testing/explanations/build-pass-not-check-pass.md)
+- [Frequently asked questions (FAQs)]()
+- [Known problems and pain points]()
+    - [Debuginfo test infrastructure and test suite]()
+- [Design notes]()
+- [Additional notes and resources]()
+    - [Test case minimization]()
+
+# Tidy
+
+- [Prologue](./__tidy/prologue.md)
+- [How to use this section](./__tidy/how-to-use.md)
+- [Tutorials]()
+- [How-to guides]()
+- [Reference]()
+- [Explanations]()
+
+# `rust-lang/rust` CI
+
+- [Prologue](./__ci/prologue.md)
+- [How to use this section](./__ci/how-to-use.md)
+- [Tutorials]()
+- [How-to guides]()
+- [Reference]()
+- [Explanations]()
+
+
 
 [Appendix A: Background topics](./appendix/background.md)
 
@@ -250,7 +356,7 @@
 
 [Appendix Z: HumorRust](./appendix/humorust.md)
 
----
+
 
 [pau]: ./borrow_check/region_inference/placeholders_and_universes.md
 [opaque-infer]: ./borrow_check/opaque-types-region-inference-restrictions.md
diff --git a/src/__ci/how-to-use.md b/src/__ci/how-to-use.md
new file mode 100644
index 000000000..e69de29bb
diff --git a/src/__ci/prologue.md b/src/__ci/prologue.md
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index 000000000..e69de29bb
diff --git a/src/__testing/explanations/build-pass-not-check-pass.md b/src/__testing/explanations/build-pass-not-check-pass.md
new file mode 100644
index 000000000..1bdb6bfb6
--- /dev/null
+++ b/src/__testing/explanations/build-pass-not-check-pass.md
@@ -0,0 +1,71 @@
+# Explanation: Why does my `ui` test `//@ build-pass` but not `//@ check-pass`?
+
+Alternative title: why is my `//@ build-pass` test failing in PR CI because no compiler errors are
+emitted even though locally errors are in fact emitted?
+
+To understand this situation, we need to understand why this might happen.
+
+<div class="warning">
+This particular explanation assumes that the root cause is due to PR CI forcing a check pass mode
+(to be elaborated on in the following). Double-check the [*Differential
+diagnosis*](#differential-diagnosis) section for some other possible root causes.
+</div>
+
+
+## Background contexts
+
+### Compiler errors emitted past the `--emit=metadata` phase
+
+Compiler errors get emitted at different phases of a compilation session, so if certain phases
+aren't reached, then errors that would only be emitted in those phases will not be emitted.
+
+The compiler `--emit=metadata` option (used on its own) skips encoding MIR for non-const functions
+in the crate metadata among other things as check mode doesn't need those. So if a compiler error is
+emitted only during the skipped encoding or later skipped codegen, only check-building a test source
+with `--emit=metadata` will cause the compiler error to not be emitted.
+
+(A related option is `-Zno-codegen`, which only skips codegen.)
+
+### `//@ check-pass` vs `//@ build-pass` pass modes
+
+The key difference between the two pass modes are that `//@ check-pass` invokes only
+`--emit=metadata`, whereas `//@ build-pass` will perform full crate metadata encoding and full
+codegen.
+
+### Forcing a pass mode via `compiletest`'s `--pass=check` option, and certain PR CI jobs setting `--pass=check`
+
+The reason you only observe the failure in certain PR CI jobs, such as
+[`x86_64-gnu-llvm-20`](https://github.com/rust-lang/rust/blob/864339abf952f07098dd82610256338520167d4a/src/ci/github-actions/jobs.yml#L125-L129)
+is that selected PR CI jobs have special `./x test` invocations that force pass mode to `check` to
+make sure that keeps working properly:
+
+```sh
+../x.ps1 --stage 2 test tests/ui --pass=check --host='' --target=i686-unknown-linux-gnu
+                                 #^----------
+                                 # forced `check` pass mode
+```
+
+See
+[`x86_64-gnu-llvm.sh`](https://github.com/rust-lang/rust/blob/864339abf952f07098dd82610256338520167d4a/src/ci/docker/scripts/x86_64-gnu-llvm.sh#L20).
+
+
+## Possible solutions
+
+### The `//@ ignore-pass` directive
+
+If it is *intentional* that the error is only emitted under `//@ build-pass`, then the test writer
+can opt the test out of forcing pass modes by using the `//@ ignore-pass` directive.
+
+See also: the [*Directives*](../reference/directives.md) chapter.
+
+
+## Differential diagnosis
+
+Note that there are other possible root causes, such as:
+
+- Semantic conflicts, e.g. the `HEAD` your branch was based on locally is outdated, and since then,
+  the remote `HEAD` contains commits that somehow causes your compiler error to be not emitted or
+  suppressed. In this case, rebase against latest `main`.
+- Architecture or some other environmental conditions that might cause the error to not be emitted.
+  For instance, if the error is only emitted for 64-bit architectures, but the PR CI might be
+  explicitly testing against a 32-bit architecture.
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diff --git a/src/__testing/prologue.md b/src/__testing/prologue.md
new file mode 100644
index 000000000..e2979f9b8
--- /dev/null
+++ b/src/__testing/prologue.md
@@ -0,0 +1,3 @@
+# Testing in `rust-lang/rust`
+
+TODO
diff --git a/src/__testing/reference/directives.md b/src/__testing/reference/directives.md
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