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+This directory contains unit tests for the MIR-based dataflow
+analysis.
+
+These unit tests check the dataflow analysis by embedding calls to a
+special `rustc_peek` intrinsic within the code, in tandem with an
+attribute `#[rustc_mir(rustc_peek_maybe_init)]` (\*). With that
+attribute in place, `rustc_peek` calls are a signal to the compiler to
+lookup the computed dataflow state for the Lvalue corresponding to the
+argument expression being fed to `rustc_peek`. If the dataflow state
+for that Lvalue is a 1-bit at that point in the control flow, then no
+error is emitted by the compiler at that point; if it is a 0-bit, then
+that invocation of `rustc_peek` will emit an error with the message
+"rustc_peek: bit not set".
+
+(\*): Or `#[rustc_mir(rustc_peek_maybe_uninit)]`, and perhaps other
+variants in the future.
+
+The end effect is that one can write unit tests for MIR dataflow that
+perform simple-queries of the computed dataflow state, and the tests
+should be able to be robust in the face of changes to how MIR is
+represented or constructed.
+
+----
+
+Sometimes understanding the dataflow results is difficult without
+looking at the actual MIR control-flow graph being processed with the
+corresponding GEN and KILL sets.
+
+For a graphviz-rendering with dataflow annotations, add the attribute
+`#[rustc_mir(borrowck_graphviz_postflow="/path/to/suffix.dot")]` to
+the function in question. (You can change the content of
+`"suffix.dot"` to control the filenames used for the output). This
+will generate a separate file for each dataflow analysis, adding a
+prefix (corresponding to the name of the analysis) to the filename in
+each generated output path.
+
+ * For example, the above attribute will currently cause two files to
+   be generated: `/path/to/maybe_init_suffix.dot` and
+   `/path/to/maybe_uninit_suffix.dot`.
+
+ * The generated `.dot` file shows both the computed dataflow results
+   on *entry* to each block, as well as the gen- and kill-sets that
+   were so-called "transfer functions" summarizing the effect of each
+   basic block.
+
+ * (In addition to the `borrowck_graphviz_postflow` attribute-key
+   noted above, there is also `borrowck_graphviz_preflow`; it has the
+   same interface and generates the same set of files, but it renders
+   the dataflow state after building the gen- and kill-sets but
+   *before* running the dataflow analysis itself, so each entry-set is
+   just the initial default state for that dataflow analysis. This is
+   less useful for understanding the error message output in these
+   tests.)