doc/go1.17: clarify Modules changes

Writing CL 333629 clarified my thinking about the behavioral changes
associated with lazy loading. There are really two interrelated
changes — graph pruning, and lazy loading proper — that are both made
possible by the added redundancy in the go.mod file.

(I had initially approached the whole cluster of features as “lazy
loading” because that was the starting point for the design. Graph
pruning came into the picture when we looked at how to bound the
worst-case behavior of lazy loading, but it is really the more
important of the two aspects of the design.)

Note that this change adds links to doc anchors added in CL 333629.

Fixes #36460
Fixes #47397

Change-Id: I0ef4af57f647bf5ee210ea7099191fb4befa2cc1
Reviewed-on: https://go-review.googlesource.com/c/go/+/335135
Trust: Bryan C. Mills <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>

Author: Bryan C. Mills

doc/go1.17.html

@@ -134,35 +134,54 @@ <h2 id="tools">Tools</h2>
 
 <h3 id="go-command">Go command</h3>
 
-<h4 id="lazy-loading">Lazy module loading</h4>
+<a id="lazy-loading"><!-- for existing links only --></a>
+<h4 id="graph-pruning">Pruned module graphs in <code>go 1.17</code> modules</h4>
 
 <p><!-- golang.org/issue/36460 -->
+  If a module specifies <code>go</code> <code>1.17</code> or higher, the module
+  graph includes only the <em>immediate</em> dependencies of
+  other <code>go</code> <code>1.17</code> modules, not their full transitive
+  dependencies. (See <a href="/ref/mod#graph-pruning">Module graph pruning</a>
+  for more detail.)
+</p>
+
+<p>
+  For the <code>go</code> command to correctly resolve transitive imports using
+  the pruned module graph, the <code>go.mod</code> file for each module needs to
+  include more detail about the transitive dependencies relevant to that module.
   If a module specifies <code>go</code> <code>1.17</code> or higher in its
-  <code>go.mod</code> file, its transitive requirements are now loaded lazily,
-  avoiding the need to download or read <code>go.mod</code> files for
-  otherwise-irrelevant dependencies. To support lazy loading, in Go 1.17 modules
-  the <code>go</code> command maintains <em>explicit</em> requirements in
-  the <code>go.mod</code> file for every dependency that provides any package
-  transitively imported by any package or test within the module.
-  See <a href="https://golang.org/design/36460-lazy-module-loading">the design
-  document</a> for more detail.
-  <!-- TODO(bcmills): replace the design-doc link with proper documentation. -->
+  <code>go.mod</code> file, its <codeg>go.mod</code> file now contains an
+  explicit <a href="/ref/mod#go-mod-file-require"><code>require</code>
+  directive</a> for every module that provides a transitively-imported package.
+  (In previous versions, the <code>go.mod</code> file typically only included
+  explicit requirements for <em>directly</em>-imported packages.)
+<p>
+
+<p>
+  Since the expanded <code>go.mod</code> file needed for module graph pruning
+  includes all of the dependencies needed to load the imports of any package in
+  the main module, if the main module specifies
+  <code>go</code> <code>1.17</code> or higher the <code>go</code> tool no longer
+  reads (or even downloads) <code>go.mod</code> files for dependencies if they
+  are not needed in order to complete the requested command.
+  (See <a href="/ref/mod#lazy-loading">Lazy loading</a>.)
 </p>
 
 <p><!-- golang.org/issue/45965 -->
-  Because the number of additional explicit requirements in the go.mod file may
-  be substantial, in a Go 1.17 module the newly-added requirements
-  on <em>indirect</em> dependencies are maintained in a
-  separate <code>require</code> block from the block containing direct
-  dependencies.
+  Because the number of explicit requirements may be substantially larger in an
+  expanded Go 1.17 <code>go.mod</code> file, the newly-added requirements
+  on <em>indirect</em> dependencies in a <code>go</code> <code>1.17</code>
+  module are maintained in a separate <code>require</code> block from the block
+  containing direct dependencies.
 </p>
 
 <p><!-- golang.org/issue/45094 -->
-  To facilitate the upgrade to lazy loading, the
-  <code>go</code> <code>mod</code> <code>tidy</code> subcommand now supports
-  a <code>-go</code> flag to set or change the <code>go</code> version in
-  the <code>go.mod</code> file. To enable lazy loading for an existing module
-  without changing the selected versions of its dependencies, run:
+  To facilitate the upgrade to Go 1.17 pruned module graphs, the
+  <a href="/ref/mod#go-mod-tidy"><code>go</code> <code>mod</code> <code>tidy</code></a>
+  subcommand now supports a <code>-go</code> flag to set or change
+  the <code>go</code> version in the <code>go.mod</code> file. To convert
+  the <code>go.mod</code> file for an existing module to Go 1.17 without
+  changing the selected versions of its dependencies, run:
 </p>
 
 <pre>
@@ -199,10 +218,10 @@ <h4 id="lazy-loading">Lazy module loading</h4>
 </p>
 
 <p><!-- golang.org/issue/46366 -->
-  The <code>go</code> <code>mod</code> <code>graph</code> subcommand also
-  supports the <code>-go</code> flag, which causes it to report the graph as
-  seen by the indicated Go version, showing dependencies that may otherwise be
-  pruned out by lazy loading.
+  The <a href="/ref/mod#go-mod-graph"><code>go</code> <code>mod</code> <code>graph</code></a>
+  subcommand also supports the <code>-go</code> flag, which causes it to report
+  the graph as seen by the indicated Go version, showing dependencies that may
+  otherwise be pruned out.
 </p>
 
 <h4 id="module-deprecation-comments">Module deprecation comments</h4>
@@ -270,7 +289,8 @@ <h4 id="vendor"><code>vendor</code> contents</h4>
 
 <p><!-- golang.org/issue/36876 -->
   If the main module specifies <code>go</code> <code>1.17</code> or higher,
-  <code>go</code> <code>mod</code> <code>vendor</code> now annotates
+  <a href="/ref/mod#go-mod-vendor"><code>go</code> <code>mod</code> <code>vendor</code></a>
+  now annotates
   <code>vendor/modules.txt</code> with the <code>go</code> version indicated by
   each vendored module in its own <code>go.mod</code> file. The annotated
   version is used when building the module's packages from vendored source code.