Merge changes Ief8e8ebc,Id3135db0,If1e76169 into main

* changes:
  libutil: delete unused boost context cruft
  build: remove approximately 400 seconds of CPU time (30%)
  fix: use http proxy for s3 access

doc/manual/rl-next/http-proxy-for-s3.md

@@ -0,0 +1,10 @@
+---
+synopsis: HTTP proxy environment variables are now respected for S3 binary cache stores
+issues: [fj#433]
+cls: [1788]
+category: Fixes
+credits: jade
+---
+
+Due to "legacy reasons" (according to the AWS C++ SDK docs), the AWS SDK ignores system proxy configuration by default.
+We turned it back on.

meson.build

@@ -147,6 +147,17 @@ if should_pch
   # Unlike basically everything else that takes a file, Meson requires the arguments to
   # cpp_pch : to be strings and doesn't accept files(). So absolute path it is.
   cpp_pch = [meson.project_source_root() / 'src/pch/precompiled-headers.hh']
+
+  # Saves about 400s (30% at time of writing) from compile time on-cpu, mostly
+  # by removing instantiations of nlohmann from every single damned compilation
+  # unit.
+  # There is no equivalent in gcc.
+  if cxx.get_id() == 'clang'
+    add_project_arguments(
+      '-fpch-instantiate-templates',
+      language : 'cpp',
+    )
+  endif
 else
   cpp_pch = []
 endif

src/libstore/s3-binary-cache-store.cc

@@ -140,6 +140,8 @@ ref<Aws::Client::ClientConfiguration> S3Helper::makeConfig(
     res->connectTimeoutMs = 5 * 1000;
     res->retryStrategy = std::make_shared<RetryStrategy>();
     res->caFile = settings.caFile;
+    // Use the system proxy env-vars in curl for s3, which is off by default for some reason
+    res->allowSystemProxy = true;
     return res;
 }
 

src/libutil/serialise.hh

@@ -9,8 +9,6 @@
 #include "types.hh"
 #include "file-descriptor.hh"
 
-namespace boost::context { struct stack_context; }
-
 namespace nix {
 
 
@@ -612,23 +610,6 @@ struct FramedSink : nix::BufferedSink
     };
 };
 
-/**
- * Stack allocation strategy for sinkToSource.
- * Mutable to avoid a boehm gc dependency in libutil.
- *
- * boost::context doesn't provide a virtual class, so we define our own.
- */
-struct StackAllocator {
-    virtual boost::context::stack_context allocate() = 0;
-    virtual void deallocate(boost::context::stack_context sctx) = 0;
-
-    /**
-     * The stack allocator to use in sinkToSource and potentially elsewhere.
-     * It is reassigned by the initGC() method in libexpr.
-     */
-    static StackAllocator *defaultAllocator;
-};
-
 /* Disabling GC when entering a coroutine (without the boehm patch).
    mutable to avoid boehm gc dependency in libutil.
  */