reduce the size of Env by one pointer

since `up` and `values` are both pointer-aligned the type field will
also be pointer-aligned, wasting 48 bits of space on most machines. we
can get away with removing the type field altogether by encoding some
information into the `with` expr that created the env to begin with,
reducing the GC load for the absolutely massive amount of single-entry
envs we create for lambdas. this reduces memory usage of system eval by
quite a bit (reducing heap size of our system eval from 8.4GB to 8.23GB)
and gives similar savings in eval time.

running `nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'`

before:

  Time (mean ± σ):      5.576 s ±  0.003 s    [User: 5.197 s, System: 0.378 s]
  Range (min … max):    5.572 s …  5.581 s    10 runs

after:

  Time (mean ± σ):      5.408 s ±  0.002 s    [User: 5.019 s, System: 0.388 s]
  Range (min … max):    5.405 s …  5.411 s    10 runs

doc/manual/rl-next/env-size-reduction.md

@@ -0,0 +1,7 @@
+---
+synopsis: Reduce eval memory usage and wall time
+prs: 9658
+---
+
+Reduce the size of the `Env` struct used in the evaluator by a pointer, or 8 bytes on most modern machines.
+This reduces memory usage during eval by around 2% and wall time by around 3%.

doc/manual/rl-next/with-error-reporting.md

@@ -0,0 +1,31 @@
+---
+synopsis: Better error reporting for `with` expressions
+prs: 9658
+---
+
+`with` expressions using non-attrset values to resolve variables are now reported with proper positions.
+
+Previously an incorrect `with` expression would report no position at all, making it hard to determine where the error originated:
+
+```
+nix-repl> with 1; a  
+error:
+       … <borked>
+
+         at «none»:0: (source not available)
+
+       error: value is an integer while a set was expected
+```
+
+Now position information is preserved and reported as with most other errors:
+
+```
+nix-repl> with 1; a
+error:
+       … while evaluating the first subexpression of a with expression
+         at «string»:1:1:
+            1| with 1; a
+             | ^
+
+       error: value is an integer while a set was expected
+```

src/libcmd/repl.cc

@@ -112,7 +112,7 @@ NixRepl::NixRepl(const SearchPath & searchPath, nix::ref<Store> store, ref<EvalS
     : AbstractNixRepl(state)
     , debugTraceIndex(0)
     , getValues(getValues)
-    , staticEnv(new StaticEnv(false, state->staticBaseEnv.get()))
+    , staticEnv(new StaticEnv(nullptr, state->staticBaseEnv.get()))
     , historyFile(getDataDir() + "/nix/repl-history")
 {
 }

src/libexpr/eval-inline.hh

@@ -73,8 +73,6 @@ Env & EvalState::allocEnv(size_t size)
 #endif
         env = (Env *) allocBytes(sizeof(Env) + size * sizeof(Value *));
 
-    env->type = Env::Plain;
-
     /* We assume that env->values has been cleared by the allocator; maybeThunk() and lookupVar fromWith expect this. */
 
     return *env;

src/libexpr/eval.cc

@@ -543,7 +543,7 @@ EvalState::EvalState(
     , env1AllocCache(std::allocate_shared<void *>(traceable_allocator<void *>(), nullptr))
 #endif
     , baseEnv(allocEnv(128))
-    , staticBaseEnv{std::make_shared<StaticEnv>(false, nullptr)}
+    , staticBaseEnv{std::make_shared<StaticEnv>(nullptr, nullptr)}
 {
     corepkgsFS->setPathDisplay("<nix", ">");
     internalFS->setPathDisplay("«nix-internal»", "");
@@ -781,7 +781,7 @@ void printStaticEnvBindings(const SymbolTable & st, const StaticEnv & se)
 // just for the current level of Env, not the whole chain.
 void printWithBindings(const SymbolTable & st, const Env & env)
 {
-    if (env.type == Env::HasWithAttrs) {
+    if (!env.values[0]->isThunk()) {
         std::cout << "with: ";
         std::cout << ANSI_MAGENTA;
         Bindings::iterator j = env.values[0]->attrs->begin();
@@ -835,7 +835,7 @@ void mapStaticEnvBindings(const SymbolTable & st, const StaticEnv & se, const En
     if (env.up && se.up) {
         mapStaticEnvBindings(st, *se.up, *env.up, vm);
 
-        if (env.type == Env::HasWithAttrs) {
+        if (!env.values[0]->isThunk()) {
             // add 'with' bindings.
             Bindings::iterator j = env.values[0]->attrs->begin();
             while (j != env.values[0]->attrs->end()) {
@@ -973,22 +973,23 @@ inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
 
     if (!var.fromWith) return env->values[var.displ];
 
+    // This early exit defeats the `maybeThunk` optimization for variables from `with`,
+    // The added complexity of handling this appears to be similarly in cost, or
+    // the cases where applicable were insignificant in the first place.
+    if (noEval) return nullptr;
+
+    auto * fromWith = var.fromWith;
     while (1) {
-        if (env->type == Env::HasWithExpr) {
+        forceAttrs(*env->values[0], fromWith->pos, "while evaluating the first subexpression of a with expression");
-            if (noEval) return 0;
-            Value * v = allocValue();
-            evalAttrs(*env->up, (Expr *) env->values[0], *v, noPos, "<borked>");
-            env->values[0] = v;
-            env->type = Env::HasWithAttrs;
-        }
         Bindings::iterator j = env->values[0]->attrs->find(var.name);
         if (j != env->values[0]->attrs->end()) {
             if (countCalls) attrSelects[j->pos]++;
             return j->value;
         }
-        if (!env->prevWith)
+        if (!fromWith->parentWith)
             error("undefined variable '%1%'", symbols[var.name]).atPos(var.pos).withFrame(*env, var).debugThrow<UndefinedVarError>();
-        for (size_t l = env->prevWith; l; --l, env = env->up) ;
+        for (size_t l = fromWith->prevWith; l; --l, env = env->up) ;
+        fromWith = fromWith->parentWith;
     }
 }
 
@@ -1816,9 +1817,7 @@ void ExprWith::eval(EvalState & state, Env & env, Value & v)
 {
     Env & env2(state.allocEnv(1));
     env2.up = &env;
-    env2.prevWith = prevWith;
+    env2.values[0] = attrs->maybeThunk(state, env);
-    env2.type = Env::HasWithExpr;
-    env2.values[0] = (Value *) attrs;
 
     body->eval(state, env2, v);
 }

src/libexpr/eval.hh

@@ -116,11 +116,6 @@ struct Constant
 struct Env
 {
     Env * up;
-    /**
-     * Number of of levels up to next `with` environment
-     */
-    unsigned short prevWith:14;
-    enum { Plain = 0, HasWithExpr, HasWithAttrs } type:2;
     Value * values[0];
 };
 

src/libexpr/nixexpr.cc

@@ -333,6 +333,8 @@ void ExprVar::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
     if (es.debugRepl)
         es.exprEnvs.insert(std::make_pair(this, env));
 
+    fromWith = nullptr;
+
     /* Check whether the variable appears in the environment.  If so,
        set its level and displacement. */
     const StaticEnv * curEnv;
@@ -344,7 +346,6 @@ void ExprVar::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
         } else {
             auto i = curEnv->find(name);
             if (i != curEnv->vars.end()) {
-                fromWith = false;
                 this->level = level;
                 displ = i->second;
                 return;
@@ -360,7 +361,8 @@ void ExprVar::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
             .msg = hintfmt("undefined variable '%1%'", es.symbols[name]),
             .errPos = es.positions[pos]
         });
-    fromWith = true;
+    for (auto * e = env.get(); e && !fromWith; e = e->up)
+        fromWith = e->isWith;
     this->level = withLevel;
 }
 
@@ -393,7 +395,7 @@ void ExprAttrs::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv>
         es.exprEnvs.insert(std::make_pair(this, env));
 
     if (recursive) {
-        auto newEnv = std::make_shared<StaticEnv>(false, env.get(), recursive ? attrs.size() : 0);
+        auto newEnv = std::make_shared<StaticEnv>(nullptr, env.get(), recursive ? attrs.size() : 0);
 
         Displacement displ = 0;
         for (auto & i : attrs)
@@ -435,7 +437,7 @@ void ExprLambda::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv>
         es.exprEnvs.insert(std::make_pair(this, env));
 
     auto newEnv = std::make_shared<StaticEnv>(
-        false, env.get(),
+        nullptr, env.get(),
         (hasFormals() ? formals->formals.size() : 0) +
         (!arg ? 0 : 1));
 
@@ -471,7 +473,7 @@ void ExprLet::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
     if (es.debugRepl)
         es.exprEnvs.insert(std::make_pair(this, env));
 
-    auto newEnv = std::make_shared<StaticEnv>(false, env.get(), attrs->attrs.size());
+    auto newEnv = std::make_shared<StaticEnv>(nullptr, env.get(), attrs->attrs.size());
 
     Displacement displ = 0;
     for (auto & i : attrs->attrs)
@@ -490,6 +492,10 @@ void ExprWith::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
     if (es.debugRepl)
         es.exprEnvs.insert(std::make_pair(this, env));
 
+    parentWith = nullptr;
+    for (auto * e = env.get(); e && !parentWith; e = e->up)
+        parentWith = e->isWith;
+
     /* Does this `with' have an enclosing `with'?  If so, record its
        level so that `lookupVar' can look up variables in the previous
        `with' if this one doesn't contain the desired attribute. */
@@ -506,7 +512,7 @@ void ExprWith::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
         es.exprEnvs.insert(std::make_pair(this, env));
 
     attrs->bindVars(es, env);
-    auto newEnv = std::make_shared<StaticEnv>(true, env.get());
+    auto newEnv = std::make_shared<StaticEnv>(this, env.get());
     body->bindVars(es, newEnv);
 }
 

src/libexpr/nixexpr.hh

@@ -138,6 +138,7 @@ std::ostream & operator << (std::ostream & str, const Pos & pos);
 struct Env;
 struct Value;
 class EvalState;
+struct ExprWith;
 struct StaticEnv;
 
 
@@ -226,8 +227,11 @@ struct ExprVar : Expr
     Symbol name;
 
     /* Whether the variable comes from an environment (e.g. a rec, let
-       or function argument) or from a "with". */
+       or function argument) or from a "with".
-    bool fromWith;
+
+       `nullptr`: Not from a `with`.
+       Valid pointer: the nearest, innermost `with` expression to query first. */
+    ExprWith * fromWith;
 
     /* In the former case, the value is obtained by going `level`
        levels up from the current environment and getting the
@@ -385,6 +389,7 @@ struct ExprWith : Expr
     PosIdx pos;
     Expr * attrs, * body;
     size_t prevWith;
+    ExprWith * parentWith;
     ExprWith(const PosIdx & pos, Expr * attrs, Expr * body) : pos(pos), attrs(attrs), body(body) { };
     PosIdx getPos() const override { return pos; }
     COMMON_METHODS
@@ -478,14 +483,14 @@ extern ExprBlackHole eBlackHole;
    runtime. */
 struct StaticEnv
 {
-    bool isWith;
+    ExprWith * isWith;
     const StaticEnv * up;
 
     // Note: these must be in sorted order.
     typedef std::vector<std::pair<Symbol, Displacement>> Vars;
     Vars vars;
 
-    StaticEnv(bool isWith, const StaticEnv * up, size_t expectedSize = 0) : isWith(isWith), up(up) {
+    StaticEnv(ExprWith * isWith, const StaticEnv * up, size_t expectedSize = 0) : isWith(isWith), up(up) {
         vars.reserve(expectedSize);
     };
 

src/libexpr/primops.cc

@@ -214,7 +214,7 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
             Env * env = &state.allocEnv(vScope->attrs->size());
             env->up = &state.baseEnv;
 
-            auto staticEnv = std::make_shared<StaticEnv>(false, state.staticBaseEnv.get(), vScope->attrs->size());
+            auto staticEnv = std::make_shared<StaticEnv>(nullptr, state.staticBaseEnv.get(), vScope->attrs->size());
 
             unsigned int displ = 0;
             for (auto & attr : *vScope->attrs) {