keep copies of parser inputs that are in-memory only

the parser modifies its inputs, which means that sharing them between
the error context reporting system and the parser itself can confuse the
reporting system. usually this led to early truncation of error context
reports which, while not dangerous, can be quite confusing.

src/libexpr/eval.cc

@@ -2777,9 +2777,12 @@ Expr * EvalState::parseExprFromFile(const SourcePath & path, std::shared_ptr<Sta
 
 Expr * EvalState::parseExprFromString(std::string s_, const SourcePath & basePath, std::shared_ptr<StaticEnv> & staticEnv)
 {
-    auto s = make_ref<std::string>(std::move(s_));
-    s->append("\0\0", 2);
-    return parse(s->data(), s->size(), Pos::String{.source = s}, basePath, staticEnv);
+    // NOTE this method (and parseStdin) must take care to *fully copy* their input
+    // into their respective Pos::Origin until the parser stops overwriting its input
+    // data.
+    auto s = make_ref<std::string>(s_);
+    s_.append("\0\0", 2);
+    return parse(s_.data(), s_.size(), Pos::String{.source = s}, basePath, staticEnv);
 }
 
 
@@ -2791,12 +2794,15 @@ Expr * EvalState::parseExprFromString(std::string s, const SourcePath & basePath
 
 Expr * EvalState::parseStdin()
 {
+    // NOTE this method (and parseExprFromString) must take care to *fully copy* their
+    // input into their respective Pos::Origin until the parser stops overwriting its
+    // input data.
     //Activity act(*logger, lvlTalkative, "parsing standard input");
     auto buffer = drainFD(0);
     // drainFD should have left some extra space for terminators
     buffer.append("\0\0", 2);
-    auto s = make_ref<std::string>(std::move(buffer));
-    return parse(s->data(), s->size(), Pos::Stdin{.source = s}, rootPath("."), staticBaseEnv);
+    auto s = make_ref<std::string>(buffer);
+    return parse(buffer.data(), buffer.size(), Pos::Stdin{.source = s}, rootPath("."), staticBaseEnv);
 }
 
 

tests/functional/lang/parse-fail-dup-attrs-1.err.exp

@@ -3,3 +3,4 @@ error: attribute 'x' already defined at «stdin»:1:3
             2|   y = 456;
             3|   x = 789;
              |   ^
+            4| }

tests/functional/lang/parse-fail-dup-attrs-2.err.exp

@@ -3,3 +3,4 @@ error: attribute 'x' already defined at «stdin»:9:5
             9|     x = 789;
            10|     inherit (as) x;
              |                 ^
+           11|   };

tests/functional/lang/parse-fail-dup-attrs-3.err.exp

@@ -3,3 +3,4 @@ error: attribute 'x' already defined at «stdin»:9:5
             9|     x = 789;
            10|     inherit (as) x;
              |                 ^
+           11|   };

tests/functional/lang/parse-fail-dup-attrs-4.err.exp

@@ -3,3 +3,4 @@ error: attribute 'services.ssh.port' already defined at «stdin»:2:3
             2|   services.ssh.port = 22;
             3|   services.ssh.port = 23;
              |   ^
+            4| }

tests/functional/lang/parse-fail-dup-attrs-7.err.exp

@@ -3,3 +3,4 @@ error: attribute 'x' already defined at «stdin»:6:12
             6|     inherit x;
             7|     inherit x;
              |            ^
+            8|   };

tests/functional/lang/parse-fail-undef-var-2.err.exp

@@ -1,5 +1,6 @@
 error: syntax error, unexpected ':', expecting '}'
        at «stdin»:3:13:
             2|
-            3|   f = {x, y :
+            3|   f = {x, y : ["baz" "bar" z "bat"]}: x + y;
              |             ^
+            4|

tests/functional/lang/parse-fail-utf8.err.exp

@@ -1,4 +1,5 @@
 error: syntax error, unexpected invalid token, expecting end of file
        at «stdin»:1:5:
-            1| 123 ﾃ
+            1| 123 é 4
              |     ^
+            2|