[inlining] Revised inlining implementation

language/documentation/book/src/functions.md

@@ -509,3 +509,47 @@ Using `return` without an argument is shorthand for `return ()`. That is, the fo
 fun foo() { return }
 fun foo() { return () }
 ```
+
+## Inline Functions
+
+Inline functions are functions which are expanded at caller side instead
+of compiled into Move bytecode. This allows to safe gas and may lead
+to faster execution. For example, one can define an inline function
+
+```move=
+inline fun percent(x: u64, y: u64):u64 { x * 100 / y }
+```
+
+Now, when call `percent(2, 200)` the compiler will inline the function
+definition as if the user has written `2 * 100 / 200`.
+
+### Function Parameters
+
+Inline functions support _function parameters_. This allows
+to define higher-order functions in Move which can comprehend common
+programming patterns. As inline functions are expanded at compilation time,
+this feature of function parameters can be supported without direct
+support for it in Move bytecode.
+
+Consider the following function (from the `vector` module):
+
+```move=
+/// Fold the function over the elements. For example, `fold(vector[1,2,3], 0, f)` will execute
+/// `f(f(f(0, 1), 2), 3)`
+public inline fun fold<Accumulator, Element>(
+    v: vector<Element>,
+    init: Accumulator,
+    f: |Accumulator,Element|Accumulator
+): Accumulator {
+  let accu = init;
+  foreach(v, |elem| accu = g(accu, elem));
+  accu
+}
+```
+
+Here, `foreach` is itself an inline function.
+
+In general, only lambda expressions can be passed to function parameters.
+Similar as the inline function itself, those lambdas are expanded at caller
+side. Notice that a lambda can access variables in the context, as in the
+example the variable `accu`.

language/move-compiler/src/diagnostics/codes.rs

@@ -244,7 +244,6 @@ codes!(
     // errors for inlining
     Inlining: [
         Recursion: { msg: "recursion during function inlining not allowed", severity: BlockingError },
-        InvalidBorrow: { msg: "inlined parameter cannot be borrowed", severity: BlockingError },
     ],
 );
 

language/move-compiler/src/inlining/mod.rs

@@ -3,3 +3,4 @@
 // SPDX-License-Identifier: Apache-2.0
 
 pub mod translate;
+mod visitor;