Documenting bytes to fixed bytes conversion.

Co-authored-by: chriseth <[email protected]>

Co-authored-by: Alex Beregszaszi <[email protected]>

Changelog.md

@@ -1,6 +1,7 @@
 ### 0.8.5 (unreleased)
 
 Language Features:
+* Allowing conversion from ``bytes`` and ``bytes`` slices to ``bytes1``/.../``bytes32``.
 
 
 Compiler Features:

docs/types/conversion.rst

@@ -99,6 +99,27 @@ rules explicit::
     uint8 d = uint8(uint16(a)); // d will be 0x34
     uint8 e = uint8(bytes1(a)); // e will be 0x12
 
+``bytes`` arrays and ``bytes`` calldata slices can be converted explicitly to fixed bytes types (``bytes1``/.../``bytes32``).
+In case the array is longer than the target fixed bytes type, truncation at the end will happen.
+If the array is shorter than the target type, it will be padded with zeros at the end.
+
+::
+
+    // SPDX-License-Identifier: GPL-3.0
+    pragma solidity ^0.8.5;
+
+    contract C {
+        bytes s = "abcdefgh";
+        function f(bytes calldata c, bytes memory m) public view returns (bytes16, bytes3) {
+            require(c.length == 16, "");
+            bytes16 b = bytes16(m);  // if length of m is greater than 16, truncation will happen
+            b = bytes16(s);  // padded on the right, so result is "abcdefgh\0\0\0\0\0\0\0\0"
+            bytes3 b1 = bytes3(s); // truncated, b1 equals to "abc"
+            b = bytes16(c[:8]);  // also padded with zeros
+            return (b, b1);
+        }
+    }
+
 .. _types-conversion-literals:
 
 Conversions between Literals and Elementary Types

docs/types/reference-types.rst

@@ -492,7 +492,7 @@ Array slices are useful to ABI-decode secondary data passed in function paramete
 ::
 
     // SPDX-License-Identifier: GPL-3.0
-    pragma solidity >=0.7.0 <0.9.0;
+    pragma solidity >0.8.4 <0.9.0;
     contract Proxy {
         /// @dev Address of the client contract managed by proxy i.e., this contract
         address client;
@@ -504,13 +504,9 @@ Array slices are useful to ABI-decode secondary data passed in function paramete
         /// Forward call to "setOwner(address)" that is implemented by client
         /// after doing basic validation on the address argument.
         function forward(bytes calldata _payload) external {
-            // Since ABI decoding requires padded data, we cannot
-            // use abi.decode(_payload[:4], (bytes4)).
-            bytes4 sig =
-                _payload[0] |
-                (bytes4(_payload[1]) >> 8) |
-                (bytes4(_payload[2]) >> 16) |
-                (bytes4(_payload[3]) >> 24);
+            bytes4 sig = bytes4(_payload[:4]);
+            // Due to truncating behaviour, bytes4(_payload) performs identically.
+            // bytes4 sig = bytes4(_payload);
             if (sig == bytes4(keccak256("setOwner(address)"))) {
                 address owner = abi.decode(_payload[4:], (address));
                 require(owner != address(0), "Address of owner cannot be zero.");