add 2.0 changelog (#21107)

[backport: 2.0] add v2.0 changelog

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+# v2.0.0 - yyyy-mm-dd
+
+
+## Changes affecting backward compatibility
+- `httpclient.contentLength` default to `-1` if the Content-Length header is not set in the response, it followed Apache HttpClient(Java), http(go) and .Net HttpWebResponse(C#) behavior. Previously it raised `ValueError`.
+
+- `addr` is now available for all addressable locations,
+  `unsafeAddr` is now deprecated and an alias for `addr`.
+
+- Certain definitions from the default `system` module have been moved to
+  the following new modules:
+
+  - `std/syncio`
+  - `std/assertions`
+  - `std/formatfloat`
+  - `std/objectdollar`
+  - `std/widestrs`
+  - `std/typedthreads`
+  - `std/sysatomics`
+
+  In the future, these definitions will be removed from the `system` module,
+  and their respective modules will have to be imported to use them.
+  Currently, to make these imports required, the `-d:nimPreviewSlimSystem` option
+  may be used.
+
+- Enabling `-d:nimPreviewSlimSystem` also removes the following deprecated
+  symbols in the `system` module:
+  - Aliases with `Error` suffix to exception types that have a `Defect` suffix
+    (see [exceptions](https://nim-lang.org/docs/exceptions.html)):
+    `ArithmeticError`, `DivByZeroError`, `OverflowError`,
+    `AccessViolationError`, `AssertionError`, `OutOfMemError`, `IndexError`,
+    `FieldError`, `RangeError`, `StackOverflowError`, `ReraiseError`,
+    `ObjectAssignmentError`, `ObjectConversionError`, `FloatingPointError`,
+    `FloatOverflowError`, `FloatUnderflowError`, `FloatInexactError`,
+    `DeadThreadError`, `NilAccessError`
+  - `addQuitProc`, replaced by `exitprocs.addExitProc`
+  - Legacy unsigned conversion operations: `ze`, `ze64`, `toU8`, `toU16`, `toU32`
+  - `TaintedString`, formerly a distinct alias to `string`
+  - `PInt32`, `PInt64`, `PFloat32`, `PFloat64`, aliases to
+    `ptr int32`, `ptr int64`, `ptr float32`, `ptr float64`
+
+- Enabling `-d:nimPreviewSlimSystem` removes the import of `channels_builtin` in
+  in the `system` module.
+
+- Enabling `-d:nimPreviewCstringConversion`, `ptr char`, `ptr array[N, char]` and `ptr UncheckedArray[N, char]` don't support conversion to cstring anymore.
+
+- The `gc:v2` option is removed.
+
+- The `mainmodule` and `m` options are removed.
+
+- The `threads:on` option is now the default.
+
+- Optional parameters in combination with `: body` syntax (RFC #405) are now opt-in via
+  `experimental:flexibleOptionalParams`.
+
+- Automatic dereferencing (experimental feature) is removed.
+
+- The `Math.trunc` polyfill for targeting Internet Explorer was
+  previously included in most JavaScript output files.
+  Now, it is only included with `-d:nimJsMathTruncPolyfill`.
+  If you are targeting Internet Explorer, you may choose to enable this option
+  or define your own `Math.trunc` polyfill using the [`emit` pragma](https://nim-lang.org/docs/manual.html#implementation-specific-pragmas-emit-pragma).
+  Nim uses `Math.trunc` for the division and modulo operators for integers.
+
+- `shallowCopy` and `shallow` are removed for ARC/ORC. Use `move` when possible or combine assignment and
+`sink` for optimization purposes.
+
+- The `nimPreviewDotLikeOps` define is going to be removed or deprecated.
+
+- The `{.this.}` pragma, deprecated since 0.19, has been removed.
+- `nil` literals can no longer be directly assigned to variables or fields of `distinct` pointer types. They must be converted instead.
+  ```nim
+  type Foo = distinct ptr int
+
+  # Before:
+  var x: Foo = nil
+  # After:
+  var x: Foo = Foo(nil)
+  ```
+- Removed two type pragma syntaxes deprecated since 0.20, namely
+  `type Foo = object {.final.}`, and `type Foo {.final.} [T] = object`.
+
+- `foo a = b` now means `foo(a = b)` rather than `foo(a) = b`. This is consistent
+  with the existing behavior of `foo a, b = c` meaning `foo(a, b = c)`.
+  This decision was made with the assumption that the old syntax was used rarely;
+  if your code used the old syntax, please be aware of this change.
+
+- [Overloadable enums](https://nim-lang.github.io/Nim/manual.html#overloadable-enum-value-names) and Unicode Operators
+  are no longer experimental.
+
+- Removed the `nimIncrSeqV3` define.
+
+- `macros.getImpl` for `const` symbols now returns the full definition node
+  (as `nnkConstDef`) rather than the AST of the constant value.
+
+- Lock levels are deprecated, now a noop.
+
+- ORC is now the default memory management strategy. Use
+  `--mm:refc` for a transition period.
+
+- `strictEffects` are no longer experimental.
+  Use `legacy:laxEffects` to keep backward compatibility.
+
+- The `gorge`/`staticExec` calls will now return a descriptive message in the output
+  if the execution fails for whatever reason. To get back legacy behaviour use `-d:nimLegacyGorgeErrors`.
+
+- Pointer to `cstring` conversion now triggers a `[PtrToCstringConv]` warning.
+  This warning will become an error in future versions! Use a `cast` operation
+  like `cast[cstring](x)` instead.
+
+- `logging` will default to flushing all log level messages. To get the legacy behaviour of only flushing Error and Fatal messages, use `-d:nimV1LogFlushBehavior`.
+
+- Object fields now support default values, see https://nim-lang.github.io/Nim/manual.html#types-default-values-for-object-fields for details.
+
+- Redefining templates with the same signature was previously
+  allowed to support certain macro code. To do this explicitly, the
+  `{.redefine.}` pragma has been added. Note that this is only for templates.
+  Implicit redefinition of templates is now deprecated and will give an error in the future.
+
+- Using an unnamed break in a block is deprecated. This warning will become an error in future versions! Use a named block with a named break instead.
+
+- Several Standard libraries are moved to nimble packages, use `nimble` to install them:
+  - `std/punycode` => `punycode`
+  - `std/asyncftpclient` => `asyncftpclient`
+  - `std/smtp` => `smtp`
+  - `std/db_common` => `db_connector/db_common`
+  - `std/db_sqlite` => `db_connector/db_sqlite`
+  - `std/db_mysql` => `db_connector/db_mysql`
+  - `std/db_postgres` => `db_connector/db_postgres`
+  - `std/db_odbc` => `db_connector/db_odbc`
+
+- Previously, calls like `foo(a, b): ...` or `foo(a, b) do: ...` where the final argument of
+  `foo` had type `proc ()` were assumed by the compiler to mean `foo(a, b, proc () = ...)`.
+  This behavior is now deprecated. Use `foo(a, b) do (): ...` or `foo(a, b, proc () = ...)` instead.
+
+- If no exception or any exception deriving from Exception but not Defect or CatchableError given in except, a `warnBareExcept` warning will be triggered.
+
+## Standard library additions and changes
+
+[//]: # "Changes:"
+- OpenSSL 3 is now supported.
+- `macros.parseExpr` and `macros.parseStmt` now accept an optional
+  filename argument for more informative errors.
+- Module `colors` expanded with missing colors from the CSS color standard.
+  `colPaleVioletRed` and `colMediumPurple` have also been changed to match the CSS color standard.
+- Fixed `lists.SinglyLinkedList` being broken after removing the last node ([#19353](https://github.com/nim-lang/Nim/pull/19353)).
+- The `md5` module now works at compile time and in JavaScript.
+- Changed `mimedb` to use an `OrderedTable` instead of `OrderedTableRef` to support `const` tables.
+- `strutils.find` now uses and defaults to `last = -1` for whole string searches,
+  making limiting it to just the first char (`last = 0`) valid.
+- `random.rand` now works with `Ordinal`s.
+- Undeprecated `os.isvalidfilename`.
+- `std/oids` now uses `int64` to store time internally (before it was int32).
+- `std/uri.Uri` dollar `$` improved, precalculates the `string` result length from the `Uri`.
+- `std/uri.Uri.isIpv6` is now exported.
+- `std/logging.ConsoleLogger` and `FileLogger` now have a `flushThreshold` attribute to set what log message levels are automatically flushed. For Nim v1 use `-d:nimFlushAllLogs` to automatically flush all message levels. Flushing all logs is the default behavior for Nim v2.
+
+
+- `std/net.IpAddress` dollar `$` improved, uses a fixed capacity for the `string` result based from the `IpAddressFamily`.
+- `std/jsfetch.newFetchOptions` now has default values for all parameters
+- `std/jsformdata` now accepts `Blob` data type.
+
+
+[//]: # "Additions:"
+- Added ISO 8601 week date utilities in `times`:
+  - Added `IsoWeekRange`, a range type for weeks in a week-based year.
+  - Added `IsoYear`, a distinct type for a week-based year in contrast to a regular year.
+  - Added a `initDateTime` overload to create a datetime from an ISO week date.
+  - Added `getIsoWeekAndYear` to get an ISO week number and week-based year from a datetime.
+  - Added `getIsoWeeksInYear` to return the number of weeks in a week-based year.
+- Added new modules which were part of `std/os`:
+  - Added `std/oserrors` for OS error reporting. Added `std/envvars` for environment variables handling.
+  - Added `std/paths`, `std/dirs`, `std/files`, `std/symlinks` and `std/appdirs`.
+  - Added `std/cmdline` for reading command line parameters.
+- Added `sep` parameter in `std/uri` to specify the query separator.
+- Added bindings to [`Array.shift`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/shift)
+  and [`queueMicrotask`](https://developer.mozilla.org/en-US/docs/Web/API/queueMicrotask)
+  in `jscore` for JavaScript targets.
+- Added `UppercaseLetters`, `LowercaseLetters`, `PunctuationChars`, `PrintableChars` sets to `std/strutils`.
+- Added `complex.sgn` for obtaining the phase of complex numbers.
+- Added `insertAdjacentText`, `insertAdjacentElement`, `insertAdjacentHTML`,
+  `after`, `before`, `closest`, `append`, `hasAttributeNS`, `removeAttributeNS`,
+  `hasPointerCapture`, `releasePointerCapture`, `requestPointerLock`,
+  `replaceChildren`, `replaceWith`, `scrollIntoViewIfNeeded`, `setHTML`,
+  `toggleAttribute`, and `matches` to `std/dom`.
+- Added [`jsre.hasIndices`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/hasIndices)
+- Added `capacity` for `string` and `seq` to return the current capacity, see https://github.com/nim-lang/RFCs/issues/460
+- Added `openArray[char]` overloads for `std/parseutils` allowing more code reuse.
+- Added `openArray[char]` overloads for `std/unicode` allowing more code reuse.
+- Added `safe` parameter to `base64.encodeMime`.
+
+[//]: # "Deprecations:"
+- Deprecated `selfExe` for Nimscript.
+- Deprecated `std/sums`.
+- Deprecated `std/base64.encode` for collections of arbitrary integer element type.
+  Now only `byte` and `char` are supported.
+
+[//]: # "Removals:"
+- Removed deprecated module `parseopt2`.
+- Removed deprecated module `sharedstrings`.
+- Removed deprecated module `dom_extensions`.
+- Removed deprecated module `LockFreeHash`.
+- Removed deprecated module `events`.
+- Removed deprecated `oids.oidToString`.
+- Removed define `nimExperimentalAsyncjsThen` for `std/asyncjs.then` and `std/jsfetch`.
+- Removed deprecated `jsre.test` and `jsre.toString`.
+- Removed deprecated `math.c_frexp`.
+- Removed deprecated `` httpcore.`==` ``.
+- Removed deprecated `std/posix.CMSG_SPACE` and `std/posix.CMSG_LEN` that takes wrong argument types.
+- Removed deprecated `osproc.poDemon`, symbol with typo.
+- Removed deprecated `tables.rightSize`.
+
+
+- Removed deprecated `posix.CLONE_STOPPED`.
+
+
+## Language changes
+
+- [Tag tracking](https://nim-lang.github.io/Nim/manual.html#effect-system-tag-tracking) supports the definition of forbidden tags by the `.forbids` pragma
+  which can be used to disable certain effects in proc types.
+- [Case statement macros](https://nim-lang.github.io/Nim/manual.html#macros-case-statement-macros) are no longer experimental,
+  meaning you no longer need to enable the experimental switch `caseStmtMacros` to use them.
+- Full command syntax and block arguments i.e. `foo a, b: c` are now allowed
+  for the right-hand side of type definitions in type sections. Previously
+  they would error with "invalid indentation".
+
+- Compile-time define changes:
+  - `defined` now accepts identifiers separated by dots, i.e. `defined(a.b.c)`.
+    In the command line, this is defined as `-d:a.b.c`. Older versions can
+    use accents as in ``defined(`a.b.c`)`` to access such defines.
+  - [Define pragmas for constants](https://nim-lang.github.io/Nim/manual.html#implementation-specific-pragmas-compileminustime-define-pragmas)
+    now support a string argument for qualified define names.
+
+    ```nim
+    # -d:package.FooBar=42
+    const FooBar {.intdefine: "package.FooBar".}: int = 5
+    echo FooBar # 42
+    ```
+
+    This was added to help disambiguate similar define names for different packages.
+    In older versions, this could only be achieved with something like the following:
+
+    ```nim
+    const FooBar = block:
+      const `package.FooBar` {.intdefine.}: int = 5
+      `package.FooBar`
+    ```
+  - A generic `define` pragma for constants has been added that interprets
+    the value of the define based on the type of the constant value.
+    See the [experimental manual](https://nim-lang.github.io/Nim/manual_experimental.html#generic-define-pragma)
+    for a list of supported types.
+
+- [Macro pragmas](https://nim-lang.github.io/Nim/manual.html#userminusdefined-pragmas-macro-pragmas) changes:
+  - Templates now accept macro pragmas.
+  - Macro pragmas for var/let/const sections have been redesigned in a way that works
+    similarly to routine macro pragmas. The new behavior is documented in the
+    [experimental manual](https://nim-lang.github.io/Nim/manual_experimental.html#extended-macro-pragmas).
+  - Pragma macros on type definitions can now return `nnkTypeSection` nodes as well as `nnkTypeDef`,
+    allowing multiple type definitions to be injected in place of the original type definition.
+
+    ```nim
+    import macros
+    macro multiply(amount: static int, s: untyped): untyped =
+      let name = $s[0].basename
+      result = newNimNode(nnkTypeSection)
+      for i in 1 .. amount:
+        result.add(newTree(nnkTypeDef, ident(name & $i), s[1], s[2]))
+    type
+      Foo = object
+      Bar {.multiply: 3.} = object
+        x, y, z: int
+      Baz = object
+    # becomes
+    type
+      Foo = object
+      Bar1 = object
+        x, y, z: int
+      Bar2 = object
+        x, y, z: int
+      Bar3 = object
+        x, y, z: int
+      Baz = object
+    ```
+
+- A new form of type inference called [top-down inference](https://nim-lang.github.io/Nim/manual_experimental.html#topminusdown-type-inference)
+  has been implemented for a variety of basic cases. For example, code like the following now compiles:
+
+  ```nim
+  let foo: seq[(float, byte, cstring)] = @[(1, 2, "abc")]
+  ```
+
+- `cstring` is now accepted as a selector in `case` statements, removing the
+  need to convert to `string`. On the JS backend, this is translated directly
+  to a `switch` statement.
+
+- Nim now supports `out` parameters and ["strict definitions"](https://nim-lang.github.io/Nim/manual_experimental.html#strict-definitions-and-nimout-parameters).
+- Nim now offers a [strict mode](https://nim-lang.github.io/Nim/manual_experimental.html#strict-case-objects) for `case objects`.
+
+
+## Compiler changes
+
+- The `gc` switch has been renamed to `mm` ("memory management") in order to reflect the
+  reality better. (Nim moved away from all techniques based on "tracing".)
+
+- Defines the `gcRefc` symbol which allows writing specific code for the refc GC.
+
+- `nim` can now compile version 1.4.0 as follows: `nim c --lib:lib --stylecheck:off compiler/nim`,
+  without requiring `-d:nimVersion140` which is now a noop.
+
+- `--styleCheck`, `--hintAsError` and `--warningAsError` now only apply to the current package.
+
+- The switch `--nimMainPrefix:prefix` has been added to add a prefix to the names of `NimMain` and
+  related functions produced on the backend. This prevents conflicts with other Nim
+  static libraries.
+
+- When compiling for Release the flag `-fno-math-errno` is used for GCC.
+
+
+## Tool changes
+
+- Nim now ships Nimble version 0.14 which added support for lock-files. Libraries are stored in `$nimbleDir/pkgs2` (it was `$nimbleDir/pkgs`).