Static linking and backtrace after Swift 5.9 (#77)

* Link binary statically for drastic image size reductions

* Copy swift-backtrace binary to final image

* Set SWIFT_BACKTRACE env var automatically to enable it

* Remove support for old versions of Swift, add Swift 6.0

* Use fast symbolication

README.md

@@ -14,10 +14,9 @@ $ heroku create --buildpack vapor/vapor
 
 $ git push heroku main
 remote: -----> Swift app detected
-remote: -----> Using Swift 5.10.1 (default)
-remote: -----> Using built-in clang (Swift 5.10.1)
+remote: -----> Using Swift 6.0 (default)
 remote: -----> Installing swiftenv
-remote: -----> Installing Swift 5.10.1
+remote: -----> Installing Swift 6.0
 ...
 ```
 
@@ -50,9 +49,9 @@ web: Run --env=production --port=$PORT
 
 ### Specify a Swift version
 
-The buildpack defaults to Swift 5.10.1 and will be updated when new Swift versions are released.
+The buildpack defaults to Swift 6.0, but is compatible with Swift 5.9 - 5.10.1.
 
-If you need to use a specific version of the Swift toolchain, including older versions – for example Swift 4.2.x to retain compatibility with Swift 3 projects, or a previous version as you run into issues with the latest – you can pin any version number using a file called `.swift-version` in the root of the project folder, or by setting a `SWIFT_VERSION` configuration variable on Heroku, then deploying again. 
+If you need to use a specific older version of the Swift toolchain from this range, you can pin the version number using a file called `.swift-version` in the root of the project folder, or by setting a `SWIFT_VERSION` configuration variable on Heroku, then deploying again. 
 
 ```shell
 $ echo '5.9.2' > .swift-version
@@ -69,7 +68,13 @@ $ git commit -m "Pin Swift version to 5.9.2" --allow-empty
 $ git push heroku main
 ```
 
-The version format used file is compatible with [swiftenv](http://github.com/kylef/swiftenv).
+#### Legacy versions
+
+If your project requires an even older version of Swift, you can use a previous release of the buildpack via an old tag.
+
+```shell
+$ heroku buildpacks:set https://github.com/vapor-community/heroku-buildpack.git#5101-release
+```
 
 ### Active build configuration
 

bin/compile

@@ -11,9 +11,7 @@ BUILD_DIR=$1
 CACHE_DIR=$2
 ENV_DIR=$3
 
-CLANG_VERSION=7.0.1
-
-SWIFT_VERSION="5.10.1"
+SWIFT_VERSION="6.0"
 SWIFT_BUILD_CONFIGURATION="release"
 SWIFT_BUILD_FLAGS=""
 
@@ -38,15 +36,7 @@ if [ -f "$ENV_DIR/SWIFT_BUILD_FLAGS" ]; then
   SWIFT_BUILD_FLAGS=`cat "$ENV_DIR/SWIFT_BUILD_FLAGS"`
 fi
 
-
 mkdir -p "$CACHE_DIR/$STACK"
-
-if [[ $SWIFT_VERSION = 5* ]]; then
-  puts-step "Using built-in clang (Swift $SWIFT_VERSION)"
-else
-  source "$BIN_DIR/steps/clang"
-fi
-
 source "$BIN_DIR/steps/swiftenv"
 
 cd $BUILD_DIR
@@ -65,3 +55,4 @@ mkdir -p $BUILD_DIR/.profile.d
 
 set-env PATH '$HOME/.swift-bin:$PATH'
 set-env LD_LIBRARY_PATH '$LD_LIBRARY_PATH:$HOME/.swift-lib'
+set-env SWIFT_BACKTRACE 'enable=yes,sanitize=yes,threads=all,images=all,interactive=no,output-to=stderr,symbolicate=fast,swift-backtrace=.swift-bin/swift-backtrace-static'

bin/steps/swift-build

@@ -1,2 +1,2 @@
 puts-step "Building package ($SWIFT_BUILD_CONFIGURATION configuration)"
-swift build $SWIFT_BUILD_FLAGS -c "$SWIFT_BUILD_CONFIGURATION"
+swift build $SWIFT_BUILD_FLAGS -c "$SWIFT_BUILD_CONFIGURATION" --static-swift-stdlib

bin/steps/swift-install

@@ -1,15 +1,14 @@
 DYNAMIC_LIBRARY_REGEX=".*\.so\(\.[0-9]+\)*\'"
+SWIFT_PREFIX="$(swiftenv prefix)"
 
 puts-step "Installing dynamic libraries"
 mkdir -p $BUILD_DIR/.swift-lib
-SWIFT_PREFIX="$(swiftenv prefix)"
-find -L $SWIFT_PREFIX/usr/lib/swift/linux -regex "$DYNAMIC_LIBRARY_REGEX" -type f -exec cp -a {} $BUILD_DIR/.swift-lib \; || true 2>/dev/null
 find -L .build/$SWIFT_BUILD_CONFIGURATION -regex "$DYNAMIC_LIBRARY_REGEX" -type f -exec cp -a {} $BUILD_DIR/.swift-lib \; || true 2>/dev/null
-cp -a /usr/lib/x86_64-linux-gnu/libatomic* $BUILD_DIR/.swift-lib
 
 puts-step "Installing binaries"
 mkdir -p $BUILD_DIR/.swift-bin
 find -L .build/$SWIFT_BUILD_CONFIGURATION ! -regex "$DYNAMIC_LIBRARY_REGEX" -type f -perm /a+x -exec cp -a {} $BUILD_DIR/.swift-bin \; || true 2>/dev/null
+cp -a $SWIFT_PREFIX/usr/libexec/swift/linux/swift-backtrace-static $BUILD_DIR/.swift-bin
 
 puts-step "Installing resources"
 find -L .build/$SWIFT_BUILD_CONFIGURATION/* -regex '.*\.resources$' -exec cp -a {} $BUILD_DIR/.swift-bin \; || true 2>/dev/null