Remove Go 1.21 polyfills (#118)

We can drop these now that the main module requires Go 1.21.

See:
- #111

---------

Co-authored-by: Prashant V <github@prashantv.com>

.github/workflows/ci.yml

@@ -18,6 +18,11 @@ on:
 permissions:
   contents: read
 
+env:
+  # Use the Go toolchain installed by setup-go
+  # https://github.com/actions/setup-go/issues/457
+  GOTOOLCHAIN: local
+
 jobs:
 
   test:
@@ -86,7 +91,7 @@ jobs:
     - uses: actions/setup-go@v5
       name: Set up Go
       with:
-        # Use the Go version specified in the go.mod for linting.
+        # Use the Go language version in go.mod for linting.
         go-version-file: go.mod
         cache: false  # managed by golangci-lint
 

CHANGELOG.md

@@ -37,7 +37,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   Example: `go build -toolexec="errtrace -required-packages pkg/..." pkg/to/build`
 
 ### Changed
-- Update `go` directive in go.mod to 1.21.
+- Update `go` directive in go.mod to 1.21, and drop compatibility with Go 1.20 and earlier.
 
 ### Fixed
 - cmd/errtrace: Don't exit with a non-zero status when `-h` is used.

cmd/errtrace/toolexec.go

@@ -309,7 +309,7 @@ func readBuildSHA() (_ string, ok bool) {
 
 // isStdLib checks if the current execution is for stdlib.
 func isStdLib(args []string) bool {
-	return slicesContains(args, "-std")
+	return slices.Contains(args, "-std")
 }
 
 func packageSelectorMatch(selector, importPath string) bool {

tree.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"runtime"
+	"slices"
 	"strings"
 )
 
@@ -72,7 +73,7 @@ loop:
 		}
 	}
 
-	sliceReverse(current.Trace)
+	slices.Reverse(current.Trace)
 	return current
 }