Add -context-only option

As described in #13, third-party implementations of `slog.Handler` can choose to take values out of
a context to supplement the attributes already used. An example of this in my day job is adding
trace IDs to enhance serviceability.

This change adds the ability to optionally identify `slog` methods that do not take a context and so
prevent the ability to take values from that context. Any matches can be resolved by appending
`Context` to the method. For example, `slog.Info` becomes `slog.Context`.

README.md

@@ -26,6 +26,7 @@ The linter has several options, so you can adjust it to your own code style.
 * Enforce using either key-value pairs or attributes for the entire project (optional)
 * Enforce using constants instead of raw keys (optional)
 * Enforce putting arguments on separate lines (optional)
+* Enforce using methods that take a context (optional)
 
 ## 📦 Install
 
@@ -98,5 +99,22 @@ slog.Info("a user has logged in",
 )
 ```
 
+### Context only
+
+The `-context-only` flag causes `sloglint` to report the use of any methods that do not take a `context.Context`.
+
+```go
+slog.Info("a user has logged in") // sloglint: methods that do not take a context should not be used"
+```
+
+This report can be fixed by using the equivalent method with a `Context` suffix.
+
+```go
+slog.InfoContext(ctx, "a user has logged in")
+```
+
+The `slog.Handler` implementations in the standard library do not currently utilise the given
+context. However, third-party implementations can create `slog.Attr` instances from the context.
+
 [1]: https://github.com/go-simpler/sloglint/releases
 [2]: https://github.com/go-simpler/sloggen

sloglint.go

@@ -21,6 +21,7 @@ type Options struct {
 	AttrOnly       bool // Enforce using attributes only (incompatible with KVOnly).
 	NoRawKeys      bool // Enforce using constants instead of raw keys.
 	ArgsOnSepLines bool // Enforce putting arguments on separate lines.
+	ContextOnly    bool // Enforce using methods that take a context.
 }
 
 // New creates a new sloglint analyzer.
@@ -58,6 +59,7 @@ func flags(opts *Options) flag.FlagSet {
 	boolVar(&opts.AttrOnly, "attr-only", "enforce using attributes only (incompatible with -kv-only)")
 	boolVar(&opts.NoRawKeys, "no-raw-keys", "enforce using constants instead of raw keys")
 	boolVar(&opts.ArgsOnSepLines, "args-on-sep-lines", "enforce putting arguments on separate lines")
+	boolVar(&opts.ContextOnly, "context-only", "enforce using methods that take a context")
 
 	return *fs
 }
@@ -113,6 +115,13 @@ func run(pass *analysis.Pass, opts *Options) {
 			return
 		}
 
+		if opts.ContextOnly {
+			arg, ok := pass.TypesInfo.Types[call.Args[0]]
+			if ok && arg.Type.String() != "context.Context" {
+				pass.Reportf(call.Pos(), "methods that do not take a context should not be used")
+			}
+		}
+
 		// NOTE: we assume that the arguments have already been validated by govet.
 		args := call.Args[argsPos:]
 		if len(args) == 0 {

sloglint_test.go

@@ -34,4 +34,9 @@ func TestAnalyzer(t *testing.T) {
 		analyzer := sloglint.New(&sloglint.Options{ArgsOnSepLines: true})
 		analysistest.Run(t, testdata, analyzer, "args_on_sep_lines")
 	})
+
+	t.Run("context only", func(t *testing.T) {
+		analyzer := sloglint.New(&sloglint.Options{ContextOnly: true})
+		analysistest.Run(t, testdata, analyzer, "context_only")
+	})
 }

testdata/src/context_only/context_only.go

@@ -0,0 +1,35 @@
+package context_only
+
+import (
+	"context"
+	"io"
+	"log/slog"
+)
+
+func tests() {
+	ctx := context.Background()
+
+	slog.Debug("msg") // want `methods that do not take a context should not be used`
+	slog.Info("msg")  // want `methods that do not take a context should not be used`
+	slog.Warn("msg")  // want `methods that do not take a context should not be used`
+	slog.Error("msg") // want `methods that do not take a context should not be used`
+
+	slog.Log(context.Background(), slog.LevelInfo, "msg")
+	slog.DebugContext(context.TODO(), "msg")
+	slog.InfoContext(context.WithoutCancel(ctx), "msg")
+	slog.WarnContext(ctx, "msg")
+	slog.ErrorContext(ctx, "msg")
+
+	logger := slog.New(slog.NewJSONHandler(io.Discard, &slog.HandlerOptions{}))
+
+	logger.Debug("msg") // want `methods that do not take a context should not be used`
+	logger.Info("msg")  // want `methods that do not take a context should not be used`
+	logger.Warn("msg")  // want `methods that do not take a context should not be used`
+	logger.Error("msg") // want `methods that do not take a context should not be used`
+
+	logger.Log(ctx, slog.LevelInfo, "msg")
+	logger.DebugContext(ctx, "msg")
+	logger.InfoContext(ctx, "msg")
+	logger.WarnContext(ctx, "msg")
+	logger.ErrorContext(ctx, "msg")
+}