gopls: add a -severity flag for gopls check

In golang/go#50764, users were reporting having to filter out noisy
diagnostics from the output of `gopls check` in CI. This is because
there was no differentiation between diagnostics that represent real
bugs, and those that are suggestions. By contrast, hint level
diagnostics are very unobtrusive in the editor.

Add a new -severity flag to control the minimum severity output by gopls
check, and set its default to "warning".

For golang/go#50764

Change-Id: I48d8bb74371fa6035fef4d2608412b986f509f7b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/651616
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>

Author: findleyr

gopls/doc/release/v0.19.0.md

@@ -1,3 +1,10 @@
+# Configuration Changes
+
+- The `gopls check` subcommant now accepts a `-severity` flag to set a minimum
+  severity for the diagnostics it reports. By default, the minimum severity
+  is "warning", so `gopls check` may report fewer diagnostics than before. Set
+  `-severity=hint` to reproduce the previous behavior.
+
 # New features
 
 ## "Eliminate dot import" code action

gopls/internal/cmd/check.go

@@ -16,7 +16,8 @@ import (
 
 // check implements the check verb for gopls.
 type check struct {
-	app *Application
+	app      *Application
+	Severity string `flag:"severity" help:"minimum diagnostic severity (hint, info, warning, or error)"`
 }
 
 func (c *check) Name() string      { return "check" }
@@ -35,6 +36,20 @@ Example: show the diagnostic results of this file:
 // Run performs the check on the files specified by args and prints the
 // results to stdout.
 func (c *check) Run(ctx context.Context, args ...string) error {
+	severityCutoff := protocol.SeverityWarning
+	switch c.Severity {
+	case "hint":
+		severityCutoff = protocol.SeverityHint
+	case "info":
+		severityCutoff = protocol.SeverityInformation
+	case "warning":
+		// default
+	case "error":
+		severityCutoff = protocol.SeverityError
+	default:
+		return fmt.Errorf("unrecognized -severity value %q", c.Severity)
+	}
+
 	if len(args) == 0 {
 		return nil
 	}
@@ -95,6 +110,9 @@ func (c *check) Run(ctx context.Context, args ...string) error {
 		file.diagnosticsMu.Unlock()
 
 		for _, diag := range diags {
+			if diag.Severity > severityCutoff { // lower severity value => greater severity, counterintuitively
+				continue
+			}
 			if err := print(file.uri, diag.Range, diag.Message); err != nil {
 				return err
 			}

gopls/internal/cmd/cmd.go

@@ -284,7 +284,7 @@ func (app *Application) internalCommands() []tool.Application {
 func (app *Application) featureCommands() []tool.Application {
 	return []tool.Application{
 		&callHierarchy{app: app},
-		&check{app: app},
+		&check{app: app, Severity: "warning"},
 		&codeaction{app: app},
 		&codelens{app: app},
 		&definition{app: app},

gopls/internal/cmd/integration_test.go

@@ -108,6 +108,12 @@ var C int
 -- c/c2.go --
 package c
 var C int
+-- d/d.go --
+package d
+
+import "io/ioutil"
+
+var _ = ioutil.ReadFile
 `)
 
 	// no files
@@ -141,6 +147,22 @@ var C int
 		res.checkStdout(`c2.go:2:5-6: C redeclared in this block`)
 		res.checkStdout(`c.go:2:5-6: - other declaration of C`)
 	}
+
+	// No deprecated (hint) diagnostic without -severity.
+	{
+		res := gopls(t, tree, "check", "./d/d.go")
+		res.checkExit(true)
+		if len(res.stdout) > 0 {
+			t.Errorf("check ./d/d.go returned unexpected output:\n%s", res.stdout)
+		}
+	}
+
+	// Deprecated (hint) diagnostics with -severity=hint
+	{
+		res := gopls(t, tree, "check", "-severity=hint", "./d/d.go")
+		res.checkExit(true)
+		res.checkStdout(`ioutil.ReadFile is deprecated`)
+	}
 }
 
 // TestCallHierarchy tests the 'call_hierarchy' subcommand (call_hierarchy.go).

gopls/internal/cmd/usage/check.hlp

@@ -6,3 +6,5 @@ Usage:
 Example: show the diagnostic results of this file:
 
 	$ gopls check internal/cmd/check.go
+  -severity=string
+    	minimum diagnostic severity (hint, info, warning, or error) (default "warning")