command/init: Restore the unconstrained provider warnings

When a provider dependency is implicit rather than explicit, or otherwise
when version constraints are lacking, we produce a warning recommending
the addition of explicit version constraints in the configuration.

This restores the warning functionality from previous Terraform versions,
adapting it slightly to account for the new provider FQN syntax and to
recommend using a required_providers block rather than version constraints
in "provider" blocks, because the latter is no longer recommended in the
documentation.

command/init.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"os"
+	"sort"
 	"strings"
 
 	"github.com/hashicorp/hcl/v2"
@@ -516,7 +517,7 @@ func (c *InitCommand) getProviders(earlyConfig *earlyconfig.Config, state *state
 	// TODO: Use a context that will be cancelled when the Terraform
 	// process receives SIGINT.
 	ctx := evts.OnContext(context.TODO())
-	_, err := inst.EnsureProviderVersions(ctx, reqs, mode)
+	selected, err := inst.EnsureProviderVersions(ctx, reqs, mode)
 	if err != nil {
 		// The errors captured in "err" should be redundant with what we
 		// received via the InstallerEvents callbacks above, so we'll
@@ -527,86 +528,31 @@ func (c *InitCommand) getProviders(earlyConfig *earlyconfig.Config, state *state
 		return true, diags
 	}
 
-	return true, diags
-
-	// TODO: Write the selections into the plugins lock file so we can be
-	// sure that future commands will use exactly those provider packages.
-	// TODO: Emit constraint suggestions for unconstrained providers.
-	/*
-		// With all the providers downloaded, we'll generate our lock file
-		// that ensures the provider binaries remain unchanged until we init
-		// again. If anything changes, other commands that use providers will
-		// fail with an error instructing the user to re-run this command.
-		available = c.providerPluginSet() // re-discover to see newly-installed plugins
-
-		// internal providers were already filtered out, since we don't need to get them.
-		chosen := chooseProviders(available, nil, requirements)
-
-		digests := map[string][]byte{}
-		for name, meta := range chosen {
-			digest, err := meta.SHA256()
-			if err != nil {
-				diags = diags.Append(fmt.Errorf("Failed to read provider plugin %s: %s", meta.Path, err))
-				return true, diags
-			}
-			digests[name] = digest
-			if c.ignorePluginChecksum {
-				digests[name] = nil
-			}
-		}
-		err := c.providerPluginsLock().Write(digests)
-		if err != nil {
-			diags = diags.Append(fmt.Errorf("failed to save provider manifest: %s", err))
-			return true, diags
-		}
+	// If any providers have "floating" versions (completely unconstrained)
+	// we'll suggest the user constrain with a pessimistic constraint to
+	// avoid implicitly adopting a later major release.
+	constraintSuggestions := make(map[string]string)
+	for addr, version := range selected {
+		req := reqs[addr]
 
-		{
-			// Purge any auto-installed plugins that aren't being used.
-			purged, err := c.providerInstaller.PurgeUnused(chosen)
-			if err != nil {
-				// Failure to purge old plugins is not a fatal error
-				c.Ui.Warn(fmt.Sprintf("failed to purge unused plugins: %s", err))
-			}
-			if purged != nil {
-				for meta := range purged {
-					log.Printf("[DEBUG] Purged unused %s plugin %s", meta.Name, meta.Path)
-				}
-			}
+		if len(req) == 0 {
+			constraintSuggestions[addr.ForDisplay()] = "~> " + version.String()
 		}
-
-		// If any providers have "floating" versions (completely unconstrained)
-		// we'll suggest the user constrain with a pessimistic constraint to
-		// avoid implicitly adopting a later major release.
-		constraintSuggestions := make(map[string]discovery.ConstraintStr)
-		for name, meta := range chosen {
-			req := requirements[name]
-			if req == nil {
-				// should never happen, but we don't want to crash here, so we'll
-				// be cautious.
-				continue
-			}
-
-			if req.Versions.Unconstrained() && meta.Version != discovery.VersionZero {
-				// meta.Version.MustParse is safe here because our "chosen" metas
-				// were already filtered for validity of versions.
-				constraintSuggestions[name] = meta.Version.MustParse().MinorUpgradeConstraintStr()
-			}
+	}
+	if len(constraintSuggestions) != 0 {
+		names := make([]string, 0, len(constraintSuggestions))
+		for name := range constraintSuggestions {
+			names = append(names, name)
 		}
-		if len(constraintSuggestions) != 0 {
-			names := make([]string, 0, len(constraintSuggestions))
-			for name := range constraintSuggestions {
-				names = append(names, name)
-			}
-			sort.Strings(names)
+		sort.Strings(names)
 
-			c.Ui.Output(outputInitProvidersUnconstrained)
-			for _, name := range names {
-				c.Ui.Output(fmt.Sprintf("* provider.%s: version = %q", name, constraintSuggestions[name]))
-			}
+		c.Ui.Output(outputInitProvidersUnconstrained)
+		for _, name := range names {
+			c.Ui.Output(fmt.Sprintf("* %s: version = %q", name, constraintSuggestions[name]))
 		}
+	}
 
-		return true, diags
-	*/
+	return true, diags
 }
 
 // backendConfigOverrideBody interprets the raw values of -backend-config
@@ -834,9 +780,8 @@ The following providers do not have any version constraints in configuration,
 so the latest version was installed.
 
 To prevent automatic upgrades to new major versions that may contain breaking
-changes, it is recommended to add version = "..." constraints to the
-corresponding provider blocks in configuration, with the constraint strings
-suggested below.
+changes, we recommend adding version constraints in a required_providers block
+in your configuration, with the constraint strings suggested below.
 `
 
 const errDiscoveryServiceUnreachable = `