cli: Improved error for invalid -var "foo = bar"

When specifying variable values on the command line, name-value pairs
must be joined with an equals sign, without surrounding spaces.
Previously Terraform would interpret "foo = bar" as assigning the value
" bar" to the variable named "foo ". This is never valid, as variable
names may not include whitespace.

This commit looks for this specific error and returns a diagnostic with
a suggestion for correcting it. We cannot simply trim whitespace,
because it is valid to write "foo= bar" to assign the value " bar" to
the variable "foo", as unlikely as it seems.

internal/command/meta_vars.go

@@ -102,6 +102,14 @@ func (m *Meta) collectVariableValues() (map[string]backend.UnparsedVariableValue
 			}
 			name := raw[:eq]
 			rawVal := raw[eq+1:]
+			if strings.HasSuffix(name, " ") {
+				diags = diags.Append(tfdiags.Sourceless(
+					tfdiags.Error,
+					"Invalid -var option",
+					fmt.Sprintf("Variable name %q is invalid due to trailing space. Did you mean -var=\"%s=%s\"?", name, strings.TrimSuffix(name, " "), strings.TrimPrefix(rawVal, " ")),
+				))
+				continue
+			}
 			ret[name] = unparsedVariableValueString{
 				str:        rawVal,
 				name:       name,

internal/command/plan_test.go

@@ -575,6 +575,51 @@ func TestPlan_vars(t *testing.T) {
 	}
 }
 
+func TestPlan_varsInvalid(t *testing.T) {
+	testCases := []struct {
+		args    []string
+		wantErr string
+	}{
+		{
+			[]string{"-var", "foo"},
+			`The given -var option "foo" is not correctly specified.`,
+		},
+		{
+			[]string{"-var", "foo = bar"},
+			`Variable name "foo " is invalid due to trailing space.`,
+		},
+	}
+
+	// Create a temporary working directory that is empty
+	td := t.TempDir()
+	testCopyDir(t, testFixturePath("plan-vars"), td)
+	defer testChdir(t, td)()
+
+	for _, tc := range testCases {
+		t.Run(strings.Join(tc.args, " "), func(t *testing.T) {
+			p := planVarsFixtureProvider()
+			view, done := testView(t)
+			c := &PlanCommand{
+				Meta: Meta{
+					testingOverrides: metaOverridesForProvider(p),
+					View:             view,
+				},
+			}
+
+			code := c.Run(tc.args)
+			output := done(t)
+			if code != 1 {
+				t.Fatalf("bad: %d\n\n%s", code, output.Stdout())
+			}
+
+			got := output.Stderr()
+			if !strings.Contains(got, tc.wantErr) {
+				t.Fatalf("bad error output, want %q, got:\n%s", tc.wantErr, got)
+			}
+		})
+	}
+}
+
 func TestPlan_varsUnset(t *testing.T) {
 	// Create a temporary working directory that is empty
 	td := t.TempDir()