cmd/deps: add warning when not using runtime dependencies.

This should provide clarify as to why the output may differ from a
formula's declared dependencies.

Library/Homebrew/cmd/deps.rb

@@ -18,8 +18,7 @@ module Homebrew
 
           If any version of each formula argument is installed and no other options
           are passed, this command displays their actual runtime dependencies (similar
-          to `brew linkage`), which may differ from the current versions' stated
-          dependencies if the installed versions are outdated.
+          to `brew linkage`), which may differ from a formula's declared dependencies.
 
           *Note:* `--missing` and `--skip-recommended` have precedence over `--include-*`.
         EOS
@@ -97,21 +96,45 @@ module Homebrew
         Formulary.enable_factory_cache!
 
         SimulateSystem.with(os:, arch:) do
-          recursive = !args.direct?
-          installed = args.installed? || dependents(args.named.to_formulae_and_casks).all?(&:any_version_installed?)
+          @use_runtime_dependencies = true
 
-          @use_runtime_dependencies = installed && recursive &&
-                                      !args.tree? &&
-                                      !args.graph? &&
-                                      !args.HEAD? &&
-                                      !args.include_implicit? &&
-                                      !args.include_build? &&
-                                      !args.include_test? &&
-                                      !args.include_optional? &&
-                                      !args.skip_recommended? &&
-                                      !args.missing? &&
-                                      args.os.nil? &&
-                                      args.arch.nil?
+          installed = args.installed? || dependents(args.named.to_formulae_and_casks).all?(&:any_version_installed?)
+          unless installed
+            not_using_runtime_dependencies_reason = if args.installed?
+              "not all the named formulae were installed"
+            else
+              "`--installed` was not passed"
+            end
+
+            @use_runtime_dependencies = false
+          end
+
+          %w[direct tree graph HEAD skip_recommended missing
+             include_implicit include_build include_test include_optional].each do |arg|
+            next unless args.public_send("#{arg}?")
+
+            not_using_runtime_dependencies_reason = "--#{arg.tr("_", "-")} was passed"
+
+            @use_runtime_dependencies = false
+          end
+
+          %w[os arch].each do |arg|
+            next if args.public_send(arg).nil?
+
+            not_using_runtime_dependencies_reason = "--#{arg.tr("_", "-")} was passed"
+
+            @use_runtime_dependencies = false
+          end
+
+          if !@use_runtime_dependencies && !Homebrew::EnvConfig.no_env_hints?
+            opoo <<~EOS
+              `brew deps` is not the actual runtime dependencies because #{not_using_runtime_dependencies_reason}!
+              This means dependencies may differ from a formula's declared dependencies.
+              Hide these hints with HOMEBREW_NO_ENV_HINTS (see `man brew`).
+            EOS
+          end
+
+          recursive = !args.direct?
 
           if args.tree? || args.graph?
             dependents = if args.named.present?

Library/Homebrew/test/cmd/deps_spec.rb

@@ -37,6 +37,6 @@ RSpec.describe Homebrew::Cmd::Deps do
     expect { brew "deps", "baz", "--include-test", "--missing", "--skip-recommended" }
       .to be_a_success
       .and output("bar\nfoo\ntest\n").to_stdout
-      .and not_to_output.to_stderr
+      .and output(/not the actual runtime dependencies/).to_stderr
   end
 end