feat: lake: empty build detection & --allow-empty (#13500)

This PR adds a check for empty `lake build` invocations (as an empty
build usually indicates a misconfiguration). Builds with no jobs will
now print "Nothing to build." and invocations of `lake build` with no
default targets configured will produce a warning. This will be promoted
to an error in the future. The warning (and future error) can be
suppressed with the new `--allow-empty` CLI option.

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lake/Lake/Build/Run.lean

@@ -284,6 +284,9 @@ def reportResult (cfg : BuildConfig) (out : IO.FS.Stream) (result : MonitorResul
   if result.failures.isEmpty then
     if cfg.showProgress && cfg.showSuccess then
       let numJobs := result.numJobs
+      if numJobs == 0 then
+        print! out "Nothing to build.\n"
+      else
         let jobs := if numJobs == 1 then "1 job" else s!"{numJobs} jobs"
         if cfg.noBuild then
           print! out s!"All targets up-to-date ({jobs}).\n"

src/lake/Lake/CLI/Help.lean

@@ -55,6 +55,7 @@ BASIC OPTIONS:
   --packages=file       JSON file of package entries that override the manifest
   --reconfigure, -R     elaborate configuration files instead of using OLeans
   --keep-toolchain      do not update toolchain on workspace update
+  --allow-empty         accept bare builds with no default targets configured
   --no-build            exit immediately if a build target is not up-to-date
   --no-cache            build packages locally; do not download build caches
   --try-cache           attempt to download build caches for supported packages

src/lake/Lake/CLI/Main.lean

@@ -56,6 +56,7 @@ public structure LakeOptions where
   reconfigure : Bool := false
   oldMode : Bool := false
   trustHash : Bool := true
+  allowEmpty : Bool := false
   noBuild : Bool := false
   noCache : Option Bool := none
   failLv : LogLevel := .error
@@ -247,6 +248,7 @@ def lakeLongOption : (opt : String) → CliM PUnit
 | "--old"         => modifyThe LakeOptions ({· with oldMode := true})
 | "--text"        => modifyThe LakeOptions ({· with outFormat := .text})
 | "--json"        => modifyThe LakeOptions ({· with outFormat := .json})
+| "--allow-empty" => modifyThe LakeOptions ({· with allowEmpty := true})
 | "--no-build"    => modifyThe LakeOptions ({· with noBuild := true})
 | "--no-cache"    => modifyThe LakeOptions ({· with noCache := true})
 | "--try-cache"   => modifyThe LakeOptions ({· with noCache := false})
@@ -845,6 +847,12 @@ protected def build : CliM PUnit := do
   let ws ← loadWorkspace config
   let targetSpecs ← takeArgs
   let specs ← parseTargetSpecs ws targetSpecs
+  if specs.isEmpty && !opts.allowEmpty then
+    logWarning "no targets specified and no default targets configured\
+      \n  Note: This will be an error in a future version of Lake.\
+      \n  Hint: This warning (or error) can be suppressed with '--allow-empty'."
+    if opts.failLv ≤ .warning then
+      exit 1
   specs.forM fun spec =>
     unless spec.buildable do
       throw <| .invalidBuildTarget spec.info.key.toSimpleString

tests/lake/tests/emptyBuild/clean.sh

@@ -0,0 +1 @@
+rm -rf .lake lake-manifest.json produced.out

tests/lake/tests/emptyBuild/lakefile.lean

@@ -0,0 +1,7 @@
+import Lake
+open Lake DSL
+
+package test
+
+lean_lib Lib where
+  globs := #[]

tests/lake/tests/emptyBuild/lakefile.toml

@@ -0,0 +1,5 @@
+name = "test"
+
+[[lean_lib]]
+name = "Lib"
+globs = []

tests/lake/tests/emptyBuild/test.sh

@@ -0,0 +1,47 @@
+#!/usr/bin/env bash
+source ../common.sh
+
+# This test covers the behavior of `lake build`
+# with no default targets configured.
+
+test_empty_build() {
+cfg_file=$1
+test_run -f $cfg_file update
+test_out_diff <(cat << 'EOF'
+warning: no targets specified and no default targets configured
+  Note: This will be an error in a future version of Lake.
+  Hint: This warning (or error) can be suppressed with '--allow-empty'.
+Nothing to build.
+EOF
+) -f $cfg_file build
+test_err_diff <(cat << 'EOF'
+warning: no targets specified and no default targets configured
+  Note: This will be an error in a future version of Lake.
+  Hint: This warning (or error) can be suppressed with '--allow-empty'.
+EOF
+) -f $cfg_file build --wfail
+test_out_diff <(cat << 'EOF'
+Nothing to build.
+EOF
+) -f $cfg_file build --allow-empty --wfail
+# Test the warning is not printed on a regular build.
+# The configurations use `globs = []` to minimize build variance,
+# and to verify that empty globs do not count as no jobs.
+test_out_diff <(cat << 'EOF'
+Build completed successfully (1 job).
+EOF
+) -f $cfg_file build Lib
+}
+
+# Test Lean configuration with no default targets
+./clean.sh
+echo "# TEST: lakefile.lean"
+test_empty_build lakefile.lean
+
+# Test TOML configuration with no default targets
+./clean.sh
+echo "# TEST: lakefile.toml"
+test_empty_build lakefile.toml
+
+# Cleanup
+rm -f produced.*