Improve error message for unresolved toolchains

Closes bazelbuild#15135.

PiperOrigin-RevId: 447028054

src/main/java/com/google/devtools/build/lib/skyframe/ToolchainResolutionFunction.java

@@ -24,7 +24,6 @@
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.ImmutableSetMultimap;
-import com.google.common.collect.Iterables;
 import com.google.common.collect.Table;
 import com.google.devtools.build.lib.analysis.PlatformConfiguration;
 import com.google.devtools.build.lib.analysis.config.BuildConfigurationValue;
@@ -618,20 +617,14 @@ protected Code getDetailedCode() {
     }
 
     private static String getMessage(List<Label> missingToolchainTypes) {
-      if (missingToolchainTypes.size() == 1
-          && Iterables.getOnlyElement(missingToolchainTypes)
-              .toString()
-              .equals("@bazel_tools//tools/cpp:toolchain_type")) {
-        return "No matching toolchains found for types @bazel_tools//tools/cpp:toolchain_type. "
-            + "Maybe --incompatible_use_cc_configure_from_rules_cc has been flipped and there "
-            + "is no default C++ toolchain added in the WORKSPACE file? See "
-            + "https://github.com/bazelbuild/bazel/issues/10134 for details and migration "
-            + "instructions.";
-      }
-
+      ImmutableList<String> labelStrings =
+          missingToolchainTypes.stream().map(Label::toString).collect(toImmutableList());
       return String.format(
-          "no matching toolchains found for types %s",
-          missingToolchainTypes.stream().map(Label::toString).collect(joining(", ")));
+          "No matching toolchains found for types %s."
+              + "\nTo debug, rerun with --toolchain_resolution_debug='%s'"
+              + "\nIf platforms or toolchains are a new concept for you, we'd encourage reading "
+              + "https://bazel.build/concepts/platforms-intro.",
+          String.join(", ", labelStrings), String.join("|", labelStrings));
     }
   }
 

src/test/java/com/google/devtools/build/lib/skyframe/ToolchainResolutionFunctionTest.java

@@ -189,7 +189,7 @@ public void resolve_mandatory_missing() throws Exception {
         .hasErrorEntryForKeyThat(key)
         .hasExceptionThat()
         .hasMessageThat()
-        .contains("no matching toolchains found for types //toolchain:test_toolchain");
+        .contains("No matching toolchains found for types //toolchain:test_toolchain");
   }
 
   @Test

src/test/shell/integration/toolchain_test.sh

@@ -438,7 +438,7 @@ use_toolchain(
 EOF
 
   bazel build "//${pkg}/demo:use" &> $TEST_log && fail "Build failure expected"
-  expect_log "While resolving toolchains for target //${pkg}/demo:use: no matching toolchains found for types //${pkg}/toolchain:test_toolchain"
+  expect_log "While resolving toolchains for target //${pkg}/demo:use: No matching toolchains found for types //${pkg}/toolchain:test_toolchain."
 }
 
 function test_multiple_toolchain_use_in_rule {
@@ -572,7 +572,7 @@ use_toolchains(
 EOF
 
   bazel build "//${pkg}/demo:use" &> $TEST_log && fail "Build failure expected"
-  expect_log "While resolving toolchains for target //${pkg}/demo:use: no matching toolchains found for types //${pkg}/toolchain:test_toolchain_2"
+  expect_log "While resolving toolchains for target //${pkg}/demo:use: No matching toolchains found for types //${pkg}/toolchain:test_toolchain_2."
 }
 
 function test_toolchain_use_in_rule_non_required_toolchain {
@@ -827,7 +827,7 @@ EOF
     --host_platform="//${pkg}:platform1" \
     --platforms="//${pkg}:platform1" \
     "//${pkg}/demo:use" &> $TEST_log && fail "Build failure expected"
-  expect_log "While resolving toolchains for target //${pkg}/demo:use: no matching toolchains found for types //${pkg}/toolchain:test_toolchain"
+  expect_log "While resolving toolchains for target //${pkg}/demo:use: No matching toolchains found for types //${pkg}/toolchain:test_toolchain."
   expect_not_log 'Using toolchain: rule message:'
 }