Lazify is_really_default condition in the RustdocGUI bootstrap step

The `RustdocGUI::should_run` condition spawns `npm list` several times
which adds up to seconds of wall-time.
Evaluate the condition lazily to to keep `./x.py test tidy` and similar
short-running tasks fast.

Cargo.lock

@@ -179,6 +179,7 @@ dependencies = [
  "libc",
  "merge",
  "num_cpus",
+ "once_cell",
  "opener",
  "pretty_assertions",
  "serde",

src/bootstrap/Cargo.toml

@@ -50,6 +50,7 @@ time = "0.1"
 ignore = "0.4.10"
 opener = "0.4"
 merge = "0.1.0"
+once_cell = "1.7.2"
 
 [target.'cfg(windows)'.dependencies.winapi]
 version = "0.3"

src/bootstrap/builder.rs

@@ -29,6 +29,8 @@ use crate::util::{self, add_dylib_path, add_link_lib_path, exe, libdir};
 use crate::{Build, DocTests, GitRepo, Mode};
 
 pub use crate::Compiler;
+// FIXME: replace with std::lazy after it gets stabilized and reaches beta
+use once_cell::sync::Lazy;
 
 pub struct Builder<'a> {
     pub build: &'a Build,
@@ -195,7 +197,7 @@ impl StepDescription {
 
         if paths.is_empty() || builder.config.include_default_paths {
             for (desc, should_run) in v.iter().zip(&should_runs) {
-                if desc.default && should_run.is_really_default {
+                if desc.default && should_run.is_really_default() {
                     for pathset in &should_run.paths {
                         desc.maybe_run(builder, pathset);
                     }
@@ -228,7 +230,11 @@ impl StepDescription {
     }
 }
 
-#[derive(Clone)]
+enum ReallyDefault<'a> {
+    Bool(bool),
+    Lazy(Lazy<bool, Box<dyn Fn() -> bool + 'a>>),
+}
+
 pub struct ShouldRun<'a> {
     pub builder: &'a Builder<'a>,
     // use a BTreeSet to maintain sort order
@@ -236,7 +242,7 @@ pub struct ShouldRun<'a> {
 
     // If this is a default rule, this is an additional constraint placed on
     // its run. Generally something like compiler docs being enabled.
-    is_really_default: bool,
+    is_really_default: ReallyDefault<'a>,
 }
 
 impl<'a> ShouldRun<'a> {
@@ -244,15 +250,27 @@ impl<'a> ShouldRun<'a> {
         ShouldRun {
             builder,
             paths: BTreeSet::new(),
-            is_really_default: true, // by default no additional conditions
+            is_really_default: ReallyDefault::Bool(true), // by default no additional conditions
         }
     }
 
     pub fn default_condition(mut self, cond: bool) -> Self {
-        self.is_really_default = cond;
+        self.is_really_default = ReallyDefault::Bool(cond);
         self
     }
 
+    pub fn lazy_default_condition(mut self, lazy_cond: Box<dyn Fn() -> bool + 'a>) -> Self {
+        self.is_really_default = ReallyDefault::Lazy(Lazy::new(lazy_cond));
+        self
+    }
+
+    pub fn is_really_default(&self) -> bool {
+        match &self.is_really_default {
+            ReallyDefault::Bool(val) => *val,
+            ReallyDefault::Lazy(lazy) => *lazy.deref(),
+        }
+    }
+
     /// Indicates it should run if the command-line selects the given crate or
     /// any of its (local) dependencies.
     ///

src/bootstrap/test.rs

@@ -806,15 +806,15 @@ impl Step for RustdocGUI {
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
         let builder = run.builder;
         let run = run.suite_path("src/test/rustdoc-gui");
-        run.default_condition(
+        run.lazy_default_condition(Box::new(move || {
             builder.config.nodejs.is_some()
                 && builder
                     .config
                     .npm
                     .as_ref()
                     .map(|p| check_if_browser_ui_test_is_installed(p))
-                    .unwrap_or(false),
-        )
+                    .unwrap_or(false)
+        }))
     }
 
     fn make_run(run: RunConfig<'_>) {