Implement cargo:rerun-if-env-changed=FOO

src/cargo/ops/cargo_compile.rs

@@ -703,6 +703,7 @@ fn scrape_target_config(config: &Config, triple: &str)
             env: Vec::new(),
             metadata: Vec::new(),
             rerun_if_changed: Vec::new(),
+            rerun_if_env_changed: Vec::new(),
             warnings: Vec::new(),
         };
         // We require deterministic order of evaluation, so we must sort the pairs by key first.
@@ -745,7 +746,9 @@ fn scrape_target_config(config: &Config, triple: &str)
                         output.env.push((name.clone(), val.to_string()));
                     }
                 }
-                "warning" | "rerun-if-changed" => {
+                "warning" |
+                "rerun-if-changed" |
+                "rerun-if-env-changed" => {
                     bail!("`{}` is not supported in build script overrides", k);
                 }
                 _ => {

src/cargo/ops/cargo_rustc/context.rs

@@ -17,7 +17,7 @@ use util::{self, internal, Config, profile, Cfg, CfgExpr};
 use util::errors::{CargoResult, CargoResultExt};
 
 use super::TargetConfig;
-use super::custom_build::{BuildState, BuildScripts};
+use super::custom_build::{BuildState, BuildScripts, BuildDeps};
 use super::fingerprint::Fingerprint;
 use super::layout::Layout;
 use super::links::Links;
@@ -38,7 +38,7 @@ pub struct Context<'a, 'cfg: 'a> {
     pub compilation: Compilation<'cfg>,
     pub packages: &'a PackageSet<'cfg>,
     pub build_state: Arc<BuildState>,
-    pub build_explicit_deps: HashMap<Unit<'a>, (PathBuf, Vec<String>)>,
+    pub build_explicit_deps: HashMap<Unit<'a>, BuildDeps>,
     pub fingerprints: HashMap<Unit<'a>, Arc<Fingerprint>>,
     pub compiled: HashSet<Unit<'a>>,
     pub build_config: BuildConfig,

src/cargo/ops/cargo_rustc/custom_build.rs

@@ -28,6 +28,8 @@ pub struct BuildOutput {
     pub metadata: Vec<(String, String)>,
     /// Paths to trigger a rerun of this build script.
     pub rerun_if_changed: Vec<String>,
+    /// Environment variables which, when changed, will cause a rebuild.
+    pub rerun_if_env_changed: Vec<String>,
     /// Warnings generated by this build,
     pub warnings: Vec<String>,
 }
@@ -59,6 +61,12 @@ pub struct BuildScripts {
     pub plugins: BTreeSet<PackageId>,
 }
 
+pub struct BuildDeps {
+    pub build_script_output: PathBuf,
+    pub rerun_if_changed: Vec<String>,
+    pub rerun_if_env_changed: Vec<String>,
+}
+
 /// Prepares a `Work` that executes the target as a custom build script.
 ///
 /// The `req` given is the requirement which this run of the build script will
@@ -181,11 +189,8 @@ fn build_work<'a, 'cfg>(cx: &mut Context<'a, 'cfg>, unit: &Unit<'a>)
     // Check to see if the build script has already run, and if it has keep
     // track of whether it has told us about some explicit dependencies
     let prev_output = BuildOutput::parse_file(&output_file, &pkg_name).ok();
-    let rerun_if_changed = match prev_output {
-        Some(ref prev) => prev.rerun_if_changed.clone(),
-        None => Vec::new(),
-    };
-    cx.build_explicit_deps.insert(*unit, (output_file.clone(), rerun_if_changed));
+    let deps = BuildDeps::new(&output_file, prev_output.as_ref());
+    cx.build_explicit_deps.insert(*unit, deps);
 
     fs::create_dir_all(&script_output)?;
     fs::create_dir_all(&build_output)?;
@@ -246,8 +251,6 @@ fn build_work<'a, 'cfg>(cx: &mut Context<'a, 'cfg>, unit: &Unit<'a>)
 
         })?;
 
-        paths::write(&output_file, &output.stdout)?;
-        paths::write(&err_file, &output.stderr)?;
 
         // After the build command has finished running, we need to be sure to
         // remember all of its output so we can later discover precisely what it
@@ -256,6 +259,8 @@ fn build_work<'a, 'cfg>(cx: &mut Context<'a, 'cfg>, unit: &Unit<'a>)
         // This is also the location where we provide feedback into the build
         // state informing what variables were discovered via our script as
         // well.
+        paths::write(&output_file, &output.stdout)?;
+        paths::write(&err_file, &output.stderr)?;
         let parsed_output = BuildOutput::parse(&output.stdout, &pkg_name)?;
 
         if json_messages {
@@ -337,6 +342,7 @@ impl BuildOutput {
         let mut env = Vec::new();
         let mut metadata = Vec::new();
         let mut rerun_if_changed = Vec::new();
+        let mut rerun_if_env_changed = Vec::new();
         let mut warnings = Vec::new();
         let whence = format!("build script of `{}`", pkg_name);
 
@@ -378,6 +384,7 @@ impl BuildOutput {
                 "rustc-env" => env.push(BuildOutput::parse_rustc_env(value, &whence)?),
                 "warning" => warnings.push(value.to_string()),
                 "rerun-if-changed" => rerun_if_changed.push(value.to_string()),
+                "rerun-if-env-changed" => rerun_if_env_changed.push(value.to_string()),
                 _ => metadata.push((key.to_string(), value.to_string())),
             }
         }
@@ -389,6 +396,7 @@ impl BuildOutput {
             env: env,
             metadata: metadata,
             rerun_if_changed: rerun_if_changed,
+            rerun_if_env_changed: rerun_if_env_changed,
             warnings: warnings,
         })
     }
@@ -436,6 +444,20 @@ impl BuildOutput {
     }
 }
 
+impl BuildDeps {
+    pub fn new(output_file: &Path, output: Option<&BuildOutput>) -> BuildDeps {
+        BuildDeps {
+            build_script_output: output_file.to_path_buf(),
+            rerun_if_changed: output.map(|p| &p.rerun_if_changed)
+                                    .cloned()
+                                    .unwrap_or_default(),
+            rerun_if_env_changed: output.map(|p| &p.rerun_if_env_changed)
+                                        .cloned()
+                                        .unwrap_or_default(),
+        }
+    }
+}
+
 /// Compute the `build_scripts` map in the `Context` which tracks what build
 /// scripts each package depends on.
 ///