Do not rebuild on RUSTC_BOOTSTRAP changes on nightly compiler

build.rs

@@ -4,14 +4,51 @@ use std::path::Path;
 use std::process::{self, Command, Stdio};
 
 fn main() {
-    println!("cargo:rerun-if-env-changed=RUSTC_BOOTSTRAP");
+    let error_generic_member_access;
+    let consider_rustc_bootstrap;
+    if compile_probe(false) {
+        // This is a nightly or dev compiler, so it supports unstable features
+        // regardless of RUSTC_BOOTSTRAP. No need to rerun build script if
+        // RUSTC_BOOTSTRAP is changed.
+        error_generic_member_access = true;
+        consider_rustc_bootstrap = false;
+    } else if let Some(rustc_bootstrap) = env::var_os("RUSTC_BOOTSTRAP") {
+        if compile_probe(true) {
+            // This is a stable or beta compiler for which the user has set
+            // RUSTC_BOOTSTRAP to turn on unstable features. Rerun build script
+            // if they change it.
+            error_generic_member_access = true;
+            consider_rustc_bootstrap = true;
+        } else if rustc_bootstrap == "1" {
+            // This compiler does not support the generic member access API in
+            // the form that thiserror expects. No need to pay attention to
+            // RUSTC_BOOTSTRAP.
+            error_generic_member_access = false;
+            consider_rustc_bootstrap = false;
+        } else {
+            // This is a stable or beta compiler for which RUSTC_BOOTSTRAP is
+            // set to restrict the use of unstable features by this crate.
+            error_generic_member_access = false;
+            consider_rustc_bootstrap = true;
+        }
+    } else {
+        // Without RUSTC_BOOTSTRAP, this compiler does not support the generic
+        // member access API in the form that thiserror expects, but try again
+        // if the user turns on unstable features.
+        error_generic_member_access = false;
+        consider_rustc_bootstrap = true;
+    }
 
-    if compile_probe() {
+    if error_generic_member_access {
         println!("cargo:rustc-cfg=error_generic_member_access");
     }
+
+    if consider_rustc_bootstrap {
+        println!("cargo:rerun-if-env-changed=RUSTC_BOOTSTRAP");
+    }
 }
 
-fn compile_probe() -> bool {
+fn compile_probe(rustc_bootstrap: bool) -> bool {
     if env::var_os("RUSTC_STAGE").is_some() {
         // We are running inside rustc bootstrap. This is a highly non-standard
         // environment with issues such as:
@@ -37,6 +74,10 @@ fn compile_probe() -> bool {
         Command::new(rustc)
     };
 
+    if !rustc_bootstrap {
+        cmd.env_remove("RUSTC_BOOTSTRAP");
+    }
+
     cmd.stderr(Stdio::null())
         .arg("--edition=2018")
         .arg("--crate-name=thiserror")