do not display message to enable debuginfo when it's already enabled

Author: lqd

src/cargo/core/compiler/custom_build.rs

@@ -333,6 +333,13 @@ fn build_work(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Job> {
     let targets_fresh = targets.clone();
 
     let env_profile_name = unit.profile.name.to_uppercase();
+    let built_with_debuginfo = cx
+        .bcx
+        .unit_graph
+        .get(unit)
+        .and_then(|deps| deps.iter().find(|dep| dep.unit.target == unit.target))
+        .map(|dep| dep.unit.profile.debuginfo.is_turned_on())
+        .unwrap_or(false);
 
     // Prepare the unit of "dirty work" which will actually run the custom build
     // command.
@@ -412,9 +419,10 @@ fn build_work(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Job> {
                     format!("failed to run custom build command for `{}`", pkg_descr);
 
                 // If we're opting into backtraces, mention that build dependencies' backtraces can
-                // be improved by setting a higher debuginfo level.
+                // be improved by requesting debuginfo to be built, if we're not building with
+                // debuginfo already.
                 if let Ok(show_backtraces) = std::env::var("RUST_BACKTRACE") {
-                    if show_backtraces != "0" {
+                    if !built_with_debuginfo && show_backtraces != "0" {
                         build_error_context.push_str(&format!(
                             "\n\
                             note: To improve backtraces for build dependencies, set the \

tests/testsuite/build_script.rs

@@ -102,6 +102,43 @@ Caused by:
         .run();
 }
 
+#[cargo_test]
+fn custom_build_script_failed_backtraces_message_with_debuginfo() {
+    // This is the same test as `custom_build_script_failed_backtraces_message` above, this time
+    // ensuring that the message dedicated to improving backtraces by requesting debuginfo is not
+    // shown when debuginfo is already turned on.
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+
+                name = "foo"
+                version = "0.5.0"
+                authors = ["[email protected]"]
+                build = "build.rs"
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .file("build.rs", "fn main() { std::process::exit(101); }")
+        .build();
+    p.cargo("build -v")
+        .env("RUST_BACKTRACE", "1")
+        .env("CARGO_PROFILE_DEV_BUILD_OVERRIDE_DEBUG", "true")
+        .with_status(101)
+        .with_stderr(
+            "\
+[COMPILING] foo v0.5.0 ([CWD])
+[RUNNING] `rustc --crate-name build_script_build build.rs [..]--crate-type bin [..]`
+[RUNNING] `[..]/build-script-build`
+[ERROR] failed to run custom build command for `foo v0.5.0 ([CWD])`
+
+Caused by:
+  process didn't exit successfully: `[..]/build-script-build` (exit [..]: 101)",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn custom_build_env_vars() {
     let p = project()