Use better heuristic for printing Cargo specific diagnostics

compiler/rustc_codegen_ssa/src/errors.rs

@@ -362,8 +362,11 @@ impl<G: EmissionGuarantee> IntoDiagnostic<'_, G> for LinkingFailed<'_> {
         // which by now we have no way to translate.
         if contains_undefined_ref {
             diag.note(fluent::codegen_ssa_extern_funcs_not_found)
-                .note(fluent::codegen_ssa_specify_libraries_to_link)
-                .note(fluent::codegen_ssa_use_cargo_directive);
+                .note(fluent::codegen_ssa_specify_libraries_to_link);
+
+            if rustc_session::utils::was_invoked_from_cargo() {
+                diag.note(fluent::codegen_ssa_use_cargo_directive);
+            }
         }
         diag
     }

compiler/rustc_hir_typeck/src/errors.rs

@@ -293,7 +293,7 @@ pub enum HelpUseLatestEdition {
 impl HelpUseLatestEdition {
     pub fn new() -> Self {
         let edition = LATEST_STABLE_EDITION;
-        if std::env::var_os("CARGO").is_some() {
+        if rustc_session::utils::was_invoked_from_cargo() {
             Self::Cargo { edition }
         } else {
             Self::Standalone { edition }

compiler/rustc_incremental/src/persist/fs.rs

@@ -492,7 +492,7 @@ fn lock_directory(
                 lock_err,
                 session_dir,
                 is_unsupported_lock,
-                is_cargo: std::env::var_os("CARGO").map(|_| ()),
+                is_cargo: rustc_session::utils::was_invoked_from_cargo().then_some(()),
             }))
         }
     }

compiler/rustc_lint/src/context/diagnostics.rs

@@ -205,7 +205,7 @@ pub(super) fn builtin(
                 Vec::new()
             };
 
-            let is_from_cargo = std::env::var_os("CARGO").is_some();
+            let is_from_cargo = rustc_session::utils::was_invoked_from_cargo();
             let mut is_feature_cfg = name == sym::feature;
 
             if is_feature_cfg && is_from_cargo {
@@ -340,7 +340,7 @@ pub(super) fn builtin(
                 .copied()
                 .flatten()
                 .collect();
-            let is_from_cargo = std::env::var_os("CARGO").is_some();
+            let is_from_cargo = rustc_session::utils::was_invoked_from_cargo();
 
             // Show the full list if all possible values for a given name, but don't do it
             // for names as the possibilities could be very long

compiler/rustc_parse/src/errors.rs

@@ -2545,7 +2545,7 @@ pub enum HelpUseLatestEdition {
 impl HelpUseLatestEdition {
     pub fn new() -> Self {
         let edition = LATEST_STABLE_EDITION;
-        if std::env::var_os("CARGO").is_some() {
+        if rustc_session::utils::was_invoked_from_cargo() {
             Self::Cargo { edition }
         } else {
             Self::Standalone { edition }

compiler/rustc_session/src/utils.rs

@@ -1,7 +1,10 @@
 use crate::session::Session;
 use rustc_data_structures::profiling::VerboseTimingGuard;
 use rustc_fs_util::try_canonicalize;
-use std::path::{Path, PathBuf};
+use std::{
+    path::{Path, PathBuf},
+    sync::OnceLock,
+};
 
 impl Session {
     pub fn timer(&self, what: &'static str) -> VerboseTimingGuard<'_> {
@@ -158,3 +161,18 @@ pub fn extra_compiler_flags() -> Option<(Vec<String>, bool)> {
 
     if !result.is_empty() { Some((result, excluded_cargo_defaults)) } else { None }
 }
+
+/// Returns whenever rustc was launched by Cargo as opposed to another build system.
+///
+/// To be used in diagnostics to avoid printing Cargo specific suggestions to other
+/// build systems (like Bazel, Buck2, Makefile, ...).
+pub fn was_invoked_from_cargo() -> bool {
+    static FROM_CARGO: OnceLock<bool> = OnceLock::new();
+
+    // To be able to detect Cargo, we use the simplest and least intrusive
+    // way: we check whenever the `CARGO_CRATE_NAME` env is set.
+    //
+    // Note that it is common in Makefiles to define the `CARGO` env even
+    // though we may not have been called by Cargo, so we avoid using it.
+    *FROM_CARGO.get_or_init(|| std::env::var_os("CARGO_CRATE_NAME").is_some())
+}

tests/ui/async-await/suggest-switching-edition-on-await-cargo.rs

@@ -1,4 +1,4 @@
-//@ rustc-env:CARGO=/usr/bin/cargo
+//@ rustc-env:CARGO_CRATE_NAME=foo
 
 use std::pin::Pin;
 use std::future::Future;

tests/ui/check-cfg/cargo-feature.rs

@@ -4,7 +4,7 @@
 //
 //@ check-pass
 //@ revisions: some none
-//@ rustc-env:CARGO=/usr/bin/cargo
+//@ rustc-env:CARGO_CRATE_NAME=foo
 //@ compile-flags: -Z unstable-options
 //@ [none]compile-flags: --check-cfg=cfg(feature,values())
 //@ [some]compile-flags: --check-cfg=cfg(feature,values("bitcode"))

tests/ui/check-cfg/diagnotics.rs

@@ -1,7 +1,7 @@
 //@ check-pass
 //@ revisions: cargo rustc
-//@ [rustc]unset-rustc-env:CARGO
-//@ [cargo]rustc-env:CARGO=/usr/bin/cargo
+//@ [rustc]unset-rustc-env:CARGO_CRATE_NAME
+//@ [cargo]rustc-env:CARGO_CRATE_NAME=foo
 //@ compile-flags: --check-cfg=cfg(feature,values("foo")) --check-cfg=cfg(no_values) -Z unstable-options
 
 #[cfg(featur)]

tests/ui/crate-loading/missing-std.rs

@@ -1,6 +1,6 @@
 //@ compile-flags: --target x86_64-unknown-uefi
 //@ needs-llvm-components: x86
-//@ rustc-env:CARGO=/usr/bin/cargo
+//@ rustc-env:CARGO_CRATE_NAME=foo
 #![feature(no_core)]
 #![no_core]
 extern crate core;