Suggest x build library for a custom toolchain that fails to load core

Author: mu001999

compiler/rustc_metadata/messages.ftl

@@ -25,6 +25,9 @@ metadata_conflicting_alloc_error_handler =
 metadata_conflicting_global_alloc =
     the `#[global_allocator]` in {$other_crate_name} conflicts with global allocator in: {$crate_name}
 
+metadata_consider_adding_std =
+    consider adding the standard library to the sysroot with `x build library --target {$locator_triple}`
+
 metadata_consider_building_std =
     consider building the standard library from source with `cargo build -Zbuild-std`
 

compiler/rustc_metadata/src/errors.rs

@@ -623,6 +623,7 @@ pub struct CannotFindCrate {
     pub is_nightly_build: bool,
     pub profiler_runtime: Symbol,
     pub locator_triple: TargetTriple,
+    pub is_ui_testing: bool,
 }
 
 impl IntoDiagnostic<'_> for CannotFindCrate {
@@ -646,12 +647,19 @@ impl IntoDiagnostic<'_> for CannotFindCrate {
             } else {
                 diag.note(fluent::metadata_target_no_std_support);
             }
-            // NOTE: this suggests using rustup, even though the user may not have it installed.
-            // That's because they could choose to install it; or this may give them a hint which
-            // target they need to install from their distro.
+
             if self.missing_core {
-                diag.help(fluent::metadata_consider_downloading_target);
+                if env!("CFG_RELEASE_CHANNEL") == "dev" && !self.is_ui_testing {
+                    // Note: Emits the nicer suggestion only for the dev channel.
+                    diag.help(fluent::metadata_consider_adding_std);
+                } else {
+                    // NOTE: this suggests using rustup, even though the user may not have it installed.
+                    // That's because they could choose to install it; or this may give them a hint which
+                    // target they need to install from their distro.
+                    diag.help(fluent::metadata_consider_downloading_target);
+                }
             }
+
             // Suggest using #![no_std]. #[no_core] is unstable and not really supported anyway.
             // NOTE: this is a dummy span if `extern crate std` was injected by the compiler.
             // If it's not a dummy, that means someone added `extern crate std` explicitly and

compiler/rustc_metadata/src/locator.rs

@@ -1125,6 +1125,7 @@ impl CrateError {
                         is_nightly_build: sess.is_nightly_build(),
                         profiler_runtime: Symbol::intern(&sess.opts.unstable_opts.profiler_runtime),
                         locator_triple: locator.triple,
+                        is_ui_testing: sess.opts.unstable_opts.ui_testing,
                     });
                 }
             }
@@ -1141,6 +1142,7 @@ impl CrateError {
                     is_nightly_build: sess.is_nightly_build(),
                     profiler_runtime: Symbol::intern(&sess.opts.unstable_opts.profiler_runtime),
                     locator_triple: sess.opts.target_triple.clone(),
+                    is_ui_testing: sess.opts.unstable_opts.ui_testing,
                 });
             }
         }