Give a better error when std or core are missing

compiler/rustc_metadata/src/creader.rs

@@ -511,8 +511,11 @@ impl<'a> CrateLoader<'a> {
  if dep.is_none() {
  self.used_extern_options.insert(name);
  }
- self.maybe_resolve_crate(name, dep_kind, dep)
- .unwrap_or_else(|err| err.report(self.sess, span))
+ self.maybe_resolve_crate(name, dep_kind, dep).unwrap_or_else(|err| {
+ let missing_core =
+ self.maybe_resolve_crate(sym::core, CrateDepKind::Explicit, None).is_err();
+ err.report(&self.sess, span, missing_core)
+ })
  }
 
  fn maybe_resolve_crate<'b>(

compiler/rustc_metadata/src/locator.rs

@@ -790,7 +790,8 @@ pub fn find_plugin_registrar(
 ) -> (PathBuf, CrateDisambiguator) {
  match find_plugin_registrar_impl(sess, metadata_loader, name) {
  Ok(res) => res,
- Err(err) => err.report(sess, span),
+ // `core` is always available if we got as far as loading plugins.
+ Err(err) => err.report(sess, span, false),
  }
 }
 
@@ -883,7 +884,7 @@ crate enum CrateError {
 }
 
 impl CrateError {
- crate fn report(self, sess: &Session, span: Span) -> ! {
+ crate fn report(self, sess: &Session, span: Span, missing_core: bool) -> ! {
  let mut err = match self {
  CrateError::NonAsciiName(crate_name) => sess.struct_span_err(
  span,
@@ -1068,7 +1069,37 @@ impl CrateError {
  if (crate_name == sym::std || crate_name == sym::core)
  && locator.triple != TargetTriple::from_triple(config::host_triple())
  {
- err.note(&format!("the `{}` target may not be installed", locator.triple));
+ if missing_core {
+ err.note(&format!(
+ "the `{}` target may not be installed",
+ locator.triple
+ ));
+ } else {
+ err.note(&format!(
+ "the `{}` target may not support the standard library",
+ locator.triple
+ ));
+ }
+ if missing_core && std::env::var("RUSTUP_HOME").is_ok() {
+ err.help(&format!(
+ "consider downloading the target with `rustup target add {}`",
+ locator.triple
+ ));
+ }
+ // 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 `#![no_std]` won't help.
+ if !missing_core && span.is_dummy() {
+ let current_crate =
+ sess.opts.crate_name.as_deref().unwrap_or("<unknown>");
+ err.note(&format!(
+ "`std` is required by `{}` because it does not declare `#![no_std]`",
+ current_crate
+ ));
+ }
+ if sess.is_nightly_build() && std::env::var("CARGO").is_ok() {
+ err.help("consider building the standard library from source with `cargo build -Zbuild-std`");
+ }
  } else if crate_name == sym::profiler_builtins {
  err.note(&"the compiler may have been built without the profiler runtime");
  }

src/test/ui/crate-loading/missing-std.rs

@@ -0,0 +1,11 @@
+// compile-flags: --target x86_64-unknown-uefi
+// rustc-env:CARGO=/usr/bin/cargo
+// rustc-env:RUSTUP_HOME=/home/bors/.rustup
+#![no_core]
+extern crate core;
+//~^ ERROR can't find crate for `core`
+//~| NOTE can't find crate
+//~| NOTE target may not be installed
+//~| HELP consider building the standard library from source with `cargo build -Zbuild-std`
+//~| HELP consider downloading the target with `rustup target add x86_64-unknown-uefi`
+fn main() {}

src/test/ui/crate-loading/missing-std.stderr

@@ -0,0 +1,13 @@
+error[E0463]: can't find crate for `core`
+ --> $DIR/missing-std.rs:5:1
+ |
+LL | extern crate core;
+ | ^^^^^^^^^^^^^^^^^^ can't find crate
+ |
+ = note: the `x86_64-unknown-uefi` target may not be installed
+ = help: consider downloading the target with `rustup target add x86_64-unknown-uefi`
+ = help: consider building the standard library from source with `cargo build -Zbuild-std`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0463`.