Update all_traits query to load more crates.

This commit updates the `all_traits` query to speculatively load all
crates that have an `--extern` flag for the purpose of diagnostics.

Author: davidtwco

src/librustc_typeck/check/method/suggest.rs

@@ -9,7 +9,7 @@ use errors::{Applicability, DiagnosticBuilder};
 use rustc_data_structures::sync::Lrc;
 use rustc::hir::{self, ExprKind, Node, QPath};
 use rustc::hir::def::Def;
-use rustc::hir::def_id::{CRATE_DEF_INDEX, LOCAL_CRATE, DefId};
+use rustc::hir::def_id::{CRATE_DEF_INDEX, LOCAL_CRATE, CrateNum, DefId};
 use rustc::hir::map as hir_map;
 use rustc::hir::print;
 use rustc::infer::type_variable::TypeVariableOrigin;
@@ -774,7 +774,28 @@ fn compute_all_traits<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>) -> Vec<DefId>
             _ => {}
         }
     }
+
+    let mut cnums: FxHashSet<CrateNum> = FxHashSet::default();
+
+    // Attempt to load all crates that we have `--extern` flags for, this means
+    // we will be able to make suggestions for traits they define. Particularly useful
+    // in Rust 2018 as there aren't `extern crate` lines that import a crate even if it isn't
+    // otherwise used.
+    for name in tcx.extern_prelude.keys() {
+        let cnum = tcx.maybe_load_extern_crate(*name);
+        debug!("compute_all_traits: (attempt load) name={:?} cnum={:?}", name, cnum);
+        if let Some(cnum) = cnum {
+            let _ = cnums.insert(cnum);
+        }
+    }
+
+    // Add all crates that we already know about.
     for &cnum in tcx.crates().iter() {
+        let _ = cnums.insert(cnum);
+    }
+
+    for cnum in cnums {
+        debug!("compute_all_traits: cnum={:?} name={:?}", cnum, tcx.crate_name(cnum));
         let def_id = DefId {
             krate: cnum,
             index: CRATE_DEF_INDEX,

src/test/ui/rust-2018/auxiliary/baz.rs

@@ -1,5 +1,12 @@
-// This file is used as part of the local-path-suggestions.rs test.
+// This file is used as part of the local-path-suggestions.rs and
+// the trait-import-suggestions.rs test.
 
 pub mod foobar {
     pub struct Baz;
 }
+
+pub trait BazTrait {
+    fn extern_baz(&self) { }
+}
+
+impl BazTrait for u32 { }

src/test/ui/rust-2018/auxiliary/trait-import-suggestions.rs

@@ -0,0 +1,13 @@
+// edition:2018
+// aux-build:baz.rs
+// compile-flags:--extern baz
+
+// Don't use anything from baz - making suggestions from it when the only reference to it
+// is an `--extern` flag is what is tested by this test.
+
+struct Local;
+
+fn main() {
+    let local = Local;
+    local.extern_baz(); //~ ERROR no method named `extern_baz`
+}

src/test/ui/rust-2018/extern-trait-impl-suggestions.rs

@@ -0,0 +1,16 @@
+error[E0599]: no method named `extern_baz` found for type `Local` in the current scope
+  --> $DIR/extern-trait-impl-suggestions.rs:12:11
+   |
+LL | struct Local;
+   | ------------- method `extern_baz` not found for this
+...
+LL |     local.extern_baz(); //~ ERROR no method named `extern_baz`
+   |           ^^^^^^^^^^
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+   = note: the following trait defines an item `extern_baz`, perhaps you need to implement it:
+           candidate #1: `baz::BazTrait`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0599`.

src/test/ui/rust-2018/extern-trait-impl-suggestions.stderr

@@ -1,6 +1,9 @@
 // edition:2018
-// aux-build:trait-import-suggestions.rs
-// compile-flags:--extern trait-import-suggestions
+// aux-build:baz.rs
+// compile-flags:--extern baz
+
+// Don't use anything from baz - making suggestions from it when the only reference to it
+// is an `--extern` flag is one of the things tested by this test.
 
 mod foo {
     mod foobar {
@@ -26,6 +29,6 @@ mod foo {
 fn main() {
     let x: u32 = 22;
     x.bar(); //~ ERROR no method named `bar`
-    x.baz(); //~ ERROR no method named `baz`
+    x.extern_baz(); //~ ERROR no method named `extern_baz`
     let y = u32::from_str("33"); //~ ERROR no function or associated item named `from_str`
 }

src/test/ui/rust-2018/trait-import-suggestions.rs

@@ -1,5 +1,5 @@
 error[E0599]: no method named `foobar` found for type `u32` in the current scope
-  --> $DIR/trait-import-suggestions.rs:22:11
+  --> $DIR/trait-import-suggestions.rs:25:11
    |
 LL |         x.foobar(); //~ ERROR no method named `foobar`
    |           ^^^^^^
@@ -9,7 +9,7 @@ LL |         x.foobar(); //~ ERROR no method named `foobar`
            `use crate::foo::foobar::Foobar;`
 
 error[E0599]: no method named `bar` found for type `u32` in the current scope
-  --> $DIR/trait-import-suggestions.rs:28:7
+  --> $DIR/trait-import-suggestions.rs:31:7
    |
 LL |     x.bar(); //~ ERROR no method named `bar`
    |       ^^^
@@ -20,14 +20,14 @@ help: the following trait is implemented but not in scope, perhaps add a `use` f
 LL | use crate::foo::Bar;
    |
 
-error[E0599]: no method named `baz` found for type `u32` in the current scope
-  --> $DIR/trait-import-suggestions.rs:29:7
+error[E0599]: no method named `extern_baz` found for type `u32` in the current scope
+  --> $DIR/trait-import-suggestions.rs:32:7
    |
-LL |     x.baz(); //~ ERROR no method named `baz`
-   |       ^^^
+LL |     x.extern_baz(); //~ ERROR no method named `extern_baz`
+   |       ^^^^^^^^^^
 
 error[E0599]: no function or associated item named `from_str` found for type `u32` in the current scope
-  --> $DIR/trait-import-suggestions.rs:30:18
+  --> $DIR/trait-import-suggestions.rs:33:18
    |
 LL |     let y = u32::from_str("33"); //~ ERROR no function or associated item named `from_str`
    |             -----^^^^^^^^