Add parser recovery for const $ident = $expr;.

Then use the diagnostics-stealing API to stash parser
errors and enrich them with type information in typeck.

src/librustc_typeck/collect.rs

@@ -46,7 +46,7 @@ use rustc::hir::intravisit::{self, NestedVisitorMap, Visitor};
 use rustc::hir::GenericParamKind;
 use rustc::hir::{self, CodegenFnAttrFlags, CodegenFnAttrs, Unsafety};
 
-use errors::{Applicability, DiagnosticId};
+use errors::{Applicability, DiagnosticId, StashKey};
 
 struct OnlySelfBounds(bool);
 
@@ -1149,18 +1149,41 @@ fn infer_placeholder_type(
     def_id: DefId,
     body_id: hir::BodyId,
     span: Span,
+    item_ident: Ident,
 ) -> Ty<'_> {
     let ty = tcx.typeck_tables_of(def_id).node_type(body_id.hir_id);
-    let mut diag = bad_placeholder_type(tcx, span);
-    if ty != tcx.types.err {
-        diag.span_suggestion(
-            span,
-            "replace `_` with the correct type",
-            ty.to_string(),
-            Applicability::MaybeIncorrect,
-        );
+
+    // If this came from a free `const` or `static mut?` item,
+    // then the user may have written e.g. `const A = 42;`.
+    // In this case, the parser has stashed a diagnostic for
+    // us to improve in typeck so we do that now.
+    match tcx.sess.diagnostic().steal_diagnostic(span, StashKey::ItemNoType) {
+        Some(mut err) => {
+            // The parser provided a sub-optimal `HasPlaceholders` suggestion for the type.
+            // We are typeck and have the real type, so remove that and suggest the actual type.
+            err.suggestions.clear();
+            err.span_suggestion(
+                span,
+                "provide a type for the item",
+                format!("{}: {}", item_ident, ty),
+                Applicability::MachineApplicable,
+            )
+            .emit();
+        }
+        None => {
+            let mut diag = bad_placeholder_type(tcx, span);
+            if ty != tcx.types.err {
+                diag.span_suggestion(
+                    span,
+                    "replace `_` with the correct type",
+                    ty.to_string(),
+                    Applicability::MaybeIncorrect,
+                );
+            }
+            diag.emit();
+        }
     }
-    diag.emit();
+
     ty
 }
 
@@ -1192,7 +1215,7 @@ pub fn checked_type_of(tcx: TyCtxt<'_>, def_id: DefId, fail: bool) -> Option<Ty<
             TraitItemKind::Const(ref ty, body_id)  => {
                 body_id.and_then(|body_id| {
                     if let hir::TyKind::Infer = ty.node {
-                        Some(infer_placeholder_type(tcx, def_id, body_id, ty.span))
+                        Some(infer_placeholder_type(tcx, def_id, body_id, ty.span, item.ident))
                     } else {
                         None
                     }
@@ -1214,7 +1237,7 @@ pub fn checked_type_of(tcx: TyCtxt<'_>, def_id: DefId, fail: bool) -> Option<Ty<
             }
             ImplItemKind::Const(ref ty, body_id) => {
                 if let hir::TyKind::Infer = ty.node {
-                    infer_placeholder_type(tcx, def_id, body_id, ty.span)
+                    infer_placeholder_type(tcx, def_id, body_id, ty.span, item.ident)
                 } else {
                     icx.to_ty(ty)
                 }
@@ -1246,7 +1269,7 @@ pub fn checked_type_of(tcx: TyCtxt<'_>, def_id: DefId, fail: bool) -> Option<Ty<
                 ItemKind::Static(ref ty, .., body_id)
                 | ItemKind::Const(ref ty, body_id) => {
                     if let hir::TyKind::Infer = ty.node {
-                        infer_placeholder_type(tcx, def_id, body_id, ty.span)
+                        infer_placeholder_type(tcx, def_id, body_id, ty.span, item.ident)
                     } else {
                         icx.to_ty(ty)
                     }

src/libsyntax/parse/parser/item.rs

@@ -24,7 +24,7 @@ use crate::symbol::{kw, sym};
 use std::mem;
 use log::debug;
 use rustc_target::spec::abi::Abi;
-use errors::{Applicability, DiagnosticBuilder, DiagnosticId};
+use errors::{Applicability, DiagnosticBuilder, DiagnosticId, StashKey};
 
 /// Whether the type alias or associated type is a concrete type or an opaque type.
 #[derive(Debug)]
@@ -1477,10 +1477,23 @@ impl<'a> Parser<'a> {
         }
     }
 
+    /// Parse `["const" | ("static" "mut"?)] $ident ":" $ty = $expr` with
+    /// `["const" | ("static" "mut"?)]` already parsed and stored in `m`.
+    ///
+    /// When `m` is `"const"`, `$ident` may also be `"_"`.
     fn parse_item_const(&mut self, m: Option<Mutability>) -> PResult<'a, ItemInfo> {
         let id = if m.is_none() { self.parse_ident_or_underscore() } else { self.parse_ident() }?;
-        self.expect(&token::Colon)?;
-        let ty = self.parse_ty()?;
+
+        // Parse the type of a `const` or `static mut?` item.
+        // That is, the `":" $ty` fragment.
+        let ty = if self.token == token::Eq {
+            self.recover_missing_const_type(id, m)
+        } else {
+            // Not `=` so expect `":"" $ty` as usual.
+            self.expect(&token::Colon)?;
+            self.parse_ty()?
+        };
+
         self.expect(&token::Eq)?;
         let e = self.parse_expr()?;
         self.expect(&token::Semi)?;
@@ -1491,6 +1504,34 @@ impl<'a> Parser<'a> {
         Ok((id, item, None))
     }
 
+    /// We were supposed to parse `:` but instead, we're already at `=`.
+    /// This means that the type is missing.
+    fn recover_missing_const_type(&mut self, id: Ident, m: Option<Mutability>) -> P<Ty> {
+        // Construct the error and stash it away with the hope
+        // that typeck will later enrich the error with a type.
+        let kind = match m {
+            Some(Mutability::Mutable) => "static mut",
+            Some(Mutability::Immutable) => "static",
+            None => "const",
+        };
+        let mut err = self.struct_span_err(id.span, &format!("missing type for `{}` item", kind));
+        err.span_suggestion(
+            id.span,
+            "provide a type for the item",
+            format!("{}: <type>", id),
+            Applicability::HasPlaceholders,
+        );
+        err.stash(id.span, StashKey::ItemNoType);
+
+        // The user intended that the type be inferred,
+        // so treat this as if the user wrote e.g. `const A: _ = expr;`.
+        P(Ty {
+            node: TyKind::Infer,
+            span: id.span,
+            id: ast::DUMMY_NODE_ID,
+        })
+    }
+
     /// Parses `type Foo = Bar;` or returns `None`
     /// without modifying the parser state.
     fn eat_type(&mut self) -> Option<PResult<'a, (Ident, AliasKind, Generics)>> {

src/test/ui/suggestions/const-no-type.rs

@@ -0,0 +1,46 @@
+// In the cases below, the type is missing from the `const` and `static` items.
+//
+// Here, we test that we:
+//
+// a) Perform parser recovery.
+//
+// b) Emit a diagnostic with the actual inferred type to RHS of `=` as the suggestion.
+
+fn main() {}
+
+// These will not reach typeck:
+
+#[cfg(FALSE)]
+const C2 = 42;
+//~^ ERROR missing type for `const` item
+//~| HELP provide a type for the item
+//~| SUGGESTION C2: <type>
+
+#[cfg(FALSE)]
+static S2 = "abc";
+//~^ ERROR missing type for `static` item
+//~| HELP provide a type for the item
+//~| SUGGESTION S2: <type>
+
+#[cfg(FALSE)]
+static mut SM2 = "abc";
+//~^ ERROR missing type for `static mut` item
+//~| HELP provide a type for the item
+//~| SUGGESTION SM2: <type>
+
+// These will, so the diagnostics should be stolen by typeck:
+
+const C = 42;
+//~^ ERROR missing type for `const` item
+//~| HELP provide a type for the item
+//~| SUGGESTION C: i32
+
+static S = Vec::<String>::new();
+//~^ ERROR missing type for `static` item
+//~| HELP provide a type for the item
+//~| SUGGESTION S: std::vec::Vec<std::string::String>
+
+static mut SM = "abc";
+//~^ ERROR missing type for `static mut` item
+//~| HELP provide a type for the item
+//~| SUGGESTION &'static str

src/test/ui/suggestions/const-no-type.stderr

@@ -0,0 +1,38 @@
+error: missing type for `const` item
+  --> $DIR/const-no-type.rs:33:7
+   |
+LL | const C = 42;
+   |       ^ help: provide a type for the item: `C: i32`
+
+error: missing type for `static` item
+  --> $DIR/const-no-type.rs:38:8
+   |
+LL | static S = Vec::<String>::new();
+   |        ^ help: provide a type for the item: `S: std::vec::Vec<std::string::String>`
+
+error: missing type for `static mut` item
+  --> $DIR/const-no-type.rs:43:12
+   |
+LL | static mut SM = "abc";
+   |            ^^ help: provide a type for the item: `SM: &'static str`
+
+error: missing type for `const` item
+  --> $DIR/const-no-type.rs:14:7
+   |
+LL | const C2 = 42;
+   |       ^^ help: provide a type for the item: `C2: <type>`
+
+error: missing type for `static` item
+  --> $DIR/const-no-type.rs:20:8
+   |
+LL | static S2 = "abc";
+   |        ^^ help: provide a type for the item: `S2: <type>`
+
+error: missing type for `static mut` item
+  --> $DIR/const-no-type.rs:26:12
+   |
+LL | static mut SM2 = "abc";
+   |            ^^^ help: provide a type for the item: `SM2: <type>`
+
+error: aborting due to 6 previous errors
+