Express contracts as part of function header and lower it to the contract lang items

includes post-developed commit: do not suggest internal-only keywords as corrections to parse failures.

includes post-developed commit: removed tabs that creeped in into rustfmt tool source code.

includes post-developed commit, placating rustfmt self dogfooding.

includes post-developed commit: add backquotes to prevent markdown checking from trying to treat an attr as a markdown hyperlink/

includes post-developed commit: fix lowering to keep contracts from being erroneously inherited by nested bodies (like closures).

Rebase Conflicts:
 - compiler/rustc_parse/src/parser/diagnostics.rs
 - compiler/rustc_parse/src/parser/item.rs
 - compiler/rustc_span/src/hygiene.rs

Remove contracts keywords from diagnostic messages

compiler/rustc_parse/src/parser/generics.rs

@@ -297,6 +297,28 @@ impl<'a> Parser<'a> {
         })
     }
 
+    /// Parses a rustc-internal fn contract
+    /// (`rustc_contract_requires(WWW) rustc_contract_ensures(ZZZ)`)
+    pub(super) fn parse_contract(
+        &mut self,
+    ) -> PResult<'a, Option<rustc_ast::ptr::P<ast::FnContract>>> {
+        let requires = if self.eat_keyword_noexpect(exp!(RustcContractRequires).kw) {
+            Some(self.parse_expr()?)
+        } else {
+            None
+        };
+        let ensures = if self.eat_keyword_noexpect(exp!(RustcContractEnsures).kw) {
+            Some(self.parse_expr()?)
+        } else {
+            None
+        };
+        if requires.is_none() && ensures.is_none() {
+            Ok(None)
+        } else {
+            Ok(Some(rustc_ast::ptr::P(ast::FnContract { requires, ensures })))
+        }
+    }
+
     /// Parses an optional where-clause.
     ///
     /// ```ignore (only-for-syntax-highlight)

compiler/rustc_parse/src/parser/item.rs

@@ -213,9 +213,12 @@ impl<'a> Parser<'a> {
             self.parse_use_item()?
         } else if self.check_fn_front_matter(check_pub, case) {
             // FUNCTION ITEM
-            let (ident, sig, generics, body) =
+            let (ident, sig, generics, contract, body) =
                 self.parse_fn(attrs, fn_parse_mode, lo, vis, case)?;
-            (ident, ItemKind::Fn(Box::new(Fn { defaultness: def_(), sig, generics, body })))
+            (
+                ident,
+                ItemKind::Fn(Box::new(Fn { defaultness: def_(), sig, generics, contract, body })),
+            )
         } else if self.eat_keyword(exp!(Extern)) {
             if self.eat_keyword(exp!(Crate)) {
                 // EXTERN CRATE
@@ -2372,7 +2375,7 @@ impl<'a> Parser<'a> {
         sig_lo: Span,
         vis: &Visibility,
         case: Case,
-    ) -> PResult<'a, (Ident, FnSig, Generics, Option<P<Block>>)> {
+    ) -> PResult<'a, (Ident, FnSig, Generics, Option<P<FnContract>>, Option<P<Block>>)> {
         let fn_span = self.token.span;
         let header = self.parse_fn_front_matter(vis, case)?; // `const ... fn`
         let ident = self.parse_ident()?; // `foo`
@@ -2398,6 +2401,8 @@ impl<'a> Parser<'a> {
         // inside `parse_fn_body()`.
         let fn_params_end = self.prev_token.span.shrink_to_hi();
 
+        let contract = self.parse_contract()?;
+
         generics.where_clause = self.parse_where_clause()?; // `where T: Ord`
 
         // `fn_params_end` is needed only when it's followed by a where clause.
@@ -2409,7 +2414,7 @@ impl<'a> Parser<'a> {
         let body =
             self.parse_fn_body(attrs, &ident, &mut sig_hi, fn_parse_mode.req_body, fn_params_end)?;
         let fn_sig_span = sig_lo.to(sig_hi);
-        Ok((ident, FnSig { header, decl, span: fn_sig_span }, generics, body))
+        Ok((ident, FnSig { header, decl, span: fn_sig_span }, generics, contract, body))
     }
 
     /// Provide diagnostics when function body is not found

compiler/rustc_parse/src/parser/token_type.rs

@@ -108,6 +108,8 @@ pub enum TokenType {
     KwRef,
     KwReturn,
     KwReuse,
+    KwRustcContractEnsures,
+    KwRustcContractRequires,
     KwSafe,
     KwSelfUpper,
     KwStatic,
@@ -242,6 +244,8 @@ impl TokenType {
             KwRef,
             KwReturn,
             KwReuse,
+            KwRustcContractEnsures,
+            KwRustcContractRequires,
             KwSafe,
             KwSelfUpper,
             KwStatic,
@@ -314,6 +318,8 @@ impl TokenType {
             TokenType::KwRef => Some(kw::Ref),
             TokenType::KwReturn => Some(kw::Return),
             TokenType::KwReuse => Some(kw::Reuse),
+            TokenType::KwRustcContractEnsures => Some(kw::RustcContractEnsures),
+            TokenType::KwRustcContractRequires => Some(kw::RustcContractRequires),
             TokenType::KwSafe => Some(kw::Safe),
             TokenType::KwSelfUpper => Some(kw::SelfUpper),
             TokenType::KwStatic => Some(kw::Static),
@@ -544,6 +550,8 @@ macro_rules! exp {
     (Ref)            => { exp!(@kw, Ref,        KwRef) };
     (Return)         => { exp!(@kw, Return,     KwReturn) };
     (Reuse)          => { exp!(@kw, Reuse,      KwReuse) };
+    (RustcContractEnsures)  => { exp!(@kw, RustcContractEnsures, KwRustcContractEnsures) };
+    (RustcContractRequires) => { exp!(@kw, RustcContractRequires, KwRustcContractRequires) };
     (Safe)           => { exp!(@kw, Safe,       KwSafe) };
     (SelfUpper)      => { exp!(@kw, SelfUpper,  KwSelfUpper) };
     (Static)         => { exp!(@kw, Static,     KwStatic) };