auto merge of #17733 : jgallagher/rust/while-let, r=alexcrichton

This is *heavily* based on `if let` (#17634) by @jakub- and @kballard

This should close #17687

Author: bors

src/doc/reference.md

@@ -2488,6 +2488,8 @@ The currently implemented features of the reference compiler are:
 
 * `if_let` - Allows use of the `if let` syntax.
 
+* `while_let` - Allows use of the `while let` syntax.
+
 * `intrinsics` - Allows use of the "rust-intrinsics" ABI. Compiler intrinsics
                  are inherently unstable and no promise about them is made.
 
@@ -3494,6 +3496,18 @@ of a condition expression it expects a refutable let statement. If the value of
 expression on the right hand side of the let statement matches the pattern, the corresponding
 block will execute, otherwise flow proceeds to the first `else` block that follows.
 
+### While let loops
+
+```{.ebnf .gram}
+while_let_expr : "while" "let" pat '=' expr '{' block '}' ;
+```
+
+A `while let` loop is semantically identical to a `while` loop but in place of a
+condition expression it expects a refutable let statement. If the value of the
+expression on the right hand side of the let statement matches the pattern, the
+loop body block executes and control returns to the pattern matching statement.
+Otherwise, the while expression completes.
+
 ### Return expressions
 
 ```{.ebnf .gram}

src/librustc/diagnostics.rs

@@ -145,5 +145,6 @@ register_diagnostics!(
     E0161,
     E0162,
     E0163,
-    E0164
+    E0164,
+    E0165
 )

src/librustc/lint/builtin.rs

@@ -1082,7 +1082,8 @@ impl LintPass for UnnecessaryParens {
             ast::ExprWhile(ref cond, _, _) => (cond, "`while` condition", true),
             ast::ExprMatch(ref head, _, source) => match source {
                 ast::MatchNormal => (head, "`match` head expression", true),
-                ast::MatchIfLetDesugar => (head, "`if let` head expression", true)
+                ast::MatchIfLetDesugar => (head, "`if let` head expression", true),
+                ast::MatchWhileLetDesugar => (head, "`while let` head expression", true),
             },
             ast::ExprRet(Some(ref value)) => (value, "`return` value", false),
             ast::ExprAssign(_, ref value) => (value, "assigned value", false),

src/librustc/middle/cfg/construct.rs

@@ -259,6 +259,10 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
                 expr_exit
             }
 
+            ast::ExprWhileLet(..) => {
+                self.tcx.sess.span_bug(expr.span, "non-desugared ExprWhileLet");
+            }
+
             ast::ExprForLoop(ref pat, ref head, ref body, _) => {
                 //
                 //          [pred]

src/librustc/middle/check_match.rs

@@ -261,20 +261,32 @@ fn check_arms(cx: &MatchCheckCtxt, arms: &[(Vec<P<Pat>>, Option<&Expr>)], source
 
             match is_useful(cx, &seen, v.as_slice(), LeaveOutWitness) {
                 NotUseful => {
-                    if source == MatchIfLetDesugar {
-                        if printed_if_let_err {
-                            // we already printed an irrefutable if-let pattern error.
-                            // We don't want two, that's just confusing.
-                        } else {
+                    match source {
+                        MatchIfLetDesugar => {
+                            if printed_if_let_err {
+                                // we already printed an irrefutable if-let pattern error.
+                                // We don't want two, that's just confusing.
+                            } else {
+                                // find the first arm pattern so we can use its span
+                                let &(ref first_arm_pats, _) = &arms[0];
+                                let first_pat = first_arm_pats.get(0);
+                                let span = first_pat.span;
+                                span_err!(cx.tcx.sess, span, E0162, "irrefutable if-let pattern");
+                                printed_if_let_err = true;
+                            }
+                        },
+
+                        MatchWhileLetDesugar => {
                             // find the first arm pattern so we can use its span
                             let &(ref first_arm_pats, _) = &arms[0];
                             let first_pat = first_arm_pats.get(0);
                             let span = first_pat.span;
-                            span_err!(cx.tcx.sess, span, E0162, "irrefutable if-let pattern");
-                            printed_if_let_err = true;
-                        }
-                    } else {
-                        span_err!(cx.tcx.sess, pat.span, E0001, "unreachable pattern");
+                            span_err!(cx.tcx.sess, span, E0165, "irrefutable while-let pattern");
+                        },
+
+                        MatchNormal => {
+                            span_err!(cx.tcx.sess, pat.span, E0001, "unreachable pattern")
+                        },
                     }
                 }
                 Useful => (),

src/librustc/middle/expr_use_visitor.rs

@@ -429,6 +429,10 @@ impl<'d,'t,'tcx,TYPER:mc::Typer<'tcx>> ExprUseVisitor<'d,'t,TYPER> {
                 self.walk_block(&**blk);
             }
 
+            ast::ExprWhileLet(..) => {
+                self.tcx().sess.span_bug(expr.span, "non-desugared ExprWhileLet");
+            }
+
             ast::ExprForLoop(ref pat, ref head, ref blk, _) => {
                 // The pattern lives as long as the block.
                 debug!("walk_expr for loop case: blk id={}", blk.id);

src/librustc/middle/liveness.rs

@@ -484,6 +484,9 @@ fn visit_expr(ir: &mut IrMaps, expr: &Expr) {
       ExprIfLet(..) => {
           ir.tcx.sess.span_bug(expr.span, "non-desugared ExprIfLet");
       }
+      ExprWhileLet(..) => {
+          ir.tcx.sess.span_bug(expr.span, "non-desugared ExprWhileLet");
+      }
       ExprForLoop(ref pat, _, _, _) => {
         pat_util::pat_bindings(&ir.tcx.def_map, &**pat, |bm, p_id, sp, path1| {
             debug!("adding local variable {} from for loop with bm {:?}",
@@ -1022,6 +1025,10 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             self.propagate_through_loop(expr, WhileLoop(&**cond), &**blk, succ)
           }
 
+          ExprWhileLet(..) => {
+              self.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprWhileLet");
+          }
+
           ExprForLoop(ref pat, ref head, ref blk, _) => {
             let ln = self.propagate_through_loop(expr, ForLoop(&**pat), &**blk, succ);
             self.propagate_through_expr(&**head, ln)
@@ -1480,6 +1487,9 @@ fn check_expr(this: &mut Liveness, expr: &Expr) {
       ExprIfLet(..) => {
         this.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprIfLet");
       }
+      ExprWhileLet(..) => {
+        this.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprWhileLet");
+      }
     }
 }
 

src/librustc/middle/mem_categorization.rs

@@ -530,6 +530,9 @@ impl<'t,'tcx,TYPER:Typer<'tcx>> MemCategorizationContext<'t,TYPER> {
           ast::ExprIfLet(..) => {
             self.tcx().sess.span_bug(expr.span, "non-desugared ExprIfLet");
           }
+          ast::ExprWhileLet(..) => {
+            self.tcx().sess.span_bug(expr.span, "non-desugared ExprWhileLet");
+          }
         }
     }
 

src/librustc/middle/trans/debuginfo.rs

@@ -3496,6 +3496,11 @@ fn populate_scope_map(cx: &CrateContext,
                 })
             }
 
+            ast::ExprWhileLet(..) => {
+                cx.sess().span_bug(exp.span, "debuginfo::populate_scope_map() - \
+                                              Found unexpanded while-let.");
+            }
+
             ast::ExprForLoop(ref pattern, ref head, ref body, _) => {
                 walk_expr(cx, &**head, scope_stack, scope_map);
 

src/librustc/middle/ty.rs

@@ -3605,6 +3605,9 @@ pub fn expr_kind(tcx: &ctxt, expr: &ast::Expr) -> ExprKind {
         ast::ExprIfLet(..) => {
             tcx.sess.span_bug(expr.span, "non-desugared ExprIfLet");
         }
+        ast::ExprWhileLet(..) => {
+            tcx.sess.span_bug(expr.span, "non-desugared ExprWhileLet");
+        }
 
         ast::ExprLit(ref lit) if lit_is_str(&**lit) => {
             RvalueDpsExpr