Produce a better error for irrefutable if let patterns

Modify ast::ExprMatch to include a new value of type ast::MatchSource,
making it easy to tell whether the match was written literally or
produced via desugaring. This allows us to customize error messages
appropriately.

Author: lilyball

src/librustc/diagnostics.rs

@@ -17,6 +17,11 @@ register_diagnostic!(E0001, r##"
     one is too specific or the ordering is incorrect.
 "##)
 
+register_diagnostic!(E0159, r##"
+    This error is produced by an `if let` expression where the pattern is irrefutable.
+    An `if let` that can never fail is considered an error.
+"##)
+
 register_diagnostics!(
     E0002,
     E0003,

src/librustc/front/config.rs

@@ -206,15 +206,15 @@ fn fold_block(cx: &mut Context, b: ast::P<ast::Block>) -> ast::P<ast::Block> {
 
 fn fold_expr(cx: &mut Context, expr: Gc<ast::Expr>) -> Gc<ast::Expr> {
     let expr = match expr.node {
-        ast::ExprMatch(ref m, ref arms) => {
+        ast::ExprMatch(ref m, ref arms, source) => {
             let arms = arms.iter()
                 .filter(|a| (cx.in_cfg)(a.attrs.as_slice()))
                 .map(|a| a.clone())
                 .collect();
             box(GC) ast::Expr {
                 id: expr.id,
                 span: expr.span.clone(),
-                node: ast::ExprMatch(m.clone(), arms),
+                node: ast::ExprMatch(m.clone(), arms, source),
             }
         }
         _ => expr.clone()

src/librustc/lint/builtin.rs

@@ -1080,7 +1080,10 @@ impl LintPass for UnnecessaryParens {
         let (value, msg, struct_lit_needs_parens) = match e.node {
             ast::ExprIf(cond, _, _) => (cond, "`if` condition", true),
             ast::ExprWhile(cond, _) => (cond, "`while` condition", true),
-            ast::ExprMatch(head, _) => (head, "`match` head expression", true),
+            ast::ExprMatch(head, _, source) => match source {
+                ast::MatchNormal => (head, "`match` head expression", true),
+                ast::MatchIfLetDesugar => (head, "`if let` head expression", true)
+            },
             ast::ExprRet(Some(value)) => (value, "`return` value", false),
             ast::ExprAssign(_, value) => (value, "assigned value", false),
             ast::ExprAssignOp(_, _, value) => (value, "assigned value", false),
@@ -1192,7 +1195,7 @@ impl LintPass for UnusedMut {
 
     fn check_expr(&mut self, cx: &Context, e: &ast::Expr) {
         match e.node {
-            ast::ExprMatch(_, ref arms) => {
+            ast::ExprMatch(_, ref arms, _) => {
                 for a in arms.iter() {
                     self.check_unused_mut_pat(cx, a.pats.as_slice())
                 }

src/librustc/middle/cfg/construct.rs

@@ -327,7 +327,7 @@ impl<'a> CFGBuilder<'a> {
                 expr_exit
             }
 
-            ast::ExprMatch(ref discr, ref arms) => {
+            ast::ExprMatch(ref discr, ref arms, _) => {
                 //
                 //     [pred]
                 //       |

src/librustc/middle/check_match.rs

@@ -139,7 +139,7 @@ pub fn check_crate(tcx: &ty::ctxt, krate: &Crate) {
 fn check_expr(cx: &mut MatchCheckCtxt, ex: &Expr) {
     visit::walk_expr(cx, ex, ());
     match ex.node {
-        ExprMatch(scrut, ref arms) => {
+        ExprMatch(scrut, ref arms, source) => {
             // First, check legality of move bindings.
             for arm in arms.iter() {
                 check_legality_of_move_bindings(cx,
@@ -178,7 +178,7 @@ fn check_expr(cx: &mut MatchCheckCtxt, ex: &Expr) {
             check_for_static_nan(cx, inlined_arms.as_slice());
 
             // Fourth, check for unreachable arms.
-            check_arms(cx, inlined_arms.as_slice());
+            check_arms(cx, inlined_arms.as_slice(), source);
 
             // Finally, check if the whole match expression is exhaustive.
             // Check for empty enum, because is_useful only works on inhabited types.
@@ -251,13 +251,23 @@ fn check_for_static_nan(cx: &MatchCheckCtxt, arms: &[Arm]) {
 }
 
 // Check for unreachable patterns
-fn check_arms(cx: &MatchCheckCtxt, arms: &[Arm]) {
+fn check_arms(cx: &MatchCheckCtxt, arms: &[Arm], source: MatchSource) {
     let mut seen = Matrix(vec!());
     for arm in arms.iter() {
         for &pat in arm.pats.iter() {
             let v = vec![pat];
             match is_useful(cx, &seen, v.as_slice(), LeaveOutWitness) {
-                NotUseful => span_err!(cx.tcx.sess, pat.span, E0001, "unreachable pattern"),
+                NotUseful => {
+                    if source == MatchIfLetDesugar {
+                        // find the first arm pattern so we can use its span
+                        let first_arm = &arms[0]; // we know there's at least 1 arm
+                        let first_pat = first_arm.pats.get(0); // and it's safe to assume 1 pat
+                        let span = first_pat.span;
+                        span_err!(cx.tcx.sess, span, E0159, "irrefutable if-let pattern")
+                    } else {
+                        span_err!(cx.tcx.sess, pat.span, E0001, "unreachable pattern")
+                    }
+                }
                 Useful => (),
                 UsefulWithWitness(_) => unreachable!()
             }

src/librustc/middle/expr_use_visitor.rs

@@ -358,7 +358,7 @@ impl<'d,'t,TYPER:mc::Typer> ExprUseVisitor<'d,'t,TYPER> {
 
             ast::ExprIfLet(..) => fail!("non-desugared ExprIfLet"),
 
-            ast::ExprMatch(ref discr, ref arms) => {
+            ast::ExprMatch(ref discr, ref arms, _) => {
                 // treatment of the discriminant is handled while
                 // walking the arms:
                 self.walk_expr(&**discr);

src/librustc/middle/liveness.rs

@@ -1038,7 +1038,7 @@ impl<'a> Liveness<'a> {
             self.propagate_through_loop(expr, LoopLoop, &**blk, succ)
           }
 
-          ExprMatch(ref e, ref arms) => {
+          ExprMatch(ref e, ref arms, _) => {
             //
             //      (e)
             //       |

src/librustc/middle/trans/debuginfo.rs

@@ -3686,7 +3686,7 @@ fn populate_scope_map(cx: &CrateContext,
                 }
             }
 
-            ast::ExprMatch(ref discriminant_exp, ref arms) => {
+            ast::ExprMatch(ref discriminant_exp, ref arms, _) => {
                 walk_expr(cx, &**discriminant_exp, scope_stack, scope_map);
 
                 // For each arm we have to first walk the pattern as these might

src/librustc/middle/trans/expr.rs

@@ -739,7 +739,7 @@ fn trans_rvalue_dps_unadjusted<'a>(bcx: &'a Block<'a>,
         ast::ExprIf(ref cond, ref thn, els) => {
             controlflow::trans_if(bcx, expr.id, &**cond, thn.clone(), els, dest)
         }
-        ast::ExprMatch(ref discr, ref arms) => {
+        ast::ExprMatch(ref discr, ref arms, _) => {
             _match::trans_match(bcx, expr, &**discr, arms.as_slice(), dest)
         }
         ast::ExprBlock(ref blk) => {

src/librustc/middle/typeck/check/mod.rs

@@ -3469,7 +3469,7 @@ fn check_expr_with_unifier(fcx: &FnCtxt,
             fcx.write_nil(id);
         }
       }
-      ast::ExprMatch(ref discrim, ref arms) => {
+      ast::ExprMatch(ref discrim, ref arms, _) => {
         _match::check_match(fcx, expr, &**discrim, arms.as_slice());
       }
       ast::ExprFnBlock(_, ref decl, ref body) => {

src/librustc/middle/typeck/check/regionck.rs

@@ -591,7 +591,7 @@ fn visit_expr(rcx: &mut Rcx, expr: &ast::Expr) {
             visit::walk_expr(rcx, expr, ());
         }
 
-        ast::ExprMatch(ref discr, ref arms) => {
+        ast::ExprMatch(ref discr, ref arms, _) => {
             link_match(rcx, &**discr, arms.as_slice());
 
             visit::walk_expr(rcx, expr, ());

src/librustc/util/ppaux.rs

@@ -80,6 +80,7 @@ pub fn explain_region_and_span(cx: &ctxt, region: ty::Region)
               ast::ExprMethodCall(..) => {
                 explain_span(cx, "method call", expr.span)
               },
+              ast::ExprMatch(_, _, ast::MatchIfLetDesugar) => explain_span(cx, "if let", expr.span),
               ast::ExprMatch(..) => explain_span(cx, "match", expr.span),
               _ => explain_span(cx, "expression", expr.span)
             }

src/libsyntax/ast.rs

@@ -541,7 +541,7 @@ pub enum Expr_ {
     // Conditionless loop (can be exited with break, cont, or ret)
     // FIXME #6993: change to Option<Name> ... or not, if these are hygienic.
     ExprLoop(P<Block>, Option<Ident>),
-    ExprMatch(Gc<Expr>, Vec<Arm>),
+    ExprMatch(Gc<Expr>, Vec<Arm>, MatchSource),
     ExprFnBlock(CaptureClause, P<FnDecl>, P<Block>),
     ExprProc(P<FnDecl>, P<Block>),
     ExprUnboxedFn(CaptureClause, UnboxedClosureKind, P<FnDecl>, P<Block>),
@@ -575,6 +575,12 @@ pub enum Expr_ {
     ExprParen(Gc<Expr>)
 }
 
+#[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
+pub enum MatchSource {
+    MatchNormal,
+    MatchIfLetDesugar
+}
+
 #[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
 pub enum CaptureClause {
     CaptureByValue,

src/libsyntax/ext/build.rs

@@ -860,7 +860,7 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
 
     fn expr_match(&self, span: Span, arg: Gc<ast::Expr>,
                   arms: Vec<ast::Arm>) -> Gc<Expr> {
-        self.expr(span, ast::ExprMatch(arg, arms))
+        self.expr(span, ast::ExprMatch(arg, arms, ast::MatchNormal))
     }
 
     fn expr_if(&self, span: Span,

src/libsyntax/ext/expand.rs

@@ -122,7 +122,7 @@ fn expand_expr(e: Gc<ast::Expr>, fld: &mut MacroExpander) -> Gc<ast::Expr> {
             arms.push_all_move(else_if_arms);
             arms.push(else_arm);
 
-            let match_expr = fld.cx.expr_match(span, expr, arms);
+            let match_expr = fld.cx.expr(span, ast::ExprMatch(expr, arms, ast::MatchIfLetDesugar));
             fld.fold_expr(match_expr)
         }
 

src/libsyntax/fold.rs

@@ -1148,9 +1148,10 @@ pub fn noop_fold_expr<T: Folder>(e: Gc<Expr>, folder: &mut T) -> Gc<Expr> {
             ExprLoop(folder.fold_block(body),
                      opt_ident.map(|x| folder.fold_ident(x)))
         }
-        ExprMatch(expr, ref arms) => {
+        ExprMatch(expr, ref arms, source) => {
             ExprMatch(folder.fold_expr(expr),
-                      arms.iter().map(|x| folder.fold_arm(x)).collect())
+                      arms.iter().map(|x| folder.fold_arm(x)).collect(),
+                      source)
         }
         ExprFnBlock(capture_clause, ref decl, ref body) => {
             ExprFnBlock(capture_clause,

src/libsyntax/parse/parser.rs

@@ -38,7 +38,7 @@ use ast::{ItemEnum, ItemFn, ItemForeignMod, ItemImpl};
 use ast::{ItemMac, ItemMod, ItemStruct, ItemTrait, ItemTy, Lit, Lit_};
 use ast::{LitBool, LitChar, LitByte, LitBinary};
 use ast::{LitNil, LitStr, LitInt, Local, LocalLet};
-use ast::{MutImmutable, MutMutable, Mac_, MacInvocTT, Matcher, MatchNonterminal};
+use ast::{MutImmutable, MutMutable, Mac_, MacInvocTT, Matcher, MatchNonterminal, MatchNormal};
 use ast::{MatchSeq, MatchTok, Method, MutTy, BiMul, Mutability};
 use ast::{MethodImplItem};
 use ast::{NamedField, UnNeg, NoReturn, UnNot, P, Pat, PatEnum};
@@ -2825,7 +2825,7 @@ impl<'a> Parser<'a> {
         }
         let hi = self.span.hi;
         self.bump();
-        return self.mk_expr(lo, hi, ExprMatch(discriminant, arms));
+        return self.mk_expr(lo, hi, ExprMatch(discriminant, arms, MatchNormal));
     }
 
     pub fn parse_arm(&mut self) -> Arm {

src/libsyntax/print/pprust.rs

@@ -1519,7 +1519,7 @@ impl<'a> State<'a> {
                 try!(space(&mut self.s));
                 try!(self.print_block(&**blk));
             }
-            ast::ExprMatch(ref expr, ref arms) => {
+            ast::ExprMatch(ref expr, ref arms, _) => {
                 try!(self.cbox(indent_unit));
                 try!(self.ibox(4));
                 try!(self.word_nbsp("match"));

src/libsyntax/visit.rs

@@ -800,7 +800,7 @@ pub fn walk_expr<E: Clone, V: Visitor<E>>(visitor: &mut V, expression: &Expr, en
             visitor.visit_block(&**block, env.clone())
         }
         ExprLoop(ref block, _) => visitor.visit_block(&**block, env.clone()),
-        ExprMatch(ref subexpression, ref arms) => {
+        ExprMatch(ref subexpression, ref arms, _) => {
             visitor.visit_expr(&**subexpression, env.clone());
             for arm in arms.iter() {
                 visitor.visit_arm(arm, env.clone())

src/test/compile-fail/lint-unnecessary-parens.rs

@@ -32,6 +32,7 @@ fn main() {
     match (true) { //~ ERROR unnecessary parentheses around `match` head expression
         _ => {}
     }
+    if let 1i = (1i) {} //~ ERROR unnecessary parentheses around `if let` head expression
     let v = X { y: false };
     // struct lits needs parens, so these shouldn't warn.
     if (v == X { y: true }) {}