add guard patterns to HIR and implement lowering

compiler/rustc_ast_lowering/src/pat.rs

@@ -120,8 +120,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             self.lower_range_end(end, e2.is_some()),
                         );
                     }
-                    // FIXME(guard_patterns): lower pattern guards to HIR
+                    PatKind::Guard(inner, cond) => {
-                    PatKind::Guard(inner, _) => pattern = inner,
+                        break hir::PatKind::Guard(self.lower_pat(inner), self.lower_expr(cond));
+                    }
                     PatKind::Slice(pats) => break self.lower_pat_slice(pats),
                     PatKind::Rest => {
                         // If we reach here the `..` pattern is not semantically allowed.

compiler/rustc_hir/src/hir.rs

@@ -1385,7 +1385,7 @@ impl<'hir> Pat<'hir> {
         use PatKind::*;
         match self.kind {
             Wild | Never | Lit(_) | Range(..) | Binding(.., None) | Path(_) | Err(_) => true,
-            Box(s) | Deref(s) | Ref(s, _) | Binding(.., Some(s)) => s.walk_short_(it),
+            Box(s) | Deref(s) | Ref(s, _) | Binding(.., Some(s)) | Guard(s, _) => s.walk_short_(it),
             Struct(_, fields, _) => fields.iter().all(|field| field.pat.walk_short_(it)),
             TupleStruct(_, s, _) | Tuple(s, _) | Or(s) => s.iter().all(|p| p.walk_short_(it)),
             Slice(before, slice, after) => {
@@ -1412,7 +1412,7 @@ impl<'hir> Pat<'hir> {
         use PatKind::*;
         match self.kind {
             Wild | Never | Lit(_) | Range(..) | Binding(.., None) | Path(_) | Err(_) => {}
-            Box(s) | Deref(s) | Ref(s, _) | Binding(.., Some(s)) => s.walk_(it),
+            Box(s) | Deref(s) | Ref(s, _) | Binding(.., Some(s)) | Guard(s, _) => s.walk_(it),
             Struct(_, fields, _) => fields.iter().for_each(|field| field.pat.walk_(it)),
             TupleStruct(_, s, _) | Tuple(s, _) | Or(s) => s.iter().for_each(|p| p.walk_(it)),
             Slice(before, slice, after) => {
@@ -1564,6 +1564,9 @@ pub enum PatKind<'hir> {
     /// A literal.
     Lit(&'hir Expr<'hir>),
 
+    /// A guard pattern (e.g., `x if guard(x)`).
+    Guard(&'hir Pat<'hir>, &'hir Expr<'hir>),
+
     /// A range pattern (e.g., `1..=2` or `1..2`).
     Range(Option<&'hir Expr<'hir>>, Option<&'hir Expr<'hir>>, RangeEnd),
 

compiler/rustc_hir/src/intravisit.rs

@@ -696,6 +696,10 @@ pub fn walk_pat<'v, V: Visitor<'v>>(visitor: &mut V, pattern: &'v Pat<'v>) -> V:
             visit_opt!(visitor, visit_pat, slice_pattern);
             walk_list!(visitor, visit_pat, postpatterns);
         }
+        PatKind::Guard(subpat, condition) => {
+            try_visit!(visitor.visit_pat(subpat));
+            try_visit!(visitor.visit_expr(condition));
+        }
     }
     V::Result::output()
 }

compiler/rustc_hir_analysis/src/check/region.rs

@@ -654,6 +654,7 @@ fn resolve_local<'tcx>(
     ///        | ( ..., P&, ... )
     ///        | ... "|" P& "|" ...
     ///        | box P&
+    ///        | P& if ...
     /// ```
     fn is_binding_pat(pat: &hir::Pat<'_>) -> bool {
         // Note that the code below looks for *explicit* refs only, that is, it won't
@@ -694,7 +695,9 @@ fn resolve_local<'tcx>(
             | PatKind::TupleStruct(_, subpats, _)
             | PatKind::Tuple(subpats, _) => subpats.iter().any(|p| is_binding_pat(p)),
 
-            PatKind::Box(subpat) | PatKind::Deref(subpat) => is_binding_pat(subpat),
+            PatKind::Box(subpat) | PatKind::Deref(subpat) | PatKind::Guard(subpat, _) => {
+                is_binding_pat(subpat)
+            }
 
             PatKind::Ref(_, _)
             | PatKind::Binding(hir::BindingMode(hir::ByRef::No, _), ..)

compiler/rustc_hir_pretty/src/lib.rs

@@ -1999,6 +1999,12 @@ impl<'a> State<'a> {
                 self.commasep(Inconsistent, after, |s, p| s.print_pat(p));
                 self.word("]");
             }
+            PatKind::Guard(inner, cond) => {
+                self.print_pat(inner);
+                self.space();
+                self.word_space("if");
+                self.print_expr(cond);
+            }
             PatKind::Err(_) => {
                 self.popen();
                 self.word("/*ERROR*/");

compiler/rustc_hir_typeck/src/expr.rs

@@ -457,6 +457,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // Does not constitute a read.
             hir::PatKind::Wild => false,
 
+            // Might not constitute a read, since the condition might be false.
+            hir::PatKind::Guard(_, _) => true,
+
             // This is unnecessarily restrictive when the pattern that doesn't
             // constitute a read is unreachable.
             //

compiler/rustc_hir_typeck/src/expr_use_visitor.rs

@@ -615,6 +615,7 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
                     | PatKind::Box(_)
                     | PatKind::Deref(_)
                     | PatKind::Ref(..)
+                    | PatKind::Guard(..)
                     | PatKind::Wild
                     | PatKind::Err(_) => {
                         // If the PatKind is Or, Box, or Ref, the decision is made later
@@ -1737,7 +1738,7 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
                 }
             }
 
-            PatKind::Binding(.., Some(subpat)) => {
+            PatKind::Binding(.., Some(subpat)) | PatKind::Guard(subpat, _) => {
                 self.cat_pattern(place_with_id, subpat, op)?;
             }
 

compiler/rustc_hir_typeck/src/pat.rs

@@ -284,6 +284,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             PatKind::Struct(ref qpath, fields, has_rest_pat) => {
                 self.check_pat_struct(pat, qpath, fields, has_rest_pat, expected, pat_info)
             }
+            PatKind::Guard(pat, _) => {
+                self.check_pat(pat, expected, pat_info);
+                expected
+            }
             PatKind::Or(pats) => {
                 for pat in pats {
                     self.check_pat(pat, expected, pat_info);
@@ -422,7 +426,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // An OR-pattern just propagates to each individual alternative.
             // This is maximally flexible, allowing e.g., `Some(mut x) | &Some(mut x)`.
             // In that example, `Some(mut x)` results in `Peel` whereas `&Some(mut x)` in `Reset`.
-            | PatKind::Or(_) => AdjustMode::Pass,
+            | PatKind::Or(_)
+            // Like or-patterns, guard patterns just propogate to their subpatterns.
+            | PatKind::Guard(..) => AdjustMode::Pass,
         }
     }
 
@@ -901,6 +907,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         PatKind::Struct(..)
                         | PatKind::TupleStruct(..)
                         | PatKind::Or(..)
+                        | PatKind::Guard(..)
                         | PatKind::Tuple(..)
                         | PatKind::Slice(..) => "binding",
 

compiler/rustc_mir_build/src/thir/pattern/mod.rs

@@ -435,6 +435,9 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
 
             hir::PatKind::Or(pats) => PatKind::Or { pats: self.lower_patterns(pats) },
 
+            // FIXME(guard_patterns): implement guard pattern lowering
+            hir::PatKind::Guard(pat, _) => self.lower_pattern(pat).kind,
+
             hir::PatKind::Err(guar) => PatKind::Error(guar),
         };
 

compiler/rustc_passes/src/input_stats.rs

@@ -305,6 +305,7 @@ impl<'v> hir_visit::Visitor<'v> for StatCollector<'v> {
             Deref,
             Ref,
             Lit,
+            Guard,
             Range,
             Slice,
             Err

src/librustdoc/clean/utils.rs

@@ -320,6 +320,7 @@ pub(crate) fn name_from_pat(p: &hir::Pat<'_>) -> Symbol {
             );
             return Symbol::intern("()");
         }
+        PatKind::Guard(p, _) => return name_from_pat(&*p),
         PatKind::Range(..) => return kw::Underscore,
         PatKind::Slice(begin, ref mid, end) => {
             let begin = begin.iter().map(|p| name_from_pat(p).to_string());