Auto merge of #66129 - Nadrieril:refactor-slice-pat-usefulness, r=varkor

Refactor slice pattern usefulness checking

As a follow up to #65874, this PR changes how variable-length slice patterns are handled in usefulness checking. The objectives are: cleaning up that code to make it easier to understand, and paving the way to handling fixed-length slices more cleverly too, for #53820.

Before this, variable-length slice patterns were eagerly expanded into a union of fixed-length slices. Now they have their own special constructor, which allows expanding them a bit more lazily.
As a nice side-effect, this improves diagnostics.

This PR shows a slight performance improvement, mostly due to 149792b. This will probably have to be reverted in some way when we implement or-patterns.

Author: bors

src/librustc_mir/hair/pattern/_match.rs

@@ -18,7 +18,7 @@ use rustc::hir::{self, Pat};
 
 use std::slice;
 
-use syntax_pos::{MultiSpan, Span, DUMMY_SP};
+use syntax_pos::{MultiSpan, Span};
 
 crate fn check_match(tcx: TyCtxt<'_>, def_id: DefId) {
     let body_id = match tcx.hir().as_local_hir_id(def_id) {
@@ -491,7 +491,7 @@ fn check_not_useful(
     matrix: &Matrix<'_, 'tcx>,
     hir_id: HirId,
 ) -> Result<(), Vec<super::Pat<'tcx>>> {
-    let wild_pattern = super::Pat { ty, span: DUMMY_SP, kind: box PatKind::Wild };
+    let wild_pattern = super::Pat::wildcard_from_ty(ty);
     match is_useful(cx, matrix, &PatStack::from_pattern(&wild_pattern), ConstructWitness, hir_id) {
         NotUseful => Ok(()), // This is good, wildcard pattern isn't reachable.
         UsefulWithWitness(pats) => Err(if pats.is_empty() {

src/librustc_mir/hair/pattern/check_match.rs

@@ -26,7 +26,7 @@ use rustc_index::vec::Idx;
 use std::cmp::Ordering;
 use std::fmt;
 use syntax::ast;
-use syntax_pos::Span;
+use syntax_pos::{Span, DUMMY_SP};
 
 #[derive(Clone, Debug)]
 pub enum PatternError {
@@ -55,6 +55,11 @@ pub struct Pat<'tcx> {
     pub kind: Box<PatKind<'tcx>>,
 }
 
+impl<'tcx> Pat<'tcx> {
+    pub(crate) fn wildcard_from_ty(ty: Ty<'tcx>) -> Self {
+        Pat { ty, span: DUMMY_SP, kind: Box::new(PatKind::Wild) }
+    }
+}
 
 #[derive(Copy, Clone, Debug, PartialEq)]
 pub struct PatTyProj<'tcx> {

src/librustc_mir/hair/pattern/mod.rs

@@ -1,8 +1,8 @@
-error[E0005]: refutable pattern in function argument: `&[]`, `&[_]` and `&[_, _, _]` not covered
+error[E0005]: refutable pattern in function argument: `&[]`, `&[_]` and `&[_, _, _, ..]` not covered
   --> $DIR/const_let_refutable.rs:3:16
    |
 LL | const fn slice([a, b]: &[i32]) -> i32 {
-   |                ^^^^^^ patterns `&[]`, `&[_]` and `&[_, _, _]` not covered
+   |                ^^^^^^ patterns `&[]`, `&[_]` and `&[_, _, _, ..]` not covered
 
 error[E0723]: can only call other `const fn` within a `const fn`, but `const <&i32 as std::ops::Add>::add` is not stable as `const fn`
   --> $DIR/const_let_refutable.rs:4:5

src/test/ui/consts/const_let_refutable.stderr

@@ -6,11 +6,11 @@ LL |     match buf {
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
-error[E0004]: non-exhaustive patterns: `&[]`, `&[_]`, `&[_, _]` and 3 more not covered
+error[E0004]: non-exhaustive patterns: `&[..]` not covered
   --> $DIR/match-byte-array-patterns-2.rs:10:11
    |
 LL |     match buf {
-   |           ^^^ patterns `&[]`, `&[_]`, `&[_, _]` and 3 more not covered
+   |           ^^^ pattern `&[..]` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 

src/test/ui/pattern/usefulness/match-byte-array-patterns-2.stderr

@@ -2,7 +2,7 @@
 
 fn check(list: &[Option<()>]) {
     match list {
-    //~^ ERROR `&[_, Some(_), None, _]` not covered
+    //~^ ERROR `&[_, Some(_), .., None, _]` not covered
         &[] => {},
         &[_] => {},
         &[_, _] => {},

src/test/ui/pattern/usefulness/match-slice-patterns.rs

@@ -1,8 +1,8 @@
-error[E0004]: non-exhaustive patterns: `&[_, Some(_), None, _]` not covered
+error[E0004]: non-exhaustive patterns: `&[_, Some(_), .., None, _]` not covered
   --> $DIR/match-slice-patterns.rs:4:11
    |
 LL |     match list {
-   |           ^^^^ pattern `&[_, Some(_), None, _]` not covered
+   |           ^^^^ pattern `&[_, Some(_), .., None, _]` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 

src/test/ui/pattern/usefulness/match-slice-patterns.stderr

@@ -44,7 +44,7 @@ fn main() {
     }
     let vec = vec![0.5f32];
     let vec: &[f32] = &vec;
-    match *vec { //~ ERROR non-exhaustive patterns: `[_, _, _, _]` not covered
+    match *vec { //~ ERROR non-exhaustive patterns: `[_, _, _, _, ..]` not covered
         [0.1, 0.2, 0.3] => (),
         [0.1, 0.2] => (),
         [0.1] => (),

src/test/ui/pattern/usefulness/non-exhaustive-match.rs

@@ -66,11 +66,11 @@ LL |     match *vec {
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
-error[E0004]: non-exhaustive patterns: `[_, _, _, _]` not covered
+error[E0004]: non-exhaustive patterns: `[_, _, _, _, ..]` not covered
   --> $DIR/non-exhaustive-match.rs:47:11
    |
 LL |     match *vec {
-   |           ^^^^ pattern `[_, _, _, _]` not covered
+   |           ^^^^ pattern `[_, _, _, _, ..]` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms