Address code reviews

src/librustc_mir_build/hair/pattern/check_match.rs

@@ -12,6 +12,7 @@ use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
 use rustc_hir::{HirId, Pat};
 use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_session::config::nightly_options;
 use rustc_session::lint::builtin::BINDINGS_WITH_VARIANT_NAME;
 use rustc_session::lint::builtin::{IRREFUTABLE_LET_PATTERNS, UNREACHABLE_PATTERNS};
 use rustc_session::parse::feature_err;
@@ -468,7 +469,6 @@ fn check_exhaustive<'p, 'tcx>(
     // In the case of an empty match, replace the '`_` not covered' diagnostic with something more
     // informative.
     let mut err;
-    let joined_patterns = joined_uncovered_patterns(&witnesses);
     if is_empty_match && !non_empty_enum {
         err = create_e0004(
             cx.tcx.sess,
@@ -476,6 +476,7 @@ fn check_exhaustive<'p, 'tcx>(
             format!("non-exhaustive patterns: type `{}` is non-empty", scrut_ty),
         );
     } else {
+        let joined_patterns = joined_uncovered_patterns(&witnesses);
         err = create_e0004(
             cx.tcx.sess,
             sp,
@@ -487,16 +488,26 @@ fn check_exhaustive<'p, 'tcx>(
     adt_defined_here(cx, &mut err, scrut_ty, &witnesses);
     err.help(
         "ensure that all possible cases are being handled, \
-         possibly by adding wildcards or more match arms",
+              possibly by adding wildcards or more match arms",
     );
     err.note(&format!("the matched value is of type `{}`", scrut_ty));
     if (scrut_ty == cx.tcx.types.usize || scrut_ty == cx.tcx.types.isize)
-        && joined_patterns == "`_`"
+        && !is_empty_match
+        && witnesses.len() == 1
+        && witnesses[0].is_wildcard()
     {
-        err.note("for `usize` and `isize`, no assumptions about the maximum value are permitted");
-        err.note(
-            "to exhaustively match on either pointer-size integer type, wildcards must be used",
-        );
+        err.note(&format!(
+            "`{}` does not have a fixed maximum value, \
+                so a wildcard `_` is necessary to match exhaustively",
+            scrut_ty,
+        ));
+        if nightly_options::is_nightly_build() {
+            err.help(&format!(
+                "add `#![feature(precise_pointer_size_matching)]` \
+                    to the crate attributes to enable precise `{}` matching",
+                scrut_ty,
+            ));
+        }
     }
     err.emit();
 }

src/test/ui/feature-gates/feature-gate-precise_pointer_size_matching.stderr

@@ -6,8 +6,8 @@ LL |     match 0usize {
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `usize`
-   = note: for `usize` and `isize`, no assumptions about the maximum value are permitted
-   = note: to exhaustively match on either pointer-size integer type, wildcards must be used
+   = note: `usize` does not have a fixed maximum value, so a wildcard `_` is necessary to match exhaustively
+   = help: add `#![feature(precise_pointer_size_matching)]` to the crate attributes to enable precise `usize` matching
 
 error[E0004]: non-exhaustive patterns: `_` not covered
   --> $DIR/feature-gate-precise_pointer_size_matching.rs:10:11
@@ -17,8 +17,8 @@ LL |     match 0isize {
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `isize`
-   = note: for `usize` and `isize`, no assumptions about the maximum value are permitted
-   = note: to exhaustively match on either pointer-size integer type, wildcards must be used
+   = note: `isize` does not have a fixed maximum value, so a wildcard `_` is necessary to match exhaustively
+   = help: add `#![feature(precise_pointer_size_matching)]` to the crate attributes to enable precise `isize` matching
 
 error: aborting due to 2 previous errors
 

src/test/ui/pattern/usefulness/non-exhaustive-pattern-pointer-size-int.rs

@@ -5,8 +5,7 @@ fn main() {
         //~^ ERROR non-exhaustive patterns
         //~| NOTE pattern `_` not covered
         //~| NOTE the matched value is of type `usize`
-        //~| NOTE for `usize` and `isize`, no assumptions about the maximum value are permitted
-        //~| NOTE to exhaustively match on either pointer-size integer type, wildcards must be used
+        //~| NOTE `usize` does not have a fixed maximum value
         0 ..= usize::MAX => {}
     }
 
@@ -14,8 +13,11 @@ fn main() {
         //~^ ERROR non-exhaustive patterns
         //~| NOTE pattern `_` not covered
         //~| NOTE the matched value is of type `isize`
-        //~| NOTE for `usize` and `isize`, no assumptions about the maximum value are permitted
-        //~| NOTE to exhaustively match on either pointer-size integer type, wildcards must be used
+        //~| NOTE `isize` does not have a fixed maximum value
         isize::MIN ..= isize::MAX => {}
     }
+
+    match 7usize {}
+    //~^ ERROR non-exhaustive patterns
+    //~| NOTE the matched value is of type `usize`
 }

src/test/ui/pattern/usefulness/non-exhaustive-pattern-pointer-size-int.stderr

@@ -6,20 +6,29 @@ LL |     match 0usize {
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `usize`
-   = note: for `usize` and `isize`, no assumptions about the maximum value are permitted
-   = note: to exhaustively match on either pointer-size integer type, wildcards must be used
+   = note: `usize` does not have a fixed maximum value, so a wildcard `_` is necessary to match exhaustively
+   = help: add `#![feature(precise_pointer_size_matching)]` to the crate attributes to enable precise `usize` matching
 
 error[E0004]: non-exhaustive patterns: `_` not covered
-  --> $DIR/non-exhaustive-pattern-pointer-size-int.rs:13:11
+  --> $DIR/non-exhaustive-pattern-pointer-size-int.rs:12:11
    |
 LL |     match 0isize {
    |           ^^^^^^ pattern `_` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `isize`
-   = note: for `usize` and `isize`, no assumptions about the maximum value are permitted
-   = note: to exhaustively match on either pointer-size integer type, wildcards must be used
+   = note: `isize` does not have a fixed maximum value, so a wildcard `_` is necessary to match exhaustively
+   = help: add `#![feature(precise_pointer_size_matching)]` to the crate attributes to enable precise `isize` matching
 
-error: aborting due to 2 previous errors
+error[E0004]: non-exhaustive patterns: type `usize` is non-empty
+  --> $DIR/non-exhaustive-pattern-pointer-size-int.rs:20:11
+   |
+LL |     match 7usize {}
+   |           ^^^^^^
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+   = note: the matched value is of type `usize`
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0004`.