Explain exhaustive matching on {usize,isize} maximum values

src/librustc_mir_build/hair/pattern/check_match.rs

@@ -468,6 +468,7 @@ 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,
@@ -475,7 +476,6 @@ 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,
@@ -490,6 +490,14 @@ fn check_exhaustive<'p, 'tcx>(
          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 == "`_`"
+    {
+        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.emit();
 }
 

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

@@ -6,6 +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
 
 error[E0004]: non-exhaustive patterns: `_` not covered
   --> $DIR/feature-gate-precise_pointer_size_matching.rs:10:11
@@ -15,6 +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
 
 error: aborting due to 2 previous errors
 

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

@@ -0,0 +1,21 @@
+use std::{usize, isize};
+
+fn main() {
+    match 0usize {
+        //~^ 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
+        0 ..= usize::MAX => {}
+    }
+
+    match 0isize {
+        //~^ 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
+        isize::MIN ..= isize::MAX => {}
+    }
+}

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

@@ -0,0 +1,25 @@
+error[E0004]: non-exhaustive patterns: `_` not covered
+  --> $DIR/non-exhaustive-pattern-pointer-size-int.rs:4:11
+   |
+LL |     match 0usize {
+   |           ^^^^^^ 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 `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
+
+error[E0004]: non-exhaustive patterns: `_` not covered
+  --> $DIR/non-exhaustive-pattern-pointer-size-int.rs:13:11
+   |
+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
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0004`.