Explain exhaustive matching on {usize,isize} maximum values

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;
@@ -487,9 +488,27 @@ 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)
+        && !is_empty_match
+        && witnesses.len() == 1
+        && witnesses[0].is_wildcard()
+    {
+        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

@@ -6,6 +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: `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
@@ -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: `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

@@ -0,0 +1,23 @@
+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 `usize` does not have a fixed maximum value
+        0 ..= usize::MAX => {}
+    }
+
+    match 0isize {
+        //~^ ERROR non-exhaustive patterns
+        //~| NOTE pattern `_` not covered
+        //~| NOTE the matched value is of type `isize`
+        //~| 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

@@ -0,0 +1,34 @@
+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: `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: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: `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[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`.