Auto merge of #25743 - michaelsproul:match-diagnostics, r=nrc

Part of #24407.

Currently the diagnostics for range patterns are a bit wrong:

```rust
fn main() {
    match 5u32 {
        0 ... 10 => (),
        'a' ... 10 => (),
        10 ... 'z' => (),
        "what" ... 10 => (),
        "what" ... "well" => (),
        10 ... "what" => ()
    }
}
```

```
range.rs:4:9: 4:19 error: mismatched types in range:
 expected integral variable,
    found char [E0211]
range.rs:4         'a' ... 10 => (),
                   ^~~~~~~~~~
range.rs:4:9: 4:16 error: only char and numeric types are allowed in range [E0029]
range.rs:4         'a' ... 10 => (),
                   ^~~~~~~
range.rs:4:9: 4:19 error: mismatched types:
 expected `u32`,
    found `char`
(expected u32,
    found char) [E0308]
range.rs:4         'a' ... 10 => (),
                   ^~~~~~~~~~
range.rs:5:9: 5:19 error: mismatched types in range:
 expected char,
    found integral variable [E0211]
range.rs:5         10 ... 'z' => (),
                   ^~~~~~~~~~
range.rs:5:9: 5:15 error: only char and numeric types are allowed in range [E0029]
range.rs:5         10 ... 'z' => (),
                   ^~~~~~
range.rs:6:9: 6:22 error: mismatched types in range:
 expected integral variable,
    found &-ptr [E0211]
range.rs:6         "what" ... 10 => (),
                   ^~~~~~~~~~~~~
range.rs:6:9: 6:19 error: only char and numeric types are allowed in range [E0029]
range.rs:6         "what" ... 10 => (),
                   ^~~~~~~~~~
range.rs:6:9: 6:22 error: mismatched types:
 expected `u32`,
    found `&'static str`
(expected u32,
    found &-ptr) [E0308]
range.rs:6         "what" ... 10 => (),
                   ^~~~~~~~~~~~~
range.rs:7:9: 7:19 error: only char and numeric types are allowed in range [E0029]
range.rs:7         "what" ... "well" => (),
                   ^~~~~~~~~~
range.rs:7:9: 7:26 error: mismatched types:
 expected `u32`,
    found `&'static str`
(expected u32,
    found &-ptr) [E0308]
range.rs:7         "what" ... "well" => (),
                   ^~~~~~~~~~~~~~~~~
range.rs:8:9: 8:22 error: mismatched types in range:
 expected &-ptr,
    found integral variable [E0211]
range.rs:8         10 ... "what" => ()
                   ^~~~~~~~~~~~~
range.rs:8:9: 8:15 error: only char and numeric types are allowed in range [E0029]
range.rs:8         10 ... "what" => ()
                   ^~~~~~
error: aborting due to 12 previous errors
```

The problems here are:

1. The type of the end of the range is used to predict the type of the start (only mildly counter intuitive).
2. E0029 is erroneously generated for `char ... num` and `num ... char`.
2. `u32` is mentioned.
3. Errors which are essentially the same are reported multiple times.

I've attempted to fix this by checking the requirements in a different order. The output I've achieved for the above example is:

```
/home/michael/Temp/range.rs:4:17: 4:22 error: mismatched types in range:
 expected char,
    found integral variable [E0211]
/home/michael/Temp/range.rs:4         'a' ... 10 => (),
                                              ^~~~~
/home/michael/Temp/range.rs:5:16: 5:22 error: mismatched types in range:
 expected integral variable,
    found char [E0211]
/home/michael/Temp/range.rs:5         10 ... 'z' => (),
                                             ^~~~~~
/home/michael/Temp/range.rs:6:9: 6:19 error: only char and numeric types are allowed in range [E0029]
/home/michael/Temp/range.rs:6         "what" ... 10 => (),
                                      ^~~~~~~~~~
/home/michael/Temp/range.rs:6:9: 6:19 help: run `rustc --explain E0029` to see a detailed explanation
/home/michael/Temp/range.rs:6:9: 6:19 note: Start type: &'static str
End type: _
/home/michael/Temp/range.rs:6         "what" ... 10 => (),
                                      ^~~~~~~~~~
/home/michael/Temp/range.rs:7:9: 7:26 error: only char and numeric types are allowed in range [E0029]
/home/michael/Temp/range.rs:7         "what" ... "well" => (),
                                      ^~~~~~~~~~~~~~~~~
/home/michael/Temp/range.rs:7:9: 7:26 help: run `rustc --explain E0029` to see a detailed explanation
/home/michael/Temp/range.rs:7:9: 7:26 note: Start type: &'static str
End type: &'static str
/home/michael/Temp/range.rs:7         "what" ... "well" => (),
                                      ^~~~~~~~~~~~~~~~~
/home/michael/Temp/range.rs:8:16: 8:25 error: only char and numeric types are allowed in range [E0029]
/home/michael/Temp/range.rs:8         10 ... "what" => ()
                                             ^~~~~~~~~
/home/michael/Temp/range.rs:8:16: 8:25 help: run `rustc --explain E0029` to see a detailed explanation
/home/michael/Temp/range.rs:8:16: 8:25 note: Start type: _
End type: &'static str
/home/michael/Temp/range.rs:8         10 ... "what" => ()
                                             ^~~~~~~~~
error: aborting due to 5 previous errors
```

I think this is already tonnes better, but the `Start type/End type` stuff could be neater. I don't think there's really any need to start a `note:` block but I wanted to get some feedback on this. I'd also appreciate advice on how to print the integer types as something other than `_`.

Author: bors

src/compiletest/runtest.rs

@@ -24,7 +24,6 @@ use std::fmt;
 use std::fs::{self, File};
 use std::io::BufReader;
 use std::io::prelude::*;
-use std::iter::repeat;
 use std::net::TcpStream;
 use std::path::{Path, PathBuf};
 use std::process::{Command, Output, ExitStatus};
@@ -928,12 +927,12 @@ fn check_forbid_output(props: &TestProps,
     }
 }
 
-fn check_expected_errors(expected_errors: Vec<errors::ExpectedError> ,
+fn check_expected_errors(expected_errors: Vec<errors::ExpectedError>,
                          testfile: &Path,
                          proc_res: &ProcRes) {
 
     // true if we found the error in question
-    let mut found_flags: Vec<_> = repeat(false).take(expected_errors.len()).collect();
+    let mut found_flags = vec![false; expected_errors.len()];
 
     if proc_res.status.success() {
         fatal("process did not return an error status");
@@ -954,14 +953,10 @@ fn check_expected_errors(expected_errors: Vec<errors::ExpectedError> ,
         }
     }
 
-    // A multi-line error will have followup lines which will always
-    // start with one of these strings.
+    // A multi-line error will have followup lines which start with a space
+    // or open paren.
     fn continuation( line: &str) -> bool {
-        line.starts_with(" expected") ||
-        line.starts_with("    found") ||
-        //                1234
-        // Should have 4 spaces: see issue 18946
-        line.starts_with("(")
+        line.starts_with(" ") || line.starts_with("(")
     }
 
     // Scan and extract our error/warning messages,

src/librustc_typeck/check/_match.rs

@@ -83,41 +83,64 @@ pub fn check_pat<'a, 'tcx>(pcx: &pat_ctxt<'a, 'tcx>,
             demand::suptype(fcx, pat.span, expected, pat_ty);
         }
         ast::PatRange(ref begin, ref end) => {
-            check_expr(fcx, &**begin);
-            check_expr(fcx, &**end);
-
-            let lhs_ty = fcx.expr_ty(&**begin);
-            let rhs_ty = fcx.expr_ty(&**end);
-
-            let lhs_eq_rhs =
-                require_same_types(
-                    tcx, Some(fcx.infcx()), false, pat.span, lhs_ty, rhs_ty,
-                    || "mismatched types in range".to_string());
-
-            let numeric_or_char =
-                lhs_eq_rhs && (ty::type_is_numeric(lhs_ty) || ty::type_is_char(lhs_ty));
-
-            if numeric_or_char {
-                match const_eval::compare_lit_exprs(tcx, &**begin, &**end, Some(lhs_ty),
-                                                    |id| {fcx.item_substs()[&id].substs
-                                                             .clone()}) {
-                    Some(Ordering::Less) |
-                    Some(Ordering::Equal) => {}
-                    Some(Ordering::Greater) => {
-                        span_err!(tcx.sess, begin.span, E0030,
-                            "lower range bound must be less than upper");
-                    }
-                    None => {
-                        span_err!(tcx.sess, begin.span, E0031,
-                            "mismatched types in range");
-                    }
-                }
-            } else {
-                span_err!(tcx.sess, begin.span, E0029,
-                          "only char and numeric types are allowed in range");
+            check_expr(fcx, begin);
+            check_expr(fcx, end);
+
+            let lhs_ty = fcx.expr_ty(begin);
+            let rhs_ty = fcx.expr_ty(end);
+
+            // Check that both end-points are of numeric or char type.
+            let numeric_or_char = |t| ty::type_is_numeric(t) || ty::type_is_char(t);
+            let lhs_compat = numeric_or_char(lhs_ty);
+            let rhs_compat = numeric_or_char(rhs_ty);
+
+            if !lhs_compat || !rhs_compat {
+                let span = if !lhs_compat && !rhs_compat {
+                    pat.span
+                } else if !lhs_compat {
+                    begin.span
+                } else {
+                    end.span
+                };
+
+                // Note: spacing here is intentional, we want a space before "start" and "end".
+                span_err!(tcx.sess, span, E0029,
+                          "only char and numeric types are allowed in range patterns\n \
+                           start type: {}\n end type: {}",
+                          fcx.infcx().ty_to_string(lhs_ty),
+                          fcx.infcx().ty_to_string(rhs_ty)
+                );
+                return;
+            }
+
+            // Check that the types of the end-points can be unified.
+            let types_unify = require_same_types(
+                    tcx, Some(fcx.infcx()), false, pat.span, rhs_ty, lhs_ty,
+                    || "mismatched types in range".to_string()
+            );
+
+            // It's ok to return without a message as `require_same_types` prints an error.
+            if !types_unify {
+                return;
             }
 
-            fcx.write_ty(pat.id, lhs_ty);
+            // Now that we know the types can be unified we find the unified type and use
+            // it to type the entire expression.
+            let common_type = fcx.infcx().resolve_type_vars_if_possible(&lhs_ty);
+
+            fcx.write_ty(pat.id, common_type);
+
+            // Finally we evaluate the constants and check that the range is non-empty.
+            let get_substs = |id| fcx.item_substs()[&id].substs.clone();
+            match const_eval::compare_lit_exprs(tcx, begin, end, Some(&common_type), get_substs) {
+                Some(Ordering::Less) |
+                Some(Ordering::Equal) => {}
+                Some(Ordering::Greater) => {
+                    span_err!(tcx.sess, begin.span, E0030,
+                        "lower range bound must be less than or equal to upper");
+                }
+                None => tcx.sess.span_bug(begin.span, "literals of different types in range pat")
+            }
 
             // subtyping doesn't matter here, as the value is some kind of scalar
             demand::eqtype(fcx, pat.span, expected, lhs_ty);

src/librustc_typeck/diagnostics.rs

@@ -146,6 +146,47 @@ match d {
 ```
 "##,
 
+E0029: r##"
+In a match expression, only numbers and characters can be matched against a
+range. This is because the compiler checks that the range is non-empty at
+compile-time, and is unable to evaluate arbitrary comparison functions. If you
+want to capture values of an orderable type between two end-points, you can use
+a guard.
+
+```
+// The ordering relation for strings can't be evaluated at compile time,
+// so this doesn't work:
+match string {
+    "hello" ... "world" => ...
+    _ => ...
+}
+
+// This is a more general version, using a guard:
+match string {
+    s if s >= "hello" && s <= "world" => ...
+    _ => ...
+}
+```
+"##,
+
+E0030: r##"
+When matching against a range, the compiler verifies that the range is
+non-empty.  Range patterns include both end-points, so this is equivalent to
+requiring the start of the range to be less than or equal to the end of the
+range.
+
+For example:
+
+```
+match 5u32 {
+    // This range is ok, albeit pointless.
+    1 ... 1 => ...
+    // This range is empty, and the compiler can tell.
+    1000 ... 5 => ...
+}
+```
+"##,
+
 E0033: r##"
 This error indicates that a pointer to a trait type cannot be implicitly
 dereferenced by a pattern. Every trait defines a type, but because the
@@ -1154,9 +1195,6 @@ For more information see the [opt-in builtin traits RFC](https://github.com/rust
 }
 
 register_diagnostics! {
-    E0029,
-    E0030,
-    E0031,
     E0034, // multiple applicable methods in scope
     E0035, // does not take type parameters
     E0036, // incorrect number of type parameters given for this method

src/test/compile-fail/match-ill-type1.rs

@@ -8,22 +8,32 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-//error-pattern: lower range bound
-//error-pattern: only char and numeric types
-//error-pattern: mismatched types
-
 fn main() {
     match 5 {
-      6 ... 1 => { }
-      _ => { }
+        6 ... 1 => { }
+        _ => { }
+    };
+    //~^^^ ERROR lower range bound must be less than or equal to upper
+
+    match "wow" {
+        "bar" ... "foo" => { }
     };
+    //~^^ ERROR only char and numeric types are allowed in range
+    //~| start type: &'static str
+    //~| end type: &'static str
 
     match "wow" {
-      "bar" ... "foo" => { }
+        10 ... "what" => ()
     };
+    //~^^ ERROR only char and numeric types are allowed in range
+    //~| start type: _
+    //~| end type: &'static str
 
     match 5 {
-      'c' ... 100 => { }
-      _ => { }
+        'c' ... 100 => { }
+        _ => { }
     };
+    //~^^^ ERROR mismatched types in range
+    //~| expected char
+    //~| found integral variable
 }

src/test/compile-fail/match-range-fail.rs

@@ -0,0 +1,26 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that type inference for range patterns works correctly (is bi-directional).
+
+pub fn main() {
+    match 1 {
+        1 ... 3 => {}
+        _ => panic!("should match range")
+    }
+    match 1 {
+        1 ... 3u16 => {}
+        _ => panic!("should match range with inferred start type")
+    }
+    match 1 {
+        1u16 ... 3 => {}
+        _ => panic!("should match range with inferred end type")
+    }
+}