Accept multiple --error-format flags

src/librustc/session/config.rs

@@ -1816,7 +1816,7 @@ pub fn rustc_optgroups() -> Vec<RustcOptGroup> {
         ),
         opt::opt_s("", "sysroot", "Override the system root", "PATH"),
         opt::multi("Z", "", "Set internal debugging options", "FLAG"),
-        opt::opt_s(
+        opt::multi_s(
             "",
             "error-format",
             "How errors and other messages are produced",
@@ -1985,7 +1985,8 @@ pub fn build_session_options_and_crate_config(
     // is unstable, it will not be present. We have to use opts_present not
     // opt_present because the latter will panic.
     let error_format = if matches.opts_present(&["error-format".to_owned()]) {
-        match matches.opt_str("error-format").as_ref().map(|s| &s[..]) {
+        // Last argument takes precedence
+        match matches.opt_strs("error-format").last().map(|s| &s[..]) {
             None |
             Some("human") => ErrorOutputType::HumanReadable(HumanReadableErrorType::Default(color)),
             Some("human-annotate-rs") => {

src/test/ui/error-format-precedence.rs

@@ -0,0 +1,9 @@
+// compile-flags: --error-format=human --error-format=json
+
+fn foo(_: u32) {}
+
+fn main() {
+    foo("Bonjour".to_owned());
+    let x = 0u32;
+    x.salut();
+}

src/test/ui/error-format-precedence.stderr

@@ -0,0 +1,55 @@
+{"message":"mismatched types","code":{"code":"E0308","explanation":"
+This error occurs when the compiler was unable to infer the concrete type of a
+variable. It can occur for several cases, the most common of which is a
+mismatch in the expected type that the compiler inferred for a variable's
+initializing expression, and the actual type explicitly assigned to the
+variable.
+
+For example:
+
+```compile_fail,E0308
+let x: i32 = \"I am not a number!\";
+//     ~~~   ~~~~~~~~~~~~~~~~~~~~
+//      |             |
+//      |    initializing expression;
+//      |    compiler infers type `&str`
+//      |
+//    type `i32` assigned to variable `x`
+```
+"},"level":"error","spans":[{"file_name":"$DIR/error-format-precedence.rs","byte_start":99,"byte_end":119,"line_start":6,"line_end":6,"column_start":9,"column_end":29,"is_primary":true,"text":[{"text":"    foo(\"Bonjour\".to_owned());","highlight_start":9,"highlight_end":29}],"label":"expected u32, found struct `std::string::String`","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"expected type `u32`
+   found type `std::string::String`","code":null,"level":"note","spans":[],"children":[],"rendered":null}],"rendered":"error[E0308]: mismatched types
+  --> $DIR/error-format-precedence.rs:6:9
+   |
+LL |     foo(\"Bonjour\".to_owned());
+   |         ^^^^^^^^^^^^^^^^^^^^ expected u32, found struct `std::string::String`
+   |
+   = note: expected type `u32`
+              found type `std::string::String`
+
+"}
+{"message":"no method named `salut` found for type `u32` in the current scope","code":{"code":"E0599","explanation":"
+This error occurs when a method is used on a type which doesn't implement it:
+
+Erroneous code example:
+
+```compile_fail,E0599
+struct Mouth;
+
+let x = Mouth;
+x.chocolate(); // error: no method named `chocolate` found for type `Mouth`
+               //        in the current scope
+```
+"},"level":"error","spans":[{"file_name":"$DIR/error-format-precedence.rs","byte_start":146,"byte_end":151,"line_start":8,"line_end":8,"column_start":7,"column_end":12,"is_primary":true,"text":[{"text":"    x.salut();","highlight_start":7,"highlight_end":12}],"label":null,"suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[],"rendered":"error[E0599]: no method named `salut` found for type `u32` in the current scope
+  --> $DIR/error-format-precedence.rs:8:7
+   |
+LL |     x.salut();
+   |       ^^^^^
+
+"}
+{"message":"aborting due to 2 previous errors","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to 2 previous errors
+
+"}
+{"message":"Some errors have detailed explanations: E0308, E0599.","code":null,"level":"","spans":[],"children":[],"rendered":"Some errors have detailed explanations: E0308, E0599.
+"}
+{"message":"For more information about an error, try `rustc --explain E0308`.","code":null,"level":"","spans":[],"children":[],"rendered":"For more information about an error, try `rustc --explain E0308`.
+"}