Rollup merge of #33914 - GuillaumeGomez:improve_err_expl, r=GuillaumeGomez

Improve err expl

r? @steveklabnik

Author: GuillaumeGomez

src/librustc/diagnostics.rs

@@ -406,15 +406,58 @@ function `main()`. If there are multiple such functions, please rename one.
 "##,
 
 E0137: r##"
+More than one function was declared with the `#[main]` attribute.
+
+Erroneous code example:
+
+```compile_fail
+#![feature(main)]
+
+#[main]
+fn foo() {}
+
+#[main]
+fn f() {} // error: multiple functions with a #[main] attribute
+```
+
 This error indicates that the compiler found multiple functions with the
 `#[main]` attribute. This is an error because there must be a unique entry
-point into a Rust program.
+point into a Rust program. Example:
+
+```
+#![feature(main)]
+
+#[main]
+fn f() {} // ok!
+```
 "##,
 
 E0138: r##"
+More than one function was declared with the `#[start]` attribute.
+
+Erroneous code example:
+
+```compile_fail
+#![feature(start)]
+
+#[start]
+fn foo(argc: isize, argv: *const *const u8) -> isize {}
+
+#[start]
+fn f(argc: isize, argv: *const *const u8) -> isize {}
+// error: multiple 'start' functions
+```
+
 This error indicates that the compiler found multiple functions with the
 `#[start]` attribute. This is an error because there must be a unique entry
-point into a Rust program.
+point into a Rust program. Example:
+
+```
+#![feature(start)]
+
+#[start]
+fn foo(argc: isize, argv: *const *const u8) -> isize { 0 } // ok!
+```
 "##,
 
 // FIXME link this to the relevant turpl chapters for instilling fear of the
@@ -495,6 +538,17 @@ call to `mem::forget(v)` in case you want to avoid destructors being called.
 "##,
 
 E0152: r##"
+A lang item was redefined.
+
+Erroneous code example:
+
+```compile_fail
+#![feature(lang_items)]
+
+#[lang = "panic_fmt"]
+struct Foo; // error: duplicate lang item found: `panic_fmt`
+```
+
 Lang items are already implemented in the standard library. Unless you are
 writing a free-standing application (e.g. a kernel), you do not need to provide
 them yourself.

src/librustc_passes/diagnostics.rs

@@ -50,11 +50,36 @@ match 5u32 {
 "##,
 
 E0161: r##"
+A value was moved. However, its size was not known at compile time, and only
+values of a known size can be moved.
+
+Erroneous code example:
+
+```compile_fail
+#![feature(box_syntax)]
+
+fn main() {
+    let array: &[isize] = &[1, 2, 3];
+    let _x: Box<[isize]> = box *array;
+    // error: cannot move a value of type [isize]: the size of [isize] cannot
+    //        be statically determined
+}
+```
+
 In Rust, you can only move a value when its size is known at compile time.
 
 To work around this restriction, consider "hiding" the value behind a reference:
 either `&x` or `&mut x`. Since a reference has a fixed size, this lets you move
-it around as usual.
+it around as usual. Example:
+
+```
+#![feature(box_syntax)]
+
+fn main() {
+    let array: &[isize] = &[1, 2, 3];
+    let _x: Box<&[isize]> = box array; // ok!
+}
+```
 "##,
 
 E0265: r##"