Provide a span if main function is not present in crate

Unfortunately, the diagnotic machinery does not cope well with an empty
span which can happen if the crate is empty, in which case we merely set
a spanless note.

src/librustc/middle/entry.rs

@@ -16,16 +16,16 @@ struct EntryContext<'a, 'tcx> {
 
     map: &'a hir_map::Map<'tcx>,
 
-    /// The top-level function called 'main'.
+    /// The top-level function called `main`.
     main_fn: Option<(HirId, Span)>,
 
-    /// The function that has attribute named 'main'.
+    /// The function that has attribute named `main`.
     attr_main_fn: Option<(HirId, Span)>,
 
     /// The function that has the attribute 'start' on it.
     start_fn: Option<(HirId, Span)>,
 
-    /// The functions that one might think are 'main' but aren't, e.g.
+    /// The functions that one might think are `main` but aren't, e.g.
     /// main functions not defined at the top level. For diagnostics.
     non_main_fns: Vec<(HirId, Span)> ,
 }
@@ -88,7 +88,7 @@ fn entry_point_type(item: &Item, at_root: bool) -> EntryPointType {
                 EntryPointType::MainAttr
             } else if item.ident.name == sym::main {
                 if at_root {
-                    // This is a top-level function so can be 'main'.
+                    // This is a top-level function so can be `main`.
                     EntryPointType::MainNamed
                 } else {
                     EntryPointType::OtherMain
@@ -109,7 +109,7 @@ fn find_item(item: &Item, ctxt: &mut EntryContext<'_, '_>, at_root: bool) {
                 ctxt.main_fn = Some((item.hir_id, item.span));
             } else {
                 span_err!(ctxt.session, item.span, E0136,
-                          "multiple 'main' functions");
+                          "multiple `main` functions");
             }
         },
         EntryPointType::OtherMain => {
@@ -130,7 +130,7 @@ fn find_item(item: &Item, ctxt: &mut EntryContext<'_, '_>, at_root: bool) {
             if ctxt.start_fn.is_none() {
                 ctxt.start_fn = Some((item.hir_id, item.span));
             } else {
-                struct_span_err!(ctxt.session, item.span, E0138, "multiple 'start' functions")
+                struct_span_err!(ctxt.session, item.span, E0138, "multiple `start` functions")
                     .span_label(ctxt.start_fn.unwrap().1, "previous `start` function here")
                     .span_label(item.span, "multiple `start` functions")
                     .emit();
@@ -148,34 +148,48 @@ fn configure_main(tcx: TyCtxt<'_>, visitor: &EntryContext<'_, '_>) -> Option<(De
     } else if let Some((hir_id, _)) = visitor.main_fn {
         Some((tcx.hir().local_def_id(hir_id), EntryFnType::Main))
     } else {
-        // There is no main function.
-        let mut err = struct_err!(tcx.sess, E0601,
-            "`main` function not found in crate `{}`", tcx.crate_name(LOCAL_CRATE));
-        if !visitor.non_main_fns.is_empty() {
-            // There were some functions named 'main' though. Try to give the user a hint.
-            err.note("the main function must be defined at the crate level \
-                      but you have one or more functions named 'main' that are not \
-                      defined at the crate level. Either move the definition or \
-                      attach the `#[main]` attribute to override this behavior.");
-            for &(_, span) in &visitor.non_main_fns {
-                err.span_note(span, "here is a function named 'main'");
-            }
-            err.emit();
-        } else {
-            if let Some(ref filename) = tcx.sess.local_crate_source_file {
-                err.note(&format!("consider adding a `main` function to `{}`", filename.display()));
-            }
-            if tcx.sess.teach(&err.get_code().unwrap()) {
-                err.note("If you don't know the basics of Rust, you can go look to the Rust Book \
-                          to get started: https://doc.rust-lang.org/book/");
-            }
-            err.emit();
-        }
-
+        no_main_err(tcx, visitor);
         None
     }
 }
 
+fn no_main_err(tcx: TyCtxt<'_>, visitor: &EntryContext<'_, '_>) {
+    // There is no main function.
+    let mut err = struct_err!(tcx.sess, E0601,
+        "`main` function not found in crate `{}`", tcx.crate_name(LOCAL_CRATE));
+    let filename = &tcx.sess.local_crate_source_file;
+    let note = if !visitor.non_main_fns.is_empty() {
+        for &(_, span) in &visitor.non_main_fns {
+            err.span_note(span, "here is a function named `main`");
+        }
+        err.note("you have one or more functions named `main` not defined at the crate level");
+        err.help("either move the `main` function definitions or attach the `#[main]` attribute \
+                  to one of them");
+        // There were some functions named `main` though. Try to give the user a hint.
+        format!("the main function must be defined at the crate level{}",
+                 filename.as_ref().map(|f| format!(" (in `{}`)", f.display())).unwrap_or_default())
+    } else if let Some(filename) = filename {
+        format!("consider adding a `main` function to `{}`", filename.display())
+    } else {
+        String::from("consider adding a `main` function at the crate level")
+    };
+    let sp = tcx.hir().krate().span;
+    // The file may be empty, which leads to the diagnostic machinery not emitting this
+    // note. This is a relatively simple way to detect that case and emit a span-less
+    // note instead.
+    if let Ok(_) = tcx.sess.source_map().lookup_line(sp.lo()) {
+        err.set_span(sp);
+        err.span_label(sp, &note);
+    } else {
+        err.note(&note);
+    }
+    if tcx.sess.teach(&err.get_code().unwrap()) {
+        err.note("If you don't know the basics of Rust, you can go look to the Rust Book \
+                  to get started: https://doc.rust-lang.org/book/");
+    }
+    err.emit();
+}
+
 pub fn find_entry_point(tcx: TyCtxt<'_>) -> Option<(DefId, EntryFnType)> {
     tcx.entry_fn(LOCAL_CRATE)
 }

src/test/ui-fulldeps/hash-stable-is-unstable.rs

@@ -13,3 +13,5 @@ use rustc_macros::HashStable;
 #[derive(HashStable)]
 //~^ use of unstable library feature 'rustc_private'
 struct Test;
+
+fn main() {}

src/test/ui-fulldeps/hash-stable-is-unstable.stderr

@@ -1,7 +1,3 @@
-error[E0601]: `main` function not found in crate `hash_stable_is_unstable`
-   |
-   = note: consider adding a `main` function to `$DIR/hash-stable-is-unstable.rs`
-
 error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
   --> $DIR/hash-stable-is-unstable.rs:3:1
    |
@@ -47,7 +43,6 @@ LL | #[derive(HashStable)]
    = note: for more information, see https://github.com/rust-lang/rust/issues/27812
    = help: add `#![feature(rustc_private)]` to the crate attributes to enable
 
-error: aborting due to 6 previous errors
+error: aborting due to 5 previous errors
 
-Some errors have detailed explanations: E0601, E0658.
-For more information about an error, try `rustc --explain E0601`.
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/associated-type-bounds/inside-adt.rs

@@ -31,3 +31,5 @@ union U2 { f: Box<dyn Iterator<Item: Copy>> }
 union U3 { f: dyn Iterator<Item: 'static> }
 //~^ ERROR associated type bounds are not allowed within structs, enums, or unions
 //~| ERROR could not find defining uses
+
+fn main() {}

src/test/ui/associated-type-bounds/inside-adt.stderr

@@ -52,10 +52,6 @@ error: associated type bounds are not allowed within structs, enums, or unions
 LL | union U3 { f: dyn Iterator<Item: 'static> }
    |                            ^^^^^^^^^^^^^
 
-error[E0601]: `main` function not found in crate `inside_adt`
-   |
-   = note: consider adding a `main` function to `$DIR/inside-adt.rs`
-
 error: could not find defining uses
   --> $DIR/inside-adt.rs:5:29
    |
@@ -110,6 +106,5 @@ error: could not find defining uses
 LL | union U3 { f: dyn Iterator<Item: 'static> }
    |                            ^^^^^^^^^^^^^
 
-error: aborting due to 19 previous errors
+error: aborting due to 18 previous errors
 
-For more information about this error, try `rustc --explain E0601`.

src/test/ui/conditional-compilation/cfg-attr-cfg-2.stderr

@@ -1,6 +1,9 @@
 error[E0601]: `main` function not found in crate `cfg_attr_cfg_2`
+  --> $DIR/cfg-attr-cfg-2.rs:8:1
    |
-   = note: consider adding a `main` function to `$DIR/cfg-attr-cfg-2.rs`
+LL | / #[cfg_attr(foo, cfg(bar))]
+LL | | fn main() { }
+   | |_____________^ consider adding a `main` function to `$DIR/cfg-attr-cfg-2.rs`
 
 error: aborting due to previous error
 

src/test/ui/conditional-compilation/cfg-in-crate-1.stderr

@@ -1,6 +1,8 @@
 error[E0601]: `main` function not found in crate `cfg_in_crate_1`
+  --> $DIR/cfg-in-crate-1.rs:3:1
    |
-   = note: consider adding a `main` function to `$DIR/cfg-in-crate-1.rs`
+LL | #![cfg(bar)]
+   | ^^^^^^^^^^^^ consider adding a `main` function to `$DIR/cfg-in-crate-1.rs`
 
 error: aborting due to previous error
 

src/test/ui/continue-after-missing-main.stderr

@@ -1,6 +1,14 @@
 error[E0601]: `main` function not found in crate `continue_after_missing_main`
+  --> $DIR/continue-after-missing-main.rs:1:1
    |
-   = note: consider adding a `main` function to `$DIR/continue-after-missing-main.rs`
+LL | / #![allow(dead_code)]
+LL | |
+LL | | // error-pattern:`main` function not found in crate
+LL | |
+...  |
+LL | |
+LL | | }
+   | |_^ consider adding a `main` function to `$DIR/continue-after-missing-main.rs`
 
 error[E0623]: lifetime mismatch
   --> $DIR/continue-after-missing-main.rs:30:56

src/test/ui/elided-test.stderr

@@ -1,6 +1,10 @@
 error[E0601]: `main` function not found in crate `elided_test`
+  --> $DIR/elided-test.rs:5:1
    |
-   = note: consider adding a `main` function to `$DIR/elided-test.rs`
+LL | / #[test]
+LL | | fn main() {
+LL | | }
+   | |_^ consider adding a `main` function to `$DIR/elided-test.rs`
 
 error: aborting due to previous error
 

src/test/ui/error-codes/E0138.stderr

@@ -1,4 +1,4 @@
-error[E0138]: multiple 'start' functions
+error[E0138]: multiple `start` functions
   --> $DIR/E0138.rs:7:1
    |
 LL | fn foo(argc: isize, argv: *const *const u8) -> isize { 0 }

src/test/ui/error-codes/E0601.rs

@@ -1 +1 @@
-// Test for main function not found.
+//~ ERROR `main` function not found

src/test/ui/error-codes/E0601.stderr

@@ -1,6 +1,8 @@
 error[E0601]: `main` function not found in crate `E0601`
+  --> $DIR/E0601.rs:1:37
    |
-   = note: consider adding a `main` function to `$DIR/E0601.rs`
+LL |
+   |                                     ^ consider adding a `main` function to `$DIR/E0601.rs`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-46101.rs

@@ -2,3 +2,5 @@
 trait Foo {}
 #[derive(Foo::Anything)] //~ ERROR failed to resolve: partially resolved path in a derive macro
 struct S;
+
+fn main() {}

src/test/ui/issues/issue-46101.stderr

@@ -4,11 +4,6 @@ error[E0433]: failed to resolve: partially resolved path in a derive macro
 LL | #[derive(Foo::Anything)]
    |          ^^^^^^^^^^^^^ partially resolved path in a derive macro
 
-error[E0601]: `main` function not found in crate `issue_46101`
-   |
-   = note: consider adding a `main` function to `$DIR/issue-46101.rs`
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-Some errors have detailed explanations: E0433, E0601.
-For more information about an error, try `rustc --explain E0433`.
+For more information about this error, try `rustc --explain E0433`.

src/test/ui/issues/issue-49040.rs

@@ -1,2 +1,3 @@
 #![allow(unused_variables)]; //~ ERROR expected item, found `;`
+//~^ ERROR `main` function
 fn foo() {}

src/test/ui/issues/issue-49040.stderr

@@ -5,8 +5,12 @@ LL | #![allow(unused_variables)];
    |                            ^ help: remove this semicolon
 
 error[E0601]: `main` function not found in crate `issue_49040`
+  --> $DIR/issue-49040.rs:1:1
    |
-   = note: consider adding a `main` function to `$DIR/issue-49040.rs`
+LL | / #![allow(unused_variables)];
+LL | |
+LL | | fn foo() {}
+   | |__^ consider adding a `main` function to `$DIR/issue-49040.rs`
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-60057.rs

@@ -15,3 +15,5 @@ impl A {
         }
     }
 }
+
+fn main() {}

src/test/ui/issues/issue-60057.stderr

@@ -10,11 +10,6 @@ error[E0425]: cannot find value `banana` in this scope
 LL |             banana: banana
    |                     ^^^^^^ help: you might have meant to use the available field: `self.banana`
 
-error[E0601]: `main` function not found in crate `issue_60057`
-   |
-   = note: consider adding a `main` function to `$DIR/issue-60057.rs`
-
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0425, E0601.
-For more information about an error, try `rustc --explain E0425`.
+For more information about this error, try `rustc --explain E0425`.

src/test/ui/json-short.stderr

@@ -11,7 +11,7 @@ fn main() {
 
 If you don't know the basics of Rust, you can go look to the Rust Book to get
 started: https://doc.rust-lang.org/book/
-"},"level":"error","spans":[],"children":[{"message":"consider adding a `main` function to `$DIR/json-short.rs`","code":null,"level":"note","spans":[],"children":[],"rendered":null}],"rendered":"error[E0601]: `main` function not found in crate `json_short`
+"},"level":"error","spans":[{"file_name":"$DIR/json-short.rs","byte_start":76,"byte_end":76,"line_start":2,"line_end":2,"column_start":63,"column_end":63,"is_primary":true,"text":[{"text":"// compile-flags: --json=diagnostic-short --error-format=json","highlight_start":63,"highlight_end":63}],"label":"consider adding a `main` function to `$DIR/json-short.rs`","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[],"rendered":"$DIR/json-short.rs:2:63: error[E0601]: `main` function not found in crate `json_short`
 "}
 {"message":"aborting due to previous error","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to previous error
 "}

src/test/ui/lifetime-before-type-params.rs

@@ -7,3 +7,5 @@ fn third<T, U, 'a>() {}
 //~^ ERROR lifetime parameters must be declared prior to type parameters
 fn fourth<'a, T, 'b, U, 'c, V>() {}
 //~^ ERROR lifetime parameters must be declared prior to type parameters
+
+fn main() {}

src/test/ui/lifetime-before-type-params.stderr

@@ -22,10 +22,5 @@ error: lifetime parameters must be declared prior to type parameters
 LL | fn fourth<'a, T, 'b, U, 'c, V>() {}
    |          --------^^-----^^---- help: reorder the parameters: lifetimes, then types: `<'a, 'b, 'c, T, U, V>`
 
-error[E0601]: `main` function not found in crate `lifetime_before_type_params`
-   |
-   = note: consider adding a `main` function to `$DIR/lifetime-before-type-params.rs`
-
-error: aborting due to 5 previous errors
+error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0601`.

src/test/ui/main-wrong-location.rs

@@ -1,4 +1,5 @@
 mod m {
+//~^ ERROR `main` function not found
     // An inferred main entry point (that doesn't use #[main])
     // must appear at the top of the crate
     fn main() { }

src/test/ui/main-wrong-location.stderr

@@ -1,11 +1,21 @@
 error[E0601]: `main` function not found in crate `main_wrong_location`
+  --> $DIR/main-wrong-location.rs:1:1
    |
-   = note: the main function must be defined at the crate level but you have one or more functions named 'main' that are not defined at the crate level. Either move the definition or attach the `#[main]` attribute to override this behavior.
-note: here is a function named 'main'
-  --> $DIR/main-wrong-location.rs:4:5
+LL | / mod m {
+LL | |
+LL | |     // An inferred main entry point (that doesn't use #[main])
+LL | |     // must appear at the top of the crate
+LL | |     fn main() { }
+LL | | }
+   | |_^ the main function must be defined at the crate level (in `$DIR/main-wrong-location.rs`)
+   |
+note: here is a function named `main`
+  --> $DIR/main-wrong-location.rs:5:5
    |
 LL |     fn main() { }
    |     ^^^^^^^^^^^^^
+   = note: you have one or more functions named `main` not defined at the crate level
+   = help: either move the `main` function definitions or attach the `#[main]` attribute to one of them
 
 error: aborting due to previous error
 

src/test/ui/missing/missing-main.stderr

@@ -1,6 +1,8 @@
 error[E0601]: `main` function not found in crate `missing_main`
+  --> $DIR/missing-main.rs:2:1
    |
-   = note: consider adding a `main` function to `$DIR/missing-main.rs`
+LL | fn mian() { }
+   | ^^^^^^^^^^^^^ consider adding a `main` function to `$DIR/missing-main.rs`
 
 error: aborting due to previous error
 

src/test/ui/parser-recovery-1.rs

@@ -3,6 +3,7 @@
 // Test that we can recover from missing braces in the parser.
 
 trait Foo {
+//~^ ERROR `main` function not found
     fn bar() {
         let x = foo();
         //~^ ERROR cannot find function `foo` in this scope