Auto merge of #71487 - rcoh:71471-shebang, r=petrochenkov

Fix bug in shebang handling

Shebang handling was too agressive in stripping out the first line in cases where it is actually _not_ a shebang, but instead, valid rust (#70528). This is a second attempt at resolving this issue (the first attempt was reverted, for, among other reasons, causing an ICE in certain cases (#71372, #71471).

The behavior is now codified by a number of UI tests, but simply:
For the first line to be a shebang, the following must all be true:
1. The line must start with `#!`
2. The line must contain a non-whitespace character after `#!`
3. The next character in the file, ignoring comments & whitespace must not be `[`

I believe this is a strict superset of what we used to allow, so perhaps a crater run is unnecessary, but probably not a terrible idea.

Fixes #70528

src/test/ui/parser/shebang/issue-71471-ignore-tidy.rs

@@ -0,0 +1,2 @@
+
+#!B //~ expected `[`, found `B`

src/test/ui/parser/shebang/issue-71471-ignore-tidy.stderr

@@ -0,0 +1,8 @@
+error: expected `[`, found `B`
+  --> $DIR/issue-71471-ignore-tidy.rs:2:3
+   |
+LL | #!B
+   |   ^ expected `[`
+
+error: aborting due to previous error
+

src/test/ui/parser/shebang/multiline-attrib.rs

@@ -0,0 +1,7 @@
+#!
+[allow(unused_variables)]
+// check-pass
+
+fn main() {
+    let x = 5;
+}

src/test/ui/parser/shebang/regular-attrib.rs

@@ -0,0 +1,5 @@
+#![allow(unused_variables)]
+// check-pass
+fn main() {
+    let x = 5;
+}

src/test/ui/parser/shebang/shebang-and-attrib.rs

@@ -0,0 +1,9 @@
+#!/usr/bin/env run-cargo-script
+
+// check-pass
+#![allow(unused_variables)]
+
+
+fn main() {
+    let x = 5;
+}

src/test/ui/parser/shebang/shebang-comment.rs

@@ -0,0 +1,6 @@
+#!//bin/bash
+
+// check-pass
+fn main() {
+    println!("a valid shebang (that is also a rust comment)")
+}

src/test/ui/parser/shebang/shebang-must-start-file.rs

@@ -0,0 +1,6 @@
+// something on the first line for tidy
+#!/bin/bash  //~ expected `[`, found `/`
+
+fn main() {
+    println!("ok!");
+}

src/test/ui/parser/shebang/shebang-must-start-file.stderr

@@ -0,0 +1,8 @@
+error: expected `[`, found `/`
+  --> $DIR/shebang-must-start-file.rs:2:3
+   |
+LL | #!/bin/bash
+   |   ^ expected `[`
+
+error: aborting due to previous error
+

src/test/ui/parser/shebang/sneaky-attrib.rs

@@ -0,0 +1,16 @@
+#!//bin/bash
+
+
+// This could not possibly be a shebang & also a valid rust file, since a Rust file
+// can't start with `[`
+/*
+    [ (mixing comments to also test that we ignore both types of comments)
+
+ */
+
+[allow(unused_variables)]
+
+// check-pass
+fn main() {
+    let x = 5;
+}

src/test/ui/parser/shebang/valid-shebang.rs

@@ -0,0 +1,6 @@
+#!/usr/bin/env run-cargo-script
+
+// check-pass
+fn main() {
+    println!("Hello World!");
+}