Change:
{"message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type. <del> Erroneous code example: </del> <ins> Erroneous code examples: </ins> ```compile_fail,E0308 <del> let x: i32 = "I am not a number!"; //     ~~~   ~~~~~~~~~~~~~~~~~~~~ //      |             | //      |    initializing expression; //      |    compiler infers type `&str` //      | //    type `i32` assigned to variable `x` </del> <ins> fn plus_one(x: i32) -> i32 { x + 1 } plus_one("Not a number"); //       ^^^^^^^^^^^^^^ expected `i32`, found `&str` if "Not a bool" { // ^^^^^^^^^^^^ expected `bool`, found `&str` } let x: f32 = "Not a float"; //     ---   ^^^^^^^^^^^^^ expected `f32`, found `&str` //     | //     expected due to this </ins> ``` <del> This error occurs when the compiler is unable to infer the concrete type of a variable. It can occur in several cases, the most common being a mismatch between two types: the type the author explicitly assigned, and the type the compiler inferred. </del> <ins> This error occurs when an expression was used in a place where the compiler expected an expression of a different type. It can occur in several cases, the most common being when calling a function and passing an argument which has a different type than the matching type in the function declaration. </ins> "},"level":"error","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":621,"byte_end":622,"line_start":17,"line_end":17,"column_start":22,"column_end":23,"is_primary":true,"text":[{"text":"    let s : String = 1;  // Error in the middle of line.","highlight_start":22,"highlight_end":23}],"label":"expected struct `String`, found integer","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":612,"byte_end":618,"line_start":17,"line_end":17,"column_start":13,"column_end":19,"is_primary":false,"text":[{"text":"    let s : String = 1;  // Error in the middle of line.","highlight_start":13,"highlight_end":19}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"try using a conversion method","code":null,"level":"help","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":621,"byte_end":622,"line_start":17,"line_end":17,"column_start":22,"column_end":23,"is_primary":true,"text":[{"text":"    let s : String = 1;  // Error in the middle of line.","highlight_start":22,"highlight_end":23}],"label":null,"suggested_replacement":"1.to_string()","suggestion_applicability":"MaybeIncorrect","expansion":null}],"children":[],"rendered":null}],"rendered":"$DIR/json-bom-plus-crlf-multifile-aux.rs:17:22: error[E0308]: mismatched types "} {"message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type. <del> Erroneous code example: </del> <ins> Erroneous code examples: </ins> ```compile_fail,E0308 <del> let x: i32 = "I am not a number!"; //     ~~~   ~~~~~~~~~~~~~~~~~~~~ //      |             | //      |    initializing expression; //      |    compiler infers type `&str` //      | //    type `i32` assigned to variable `x` </del> <ins> fn plus_one(x: i32) -> i32 { x + 1 } plus_one("Not a number"); //       ^^^^^^^^^^^^^^ expected `i32`, found `&str` if "Not a bool" { // ^^^^^^^^^^^^ expected `bool`, found `&str` } let x: f32 = "Not a float"; //     ---   ^^^^^^^^^^^^^ expected `f32`, found `&str` //     | //     expected due to this </ins> ``` <del> This error occurs when the compiler is unable to infer the concrete type of a variable. It can occur in several cases, the most common being a mismatch between two types: the type the author explicitly assigned, and the type the compiler inferred. </del> <ins> This error occurs when an expression was used in a place where the compiler expected an expression of a different type. It can occur in several cases, the most common being when calling a function and passing an argument which has a different type than the matching type in the function declaration. </ins> "},"level":"error","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":681,"byte_end":682,"line_start":19,"line_end":19,"column_start":22,"column_end":23,"is_primary":true,"text":[{"text":"    let s : String = 1","highlight_start":22,"highlight_end":23}],"label":"expected struct `String`, found integer","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":672,"byte_end":678,"line_start":19,"line_end":19,"column_start":13,"column_end":19,"is_primary":false,"text":[{"text":"    let s : String = 1","highlight_start":13,"highlight_end":19}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"try using a conversion method","code":null,"level":"help","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":681,"byte_end":682,"line_start":19,"line_end":19,"column_start":22,"column_end":23,"is_primary":true,"text":[{"text":"    let s : String = 1","highlight_start":22,"highlight_end":23}],"label":null,"suggested_replacement":"1.to_string()","suggestion_applicability":"MaybeIncorrect","expansion":null}],"children":[],"rendered":null}],"rendered":"$DIR/json-bom-plus-crlf-multifile-aux.rs:19:22: error[E0308]: mismatched types "} {"message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type. <del> Erroneous code example: </del> <ins> Erroneous code examples: </ins> ```compile_fail,E0308 <del> let x: i32 = "I am not a number!"; //     ~~~   ~~~~~~~~~~~~~~~~~~~~ //      |             | //      |    initializing expression; //      |    compiler infers type `&str` //      | //    type `i32` assigned to variable `x` </del> <ins> fn plus_one(x: i32) -> i32 { x + 1 } plus_one("Not a number"); //       ^^^^^^^^^^^^^^ expected `i32`, found `&str` if "Not a bool" { // ^^^^^^^^^^^^ expected `bool`, found `&str` } let x: f32 = "Not a float"; //     ---   ^^^^^^^^^^^^^ expected `f32`, found `&str` //     | //     expected due to this </ins> ``` <del> This error occurs when the compiler is unable to infer the concrete type of a variable. It can occur in several cases, the most common being a mismatch between two types: the type the author explicitly assigned, and the type the compiler inferred. </del> <ins> This error occurs when an expression was used in a place where the compiler expected an expression of a different type. It can occur in several cases, the most common being when calling a function and passing an argument which has a different type than the matching type in the function declaration. </ins> "},"level":"error","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":745,"byte_end":746,"line_start":23,"line_end":23,"column_start":1,"column_end":2,"is_primary":true,"text":[{"text":"1;  // Error after the newline.","highlight_start":1,"highlight_end":2}],"label":"expected struct `String`, found integer","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":735,"byte_end":741,"line_start":22,"line_end":22,"column_start":13,"column_end":19,"is_primary":false,"text":[{"text":"    let s : String =","highlight_start":13,"highlight_end":19}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"try using a conversion method","code":null,"level":"help","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":745,"byte_end":746,"line_start":23,"line_end":23,"column_start":1,"column_end":2,"is_primary":true,"text":[{"text":"1;  // Error after the newline.","highlight_start":1,"highlight_end":2}],"label":null,"suggested_replacement":"1.to_string()","suggestion_applicability":"MaybeIncorrect","expansion":null}],"children":[],"rendered":null}],"rendered":"$DIR/json-bom-plus-crlf-multifile-aux.rs:23:1: error[E0308]: mismatched types "} {"message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type. <del> Erroneous code example: </del> <ins> Erroneous code examples: </ins> ```compile_fail,E0308 <del> let x: i32 = "I am not a number!"; //     ~~~   ~~~~~~~~~~~~~~~~~~~~ //      |             | //      |    initializing expression; //      |    compiler infers type `&str` //      | //    type `i32` assigned to variable `x` </del> <ins> fn plus_one(x: i32) -> i32 { x + 1 } plus_one("Not a number"); //       ^^^^^^^^^^^^^^ expected `i32`, found `&str` if "Not a bool" { // ^^^^^^^^^^^^ expected `bool`, found `&str` } let x: f32 = "Not a float"; //     ---   ^^^^^^^^^^^^^ expected `f32`, found `&str` //     | //     expected due to this </ins> ``` <del> This error occurs when the compiler is unable to infer the concrete type of a variable. It can occur in several cases, the most common being a mismatch between two types: the type the author explicitly assigned, and the type the compiler inferred. </del> <ins> This error occurs when an expression was used in a place where the compiler expected an expression of a different type. It can occur in several cases, the most common being when calling a function and passing an argument which has a different type than the matching type in the function declaration. </ins> "},"level":"error","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":801,"byte_end":809,"line_start":25,"line_end":26,"column_start":22,"column_end":6,"is_primary":true,"text":[{"text":"    let s : String = (","highlight_start":22,"highlight_end":23},{"text":"    );  // Error spanning the newline.","highlight_start":1,"highlight_end":6}],"label":"expected struct `String`, found `()`","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":792,"byte_end":798,"line_start":25,"line_end":25,"column_start":13,"column_end":19,"is_primary":false,"text":[{"text":"    let s : String = (","highlight_start":13,"highlight_end":19}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[],"rendered":"$DIR/json-bom-plus-crlf-multifile-aux.rs:25:22: error[E0308]: mismatched types "} {"message":"aborting due to 4 previous errors","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to 4 previous errors

Discussion:
I believe the error message for E0308 is incorrect. Here it is, with my emphasis on the part that seems incorrect: Isn't this error because the compiler was able to infer the type, but it was not what was expected? For example, in the example provided in the error explanation, the value assigned to is most definitely , and is expected, so the types don't match. modify labels: A-diagnostics D-incorrect D-confusing C-bug
Also the explanation says: between two types: the type the author explicitly assigned, and the type the compiler inferred. Isn't the most common case passing the wrong type to a function? Usually people do not provide a type annotation to a unless it is necessary to help the compiler figure out what concrete type a generic is. Perhaps the explanation could be clarified and expanded upon for that.
You beat me to it :)
Mentoring instructions: Change to have an up-to-date diagnostic and appropriate long description.
On it.
Proposed text:
Overall text looks good. Can you open a PR and -mention me on it?